OYSTER PARASITISM BY 

LABYRINTHOMYXA MARINA 

IN FLORIDA 

J. A. Quick, Jr. and J. G . Mackin 

PROFESSIONAL PAPERS SERIES 

Number Thirteen 

April 1971 

FLORIDA DEPARTMENT OF NATURAL RESOURCES 

MARINE RESEARCH LABORATORY 

ST. PETERSBURG, FLORIDA 
Effective July 1, 1969, agencies of the State of Florida underwent 

legislative reorganization and the Florida Board of Conservation was 

renamed the Florida Department of Natural Resources. Similarly, the 

former Division of Salt Water Fisheries became the Division of Marine 

Resources. 

Extra effort always goes into the preparation of our 

publications. Cost of production limits the number that can be 

printed. In order that the fullest possible use can be made of 

each copy, please be good enough to give this copy to your 

nearest library when you have finished with it. 
OYSTER PARASITISM BY 

LABYRINTHOMYXA MARINA 

IN FLORIDA 

J. A. Quick, Jr. and J. G. Mackin* 

Professional Papers Series 

Number Thirteen 

Florida Department of Natural Resources Marine Research Laboratory 

St. Petersburg, Florida 

* Department of Biology, Texas A. and M. University 
FLORIDA DEPARTMENT OF NATURAL RESOURCES 

REUBIN O'D. ASKEW 

Governor 

RICHARD (DICK) STONE 

Secretary of State 

ROBERT L. SHEVIN 

A ttorney General 

FRED 0. DICKINSON, JR. 

Comptroller 

THOMAS D. O'MALLEY 

Treasurer 

DOYLE CONNER 

Commissioner of Agriculture 

FLOYD T. CHRISTIAN 

Commissioner of Education 

RANDOLPH HODGES 

Executive Director 

HARMON W. SHIELDS 

Director, Div. of Marine Resources 

11 
SCIENTIFIC BOARD OF ADVISORS 

Marine Research Laboratory 

Division of Marine Resources 

Florida Department of Natural Resources 

Dr. A. F. Chestnut 

Institute of Fishery Research 

University of North Carolina 

Morehead City, North Carolina 

Dr. Albert Collier 

Oceanographic Institute 

Florida State University 

Tallahassee, Florida 

Mr. Ronald C. Phillips 

Seattle Pacific College 

Department of Botany 

Seattle, Washington 

Dr. Sammy Ray 

Texas A. & M. Marine Station 

Fort Crockett 

Galveston, Texas 

Dr. John D. Costlow, Jr. 

Duke University Marine Laboratory 

Beaufort, North Carolina 

Dr. L. B. Holthuis, Curator 

Rijksmuseum van Natuurlijke Historic 

Raamsteeg 2, Leiden 

The Netherlands 

Dr. C. Lavett Smith 

Assistant Curator 

Department of Ichthyology 

The American Museum of Natural History 

New York, New York 

Dr. Franklin Sogandares-Bernal 

Zoology Department 

Tulane University 

New Orleans, Louisiana 

Dr. Victor G. Springer 

Associate Curator 

Division of Fishes 

Smithsonian Institution 

U. S. National Museum 

Washington, D. C. 

Dr. Victor L. Loosanoff 

University of the Pacific 

Stockton, California 

Dr. J. G. Mackin Dr. W. B. Wilson 

Department of Biology Texas A. & M. Marine Station 

Texas A. & M. University Fort Crockett 

College Station, Texas Galveston, Texas 

iii 
MARINE RESEARCH LABORATORY 

FLORIDA DEPARTMENT OF NATURAL RESOURCES 

P. 0. Drawer F, St. Petersburg, Florida 33731 

Robert M. Ingle, M.S. 

Chief, Bureau of Marine Science & Technology 

Edwin A. Joyce, Jr., M.S. 

Supervisor, Marine Research Laboratory 

LABORATORY STAFF 

ICHTHYOLOGY 

Robert W. Topp, M.S. Senior Fisheries Biologist 

Frank H. Hoff, Jr., B.A Marine Biologist 

Dion Powell, B.A. Marine Sciences Technician 

Robert F. Presley, B.S. Marine Biologist 

Kenneth R. Halscott Student Research Assistant 

HISTOLOGY 

Jean Williams Marine Sciences Technician 

Rena S. Barco Marine Sciences Technician 

Sandra L. Farrington Student Research Assistant 

LARVAL FISHES 

Bonnie M. Eldred Marine Biologist 

Charles R. Futch, B.S. Marine Biologist 

PELAGIC FISHES 

Dale S. Beaumariage, B.S. Marine Biologist 

INVERTEBRATES 

William G. Lyons, B.A. Marine Sciences Technician 

David K. Camp, B.S. Marine Biologist 

Stephen P. Cobb, B.S. Marine Biologist 

Linda H. Lyons Marine Sciences Technician 

William D. Leever Student Research Assistant 

STONE CRAB RESEARCH 

Thomas Savage, M.S. Marine Biologist 

Charles E. Kalman Marine Sciences Technician 

PHYSALIA 

F. Stuart Kennedy, Jr., B.S Marine Biologist 

OYSTER BIOLOGY 

Stanley W. Morey, B.S. Marine Biologist 

Walter K. Havens, B.S. Marine Sciences Technician 

Scott A. Willis, B.S. Marine Sciences Technician 

Janie M. Staley, S.S. Marine Sciences Technician 

iv 
Jack F. Van Breedveld Marine Sciences Technician 

OYSTER PARASITES 

Joe A. Quick, Jr., B.S. Marine Biologist 

Melvin W. Bidwell, Jr. Student Research Assistant 

CLAM BIOLOGY 

Mark F. Godcharles, B.A. Marine Biologist 

MARICULTURE 

Carlton F. Rowell, B.A. Marine Biologist 

Charles E. Dugan, B.A. Marine Biologist 

ORGANIC CHEMISTRY 

Mary Ann Burklew, B.A. Marine Biologist 

STUART FIELD LABORATORY 

Ross Witham Marine Sciences Technician 

KEY WEST FIELD LABORATORY 

Sean J. Bollar, B.S. Marine Biologist 

Edward J. Little, Jr., B.S. Marine Biologist 

PENSACOLA FIELD LABORATORY 

Robert M. Gallagher, B.S. Marine Biologist 

ST. JOHNS FIELD LABORATORY (Sanford) 

Roy 0. Williams, B.S. Marine Biologist 

Gerard E. Bruger, B.S. Marine Biologist 

APALACHICOLA FIELD LABORATORY 

James G. Mills, Jr., B.A. Marine Biologist 

THERMAL ADDITION STUDIES (Crystal River) 

Churchill B. Grimes, B.A. Marine Biologist 

Joe A. Mountain, B.S. Marine Biologist 

WEST PALM BEACH FIELD LABORATORY 

John W. Jolley, Jr., B.S. Marine Biologist 

CHOCTAWHATCHEE BAY FIELD LABORATORY 

Edwin W. Irby, B.A Marine Biologist 

RESEARCH VESSEL 

Earl Girard Marine Captain 

Malcolm L. Hollinger, B.S. Marine Biologist 

Thomas A. Frakes, B.S. Marine Sciences Technician 

Walter C. Jaap, B.S. Marine Sciences Technician 

Terry R. Pulver, S.S. Marine Sciences Technician 

Andrew Kaloski, B.S. Marine Biologist 

ADMINISTRATION 

Alan H. Sellen Director of Administration 

Betty Accurso Office Supervisor 

Jane F. Biggs, B.A. Publications 

Heather Grothe Reports 
John J. Clopine, M.S.L.S. Librarian 

Marjorie S. Clopine, M.S. Library Assistant 

Cheryl Medema Personnel 

Paula Newton Budget and Financial 

Marion W. Stance11 Purchasing 

Katherine B. Stoelzel,B.A. Archives 

Linda L. Gilmore Student Assistant 

Mary E. Miller Student Assistant 

Lana S. Tester Student Assistant 

ENGINEERING 

Marco A. Omeechevarria 

Phillip Ford, Jr. 

Vladimir Janek 

George Omeechevarria Student Assistant 

TALLAHASSEE OFFICE 

William K. Whitfield, Jr., B.A. Administrative Assistant 

Hazel Jones Administrative Assistant 

Marjorie Core Records 

June Smith Secretary 

APALACHICOLA 

Joseph Martina, Jr. Field Supervisor (Div. Oyster Culture) 

vi 
CONTENTS 

ABSTRACT 1 

INTRODUCTION 1 

METHODS AND MATERIALS 1 

STATION DESCRIPTIONS 1 

LABYRINTHOMYXA DISTRIBUTION STUDY 4 

INVESTIGATIONS OF ETIOLOGY AND MORTALITY 6 

Plot Studies 6 

Calculation of Adjusted Mortalities 7 

Relation of Gapers to Oyster Death Rate 8 

Year Class Analyses 8 

RESULTS AND CONCLUSIONS 8 

PART I: STUDIES OF LABYRINTHOMYXA DISTRIBUTION 8 

COMPARISON OF ANTIBIOTIC TREATMENTS 8 

LABYRINTHOMYXA DISTRIBUTION IN FLORIDA 11 

Relationship of Labyrinthomyxa to Season 11 

Relationship of Labyrinthomyxa to Temperature 11 

Distribution of Labyrinthomyxa Intensities 11 

Relationship of Labyrinthomyxa to Salinity 13 

Relationship of Labyrinthomyxa to Oyster Size 20 

Relationship of Labyrinthomyxa to Oyster Depth 24 

Relationship of Labyrinthomyxa to Oyster Condition 24 

Annual Labyrinthomyxa Parasitism at Selected Sites 24 

PART II: STUDIES OF OYSTER MORTALITIES 27 

YEAR CLASS ANALYSIS AND ANNUAL MORTALITY 27 

Determination of Oyster Age by Shell Contours 27 

Aging Technique 27 

General Results of Year Class Analysis 28 

ETIOLOGY OF NATURAL OYSTER MORTALITIES 40 

First Year of Plot Studies 40 

Second Year of Plot Studies 40 

DETERMINATION OF MORTALITY RATES OVER SHORT PERIODS 44 

ANALYSIS OF INFECTION IN GAPERS 45 

DESCRIPTION OF A LABYRINTHOMYXA Epizootic 46 

Additional Epizootics 46 

Damage to Oysters from Labyrinthomyxa 46 

THE REDUCTION OF LABYRINTHOMYXA DAMAGE TO 

COMMERCIAL OYSTERS 48 

SUMMARY 49 

ACKNOWLEDGMENTS 53 

LITERATURE CITED 53 

vii 
TABLES 

1. Oyster condition rating key 5 

2. Evaluation of Labyrinthomyxa infection intensities in F.T.M. assays 7 

3. Results of paired Labyrinthomyxa tests in 1918 oysters 9 

4. Comparison of detection of Labyrinthomyxa at each infection intensity 10 

5. Pesticide residues in oysters at selected Labyrinthomyxa monitoring stations 20 

6. Average trace metal concentrations in oysters from selected stations 21 

FIGURES 

1. Oyster sampling stations 2, 3 

2. Shell measurements and features in 18-month oyster 6 

3. L. marina infection by season 12 

4. L. marina infection relative to temperature 13 

5. L. marina infection intensity frequency distribution 14 

6. L. marina infection relative to salinity 16 

7. Parasitism by L. marina in Suwanne Sound 17 

8. Salinity and temperature in Suwannee Sound 19 

9. Parasitism by L. marina in Crystal River 22 

10. Salinity and temperature in Crystal River 23 

11. L. marina infection relative to oyster height 25 

12. L. marina infection relative to depth 26 

13. Oyster condition and associated infection by L. marina 29 

14. Parasitism by L. marina in Tampa Bay 30 

15. Salinity and temperature in Tampa Bay 31 

16. Parasitism by L. marina in Sarasota Bay 32 

17. Salinity and temperature in Sarasota Bay 33 

18. Spatfall at two stations during 1969 34 

19. Oyster shell configurations 35, 36 

20. Oyster age calculator 37 

21. L. marina infection during Station 23 plot test 38 

22. Salinity and temperature during Station 23 plot test 38 

23. Unadjusted oyster numbers and mortality during Station 23 plot test . 39 

24. L. marina infection during Station 43 plot test 41 

25. Salinity and temperature during Station 43 plot test 41 

26. Unadjusted oyster numbers and mortalities during Station 43 plot test . 42 

27. Incidence of L. marina infection intensity detected by 

P-FS F.T.M. assay among gapers 43 

28. Labyrinthomyxa infection during an epizootic in Sarasota Bay 47 

29. Oysters grown by the mortality minimizing technique 50, 51 

viii 
OYSTER PARASITISM BY 

LABYRINTHOMYXA MARINA 

IN FLORIDA 

ABSTRACT 

The fungal oyster parasite, Labyrinthomyxa marina, was found to 

occur along the Gulf and Atlantic coasts of Florida. It was found 

concomitantly with the host throughout the year and at salinities from 

6 100 to 36 °/... High temperature was implicated as the most important 

factor contributing to high rates of parasitism. Field studies indicated 

that L. marina kills an average of over 50% of Florida's adult oysters 

yearly. Sampling and observation of the intermittent oyster epizootics 

caused by this fungus showed that mortalities could be very rapid and 

intense. Oyster mortality estimation methods were evaluated and one 

technique was developed for measuring annual death rates and another 

for estimating high, short-term oyster kills. An oyster-growing program is 

proposed for use in central Florida to obtain the most oyster meats of 

highest quality in the shortest time with the smallest effective mortality. 

INTRODUCTION 

In 1950, Mackin, Owen, and Collier 

described a new parasite of the American 

oyster, Crassostrea virginica Gmelin. This 

pathogen of uncertain relationship was 

named Dermo cystidium marinum Mackin, 

Owen and Collier, but after a thorough 

study of the life history and other aspects, 

it was placed under the genus Labyrinthorny xa, becoming Labyrinthomyxa 

.marina (Mackin, Owen, and Collier) 

Mackin and Ray (1966). 

Research increased after the development of a simple quantitative assay for L. 

marina by Ray (1952b). Workers using 

this fluid thioglycollate medium culture 

(F.T.M.) technique found oysters infected 

with the fungal parasite from New Jersey 

(Anonymous, 1958) to Mexico (Mackin, 

1962). Oyster deaths from L. marina were 

found to be particularly heavy in the 

states bordering the Gulf of Mexico 

(Mackin, 1951, 1962; Ray, 1954a, 1966a; 

Mackin and Wray, 1952; Ray, Mackin, 

and Boswell, 1953). The parasite was 

found in Florida in Pensacola Bay (Ray, 

1954b), Apalachicola Bay (Ingle and Dawson, 1953b), Cedar Keys (Ingle and 

Dawson, 1953a), and Tampa Bay 

(Stewart, 1966; Ray, 1966a). 

This laboratory began monitoring L. 

marina in Florida oysters in 1965, initially 

to determine the parasite's range beybnd 

these areas. The study was soon expanded 

to investigate the environmental limits, 

the seasonal occurrence, and the resultant 

oyster mortalities of this fungus parasite. 

METHODS AND MATERIALS 

STATION DESCRIPTIONS 

A total of 120 stations were sampled 

during our study and one or more standardized Labyrinthomyxa samplings were 

made at 91 of them. All station locations 

are shown in Figure 1; eight of these, 

however, were extremely significant to 

this study and require further definition. 

Station 15 (29. 17'30"N, 83-06'55"W): 

Located in the open Gulf about one mile 

off a large salt marsh near the mouth of 

the Suwannee River, these large oyster 

bars (reefs) are almost constantly submerged. The bottom near the bars is extremely soft, and currents readily create 

high turbidity. 

Station 20 (28° 55'00"N, 82° 41'40"W): 

Located in the mouth of Crystal River 

near the Gulf, this station is characterized 

by several small intertidal oyster bars. The 

Contribution No. 164, Florida Department of Natural Resources Marine Research Laboratory 
2 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 1. Oyster sampling stations for studies of Labyrinthomyxa marina. One or more twelve-oyster 

samples were collected from stations 1 through 91. Stations with a letter suffix were 

alternates or were never collected in this manner. Exact station locations and coordinates 

may be obtained from the Marine Research Laboratory. 

6 7 8D 

8E 

8■ 

9 

8A F 10A 1 0 

0 
F 

4 

82° 81° 800 

75B 

78A---\ 

0 78,------ 

78B 

AA 78c 

78D 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 3 
4 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

bottom near the bars is composed of gravel, shell, and firm clay. 

Station 23 (27° 50'05"N, 82°46'15"W): 

Located in a rather narrow, mangrovelined bayou in upper Boca Ciega Bay near 

Lake Seminole, this station was characterized by numerous intertidal oyster bars 

interspersed with small mangrove islands. 

A ,soft black mud bottom surrounded the 

oyster bars. Because of construction in 

the area this station is no longer productive. 

Station 43 (27°54'32"N, 82°38'00"W): 

Located in a wide, shallow lagoon in 

north Tampa Bay about three miles southwest of the Howard Frankland bridge, this 

station is characterized by many small, 

scattered intertidal oyster bars around and 

between mangrove islands. The surrounding bottom is a soft, muddy sand sparsely 

covered with Diplanthera. 

The above stations 23 and 43 were 

most important to the study because they 

were the sites of the plot tests and were 

sampled more frequently than other stations. 

Station 75 (27°30'00"N, 82°34'05"W): 

Located in the Manatee River about four 

miles from the mouth, this station is a 

long intertidal oyster bar paralleling the 

railroad bridge. It is probably an old spoil 

bank and the bottom is primarily black 

mud. 

Station 78 (27°15'56"N, 82°33'15"W): 

This station is located in a small embayment on the west side of Little Sarasota 

Bay. It is a series of small intertidal oyster 

bars near the channel leading to a private 

boat basin. The bottom is primarily sand 

with some shell. 

Station 80 (26°42'00"N, 82°09'15"W): 

This station is located in a cove surrounded by mangrove swamp on the 

northwest end of Pine Island Sound off 

Charlotte Harbor. The oysters are subtidal 

and are scattered loosely over the sand 

bottom. 

Station 81 (26°30'48"N, 81°58'45"W): 

This station is in the mouth of the Caloosahatchee River in a small mangrove 

embayment on the southeast bank. The 

oysters are intertidal and are scattered 

loosely over the sand and mud bottom, 

attached to mangrove roots, or form small 

bars. 

LABYRINTHOMYXA DISTRIBUTION STUDY 

Oysters from 91 stations in Florida 

(Figure 1) were monitored for the presence of Labyrinthomyxa from 1965 

through 1969. Six of these (Nos. 15, 20, 

75, 78, 80, and 81) were sampled monthly for data on seasonal trends of parasite 

incidence while the remainder were sampled one or more times for data on geographic range and environmental limits of 

the fungus. 

Oysters were taken at each station with 

oyster tongs, scallop dredge, or by hand 

and transported to the laboratory under 

moist, cool conditions for processing 

within ten hours of collection. Standard 

physical and hydrological data were obtained at each sampling and biological 

observations were recorded. 

Twelve oysters were removed at random from each sample lot for parasite 

analysis. The height of the upper (right) 

valve of each oyster was measured parallel 

to a tangent of the major axis of growth 

(Figure 2) with linear calipers. Each oyster was opened and the animal's condition 

rated according to an arbitrary scale (Table 1). Four tissue explants, two pieces 

each of mantle and gill, were removed just 

ventral to the labial palps for inoculation. 

Stock F.T.M. with dextrose (Difco No. 

ETM-0256-01) was rehydrated with a 

2.0% aqueous sodium chloride solution 

and dispensed in 10 ml aliquots into 17 x 

125 mm screw-capped culture tubes. One 

duplicate pair of these tubes was inoculated from each sample oyster. Every culture tube received one specimen of gill 

tissue and one of mantle tissue (Ray, 

1952b, 1966b). 

One member of each duplicate pair of 

inoculated culture tubes was fortified by 

addition of 5,000 U.S.P. units each of 

Nystatin (E. R. Squibb and Sons, New 

York, List 5915) and chloramphenicol 

(Parke, Davis, and Co., Detroit, SY65 

stock 35-65-1). The second tube of each 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 5 

TABLE 1. OYSTER CONDITION RATING KEY 

Condition Index Code No. 

Very good 1 

Good 2 

Good minus 3 

Fair plus 4 

Fair 5 

Fair minus 6 

Poor plus 7 

Poor 8 

Very poor 9 

Oyster Appearance 

Animal firm and filling shell cavity; coloration creamy 

white and evenly textured; usually ready to spawn. 

Not quite as firm or large as above; usually ready to spawn. 

Coloration less opaque, often slightly yellow or gray. 

Animal distinctly not filling shell cavity; coloration often 

mottled, with blood vessels and muscle fibers showing 

through the more translucent epithelium. 

Oyster well-developed but not opaque or tending toward 

white; grayish and translucent; flesh flaccid. 

Translucency more pronounced. 

Oyster not well-developed, darker gray, often greenish; pericardial cavity clear; small portion of shell cavity filled. 

Negative qualities more accentuated. 

Animal distinctly atrophied; coloration dark and uneven, 

very translucent; seldom more than third of shell cavity 

occupied; adductor muscle often discolored and transparent 

even in the normally white sector. 

pair received 5,000 U.S.P. units of penicillin G (E. R. Squibb and Sons, list 6735) 

plus 5.00 mg of streptomycin (E. R.. 

Squibb and Sons, list 8377). The total volume of both additions to each tube was 

0.2 ml. Inoculated, antibiotic fortified 

culture tubes were incubated in the dark 

at 25 to 30°C for seven days. 

After incubation, each piece of tissue 

was removed from the culture medium, 

blotted to remove excess liquid, teased 

onto a microscope slide, and stained three 

to ten minutes by addition of Lugol's 

aqueous iodine potassium iodide. A coverglass was pressed onto the preparation and 

examination was made at 50X and 100X 

magnifications. Infections, exhibited in 

the cultured tissue by the presence of 

darkly stained hypnospores, were classed 

quantitatively (Table 2), similar to Ray's 

(1953, 1954b) method. A representative 

code number was assigned to each infection on the basis of its intensity or the 

approximate number of cells per unit volume of host tissue. Tissue explant size and 

slide preparation thickness were kept as 

constant as possible to increase precision 

in the infection intensity measurements. 

Averaging all the infection intensity 

code numbers for any selected group of 

assayed oysters produced a number which 

reflected both the proportion of the population infected and the intensity of infection. This value is known as the weighted 

incidence (W.I.) of infection (Ray, 1953) 

and was very useful in comparing infections in various oyster populations. The 

usual calculations of incidence and average intensity of infection were also used. 

The data from this parasite distribution 

study was analyzed with the aid of an 

IBM computer. A data input card was 

punched for each oyster assayed duiing 

the study. All applicable hydrological 

data, as well as individual oyster test results and determinations, were included 

on each card. The primary computer techniques used were card sorting with respect 

to selected parameters and statistical significance verifications. 
6 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Major Axis 

of Growth 

Figure 2. Shell measurements and features in an 18-month oyster. FS - first summer's growth; SS - second summer's growth. 

INVESTIGATIONS OF ETIOLOGY AND 

MORTALITY 

Plot Studies: Relationships between 

oyster mortalities and Labyrinthomy xa 

parasitism were evaluated, using an adaptation of the tray technique (Mackin, 

1961; Hewatt and Andrews, 1954) similar 

to that of Owen (1955) and Ingle and 

Dawson (1952, 1953a, b). Locations were 

selected in oyster growing areas so that 

two very similar, adjacent plots of four 

square meters each could be delineated. 

Each square plot was cleared of oysters 

and a plastic pipe (3/4 inch PVC) was 

sunk vertically at each corner. One plot of 

each pair was then surrounded by heavy 

gauge galvanized welded fencing (1.8 x 

3.8 cm mesh) with the lower edge buried 

10 to 20 cm. At one location, a wire top 

was secured over an enclosed plot to prevent the entry of predators during high 

tides which submerged the enclosure. 

Single adult oysters were then collected 

from the surrounding area, counted, and 

replaced in approximately the original 

oyster population density. 
Appearance 

Microscopic /Macroscopic 

No hypnospores/tissue orange-brown 

Hypnospores very scattered, usually larger than average/ 

tissue still orange-brown 

Hypnospores in most fields but sometimes concentrated in 

specific areas/tissue still orange-brown 

Hypnospores common, beginning to make up a significant 

portion of each field/tissue still orange-brown 

Hypnospores numerous everywhere, seem to be occupying 

a significant volume of the tissue/darkened spots may show 

in tissue 

All fields darkened by the presence of hypnospores, seem 

to occupy about half of tissue volume/tissue often bluish, 

takes up more than usual stain 

All fields almost black with hypnospores occupying more 

than half of tissue volume; tissue often difficult to see, 

almost impossible to get thorough staining/tissue distinctly 

blue or black 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 7 

TABLE 2. EVALUATION OF LABYRINTHOMYXA INFECTION INTENSITIES 

IN F.T.M. ASSAYS 

Intensity 

Description Code 

Number 

Code 

Hypnospore 

Concentration 

Negative N 0 None in sample 

Very light VL 1 1-10/sample 

Light L 2 11-100/sample 

Light 

medium 

LM 3 99-1,000/sample 

(1-30/5 mm field)* 

Medium M 4 31-300/5 mm field 

Medium 

heavy 

MH 5 301.1,000/5 mm field 

Heavy H 6 1,001 and up/5 mm field 

200 and up/2 mm field** 

(commonly to 3,000/5 mm 

field) 

*B and L Dinoptic w/15X wide field oculars and 3.5X obj. gives 5 mm field 

**B and L Dinoptic w/15X wide field oculars and 10X obj. gives approx. 2 mm field 

The plot experiments were sampled at 

selected intervals, usually monthly. All living oysters in each plot were counted and 

returned to their respective locations to 

determine how many oysters had died 

since the sampling. Separate counts were 

made of gapers (oysters too weak to maintain shell closure), boxes (empty hinged 

shells), and all single left valves. Shells of 

these dead and dying oysters were examined for evidence of predation and removed from the plot. Hydrological measurements were taken and the sampling 

concluded by removing for analysis all 

gapers plus 12 living oysters at random 

from each plot. 

These sampled oysters were transported 

to the laboratory, measured (height), 

rated as to condition, and tested for 

Labyrinthomyxa. 

Calculation of Adjusted Mortalities: 

Month-to-month oyster mortalities were 

determined simply by subtracting the oysters counted at one sampling from those 

counted in the previous sampling. For 
8 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

longer periods, however, consideration 

had to be given to those living oysters 

removed for testing, since some of these 

oysters may have died and thus contributed to the mortality in the plot. 

To avoid this possible error in mortality 

calculations, all oysters removed for testing were considered to have died at the 

same rate as remaining oysters. The number of "surviving" test oysters was then 

added to the total living oysters to arrive 

at a more accurate figure. The resulting 

mortality rates, which include these projected deaths, are called "adjusted" mortalities. 

Relation of Gapers to Oyster Death 

Rate. Obviously, gapers should be present 

in any oyster area with a sufficiently high 

mortality rate. It was reasoned that if the 

period of time that some meat remained 

in each gaper was fairly constant, then 

there should be a quantitative relationship 

between the rate of mortality and the 

number of gapers in an area. The number 

of gapers was compared with simultaneous mortalities derived from direct 

counts of oysters to determine any such 

quantitative relationships between gapers 

and mortality. This gaper count was later 

modified to a ratio between the gapers 

and the living oysters in the study area. 

Direct determination of this gaper to living oyster ratio was made thereafter by 

randomly placing a meter square wooden 

frame on the oyster bottom and counting 

the enclosed gapers and living oysters. All 

oysters were counted except very young 

ones, or spat, whose growing lips had not 

yet angled upward from the substrate and 

whose shells had not yet begun significant 

thickening. At first, the meter square 

frame was placed inside the plots for oyster counts, but, later, these counts were 

made in the area of oyster bar surrounding the plot to better simulate routine 

field use of the technique and to provide 

better accuracy due to the increased 

number of oysters counted. Sampling in 

the plots was increased to weekly for 

short periods to obtain more accurate 

data. This method was also used at Station 78 where mortality rates were simply 

correlated with decreases in the average 

number of oysters per square meter. 

Year Class Analyses. Mackin (1961) 

used year class analysis to determine 

annual oyster mortalities in Louisiana. 

Large numbers of oysters from a designated area were measured and the resulting heights were graphed as height versus 

the frequency of occurrence. The resulting graph was then read to establish year 

classes and the number of oysters in each 

annual group determined. The annual 

mortality was computed as the difference 

in the number of oysters in each successive year class, assuming that spatfall 

remained uniform. We attempted to duplicate these earlier Louisiana successes, but 

inconsistency in our attempts to establish 

year classes by height measurements 

caused us to examine other methods of 

establishing annual groupings. 

The possible use of shell rings to establish oyster age and thus year classes was 

examined. The basic information on the 

occurrence and significance of shell rings 

and contours came from several years of 

field observations. Verification, however, 

required oysters of known age. During 

approximately two and a half years of 

sampling, several such verifications were 

possible. For example, Station 6 in north 

Florida produced a unique situation in 

which changes in water quality first killed 

all the oysters and then allowed only a 

single spatfall to survive in the fall of 

1966. Additional shell observations were 

made at Station 43 where plot studies 

provided known mortalities for comparison. Observations of spatfall at this station also aided in the evaluation of shell 

contours as a method of determining oyster age and thus, mortality. 

RESULTS AND CONCLUSIONS 

Part I: Studies of 

Labyrinthomyxa Distribution 

COMPARISON OF ANTIBIOTIC TREATMENTS 

Ray (1966a,b) demonstrated that infected oyster tissues cultured in F.T.M. 

assay higher Labyrinthomyxa intensities 

when fortified with either mycostatin or a 

mycostatin-dihydrostreptomycin-penicil- 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 9 

lin mixture than duplicate tissues cultured 

in 'F.T.M. containing a penicillin-streptomycin mixture. Also, the number of enlarged parasite cells and the size of these 

enlarged hypnospores was greater in 

media with mycostatin. 

During 1 965 P *Id 1966, we assayed 

1,918 oysters using paired F.T.M. tubes 

containing duplicate tissues but having 

one fortified with mycostatin and chloramphenicol (M+C) and the other with 

penicillin and streptomycin (P+S). Cultures containing M+C yielded an overall 

W.I. of 1.41 while the P+S culture tubes 

assayed a lower 0.99 W.I. (Table 3). The 

incidences of infection were 62% and 

56%. In addition, M+C cultures detected 

233 infections not found by the paired 

P+S tubes while the P+S cultures assayed 

only 115 not found by the simultaneous 

matched M+C fortified cultures. 

Hypnospores treated with M+C were 

larger (21% greater diameter) and appeared darker than those treated with 

P+S. In the 348 paired tests where one 

treatment was negative and the other positive (primarily low intensity infections), 

M+C treated cultures produced 34% more 

positive results than P+S. However, as the 

infections increased, the difference between P+S and M+C intensity readings 

became smaller (Table 4). It was possible 

that intensity judgments (because of the 

TABLE 3. COMPARISON OF PENICILLIN PLUS STREPTOMYCIN (P+S) 

TREATMENT WITH MYCOSTATIN PLUS CHLORAMPHENICOL 

(MTC) TREATMENT ON 1,918 OYSTERS 

M+C Added 

to F.T.M. 

Difference 

in Results 

62.04% 9.6% 

1.41 29.8% 

2.26 21.7% 

229 49.8% 

91.19% 9.6% 

P+S Added 

to F.T.M. 

Infection incidence 56.10% 

Weighted incidence 0.99 

Average intensity of 

infected oysters 1.77 

Oysters positive for 

infection in given treatment 

but negative in the other 115 

Total oysters positive in 

one or both treatments 1305 

Infection incidence derived 

from both treatments 68.04% 

Relative accuracy (% of total 

positive oysters found by 

given treatment) 82.45% 
10 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

TABLE 4. COMPARISON OF DETECTION OF LABYRINTHOMYXA AT 

EACH INFECTION INTENSITY USING THIOGLYCOLLATE CULTURE (F.T.M.) WITH PENICILLIN PLUS STREPTOMYCIN (P+S) 

AND WITH MYCOSTATIN PLUS CHLORAMPHENICOL (M+C) IN 

2,062 OYSTERS 

A — Infections of Each Intensity Found by the Two Culture Variations: 

Infection Intensity 0 1 2 3 4 

F.T.M. 

with 

No. of oyster 899 584 336 156 56 

P+S % of all oysters 43.6 28.3 16.3 7.57 2.72 

F.T.M. 

with 

No. of oysters 778 436 328 323 129 

M+C % of all oysters 37.7 21.2 15.9 15.7 6.24 

B — Infection Intensities Detected in Assay Cultures with P+S and with M+C 

in Selected Groups of Oysters: 

Oyster Groups A 

No. of oyster 

Infection intensity 

of all oysters in 

899 584 336 156 56 

each group 0 1 2 3 4 

Infection intensities 0 0 1 2 3 

found 1 1 2 3 4 

2 4 4 5 

5 

Weighted incidence 0.4 1.4 2.6 3.3 4.1 

5 6 

18 13 

0.88 0.63 

42 26 

2.02 1.24 

F G 

18 13 

5 6 

3 6 

5 

6 

5.4 6.0 

Groups 

selected & 

defined by 

P+S assay 

Results of 

M+C assay 

on all 

oysters of 

each group 

larger size and darker appearance of 

hypnospores) might have been a factor in 

this difference, but our data did not support this. It therefore seems that M+C cultures caused more hypnospores to enlarge 

at low infection levels than did P+S cultures, but this advantage almost disappeared at heavier infection levels (Table 

4). 

The P+S combination was used to fortify F.T.M. in all other assays for Labyrinthomyxa made by this laboratory. This 

was originally done in accordance with 

Ray's (1952h) recommendation and later 

continued for the sake of uniformity. Ray 

(1966b) later concluded that the M+C 

antibiotic treatment was superior. We 

agree, particularly considering the greater 

accuracy of this method as shown above, 

but our experience indicated that the 

superiority of M+C may be somewhat 

compromised in practice. The insolubility 

of mycostatin and chloramphenicol inconvenienced dispensing and sometimes interfered with microscopic observations of 

cultured tissues. The M+C seemed to 

produce some control of mycelial fungi 

but did not inhibit bacteria as well as P+S. 

Bacterial contaminants were damaging 

because sample softening and decomposition were often produced, thus complicating removal of tissues from the culture 

tubes and seriously compromising the 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 11 

quantitative aspect of the assay. This tissue softening or destruction was particularly serious in assays of gapers. Mycelial 

molds were usually restricted to the surface and did not infiltrate the sunken tissues. When fungal mycelia did invade cultured oyster tissu ?•, however, they often 

improved firmness while not interfering 

appreciably with microscopic observations 

since hyphae remained transparent. The 

anaerobic condition' of properly prepared 

F.T.M. also inhibited growth of submerged fungi but had little effect on the 

occurrence of bacteria. No significant differences could be found in the development of Labyrinthomyxa hypnospores 

between aerobic and anaerobic F.T.M. 

cultures in paired tubes from 33 infected 

oysters. The presence or absence of contaminants, likewise, seemed to have no 

effect on hypnospore enlargement per se, 

although Mackin (1962) found that 

Labyrinthomyxa marina grew better in 

culture with the mold Fusarium sp. when 

compared with controls grown in sterile 

seawater. We often encountered this 

Ascomycete as a culture contaminant. 

LAYBRINTHOMYXA DISTRIBUTION IN 

FLORIDA 

Results of the Labyrinthomyxa distribution study showed the parasite's range 

to be continuous among oyster populations through the north Florida areas 

sampled by Ray (1954b, 1966b), Ingle 

and Dawson (1953a,b), and Stewart 

(1966). We also found the parasite's range 

to be continuous down the Gulf coast 

into the Florida Keys and northward up 

the Atlantic coast to Georgia. 

A total of 2,063 oysters were tested as 

part of our study of the geographic, seasonal, and environmental distribution of 

Labyrinthomyxa. Infected oysters were 

taken from every county sampled and 

from 89 of the 91 stations (Figure 1). The 

overall W. I. was 1.02 with 1,158 oysters 

positive for the parasite to give a 56.1% 

incidence of infection. 

Relationship of Labyrinthomyxa to 

Season: The Labyrinthomyxa distribution 

study demonstrated this parasite to be 

prevalent throughout the year in Florida 

and to have an annual infection cycle of 

winter minimum and late summer maximum (Figure 3). Similar annual cycles 

have been observed by numerous authors 

(summarized by Mackin, 1962). The irregularities in our data (a February peak and 

a July decline)' coincide with the end of a 

winter drought and an unusually wet summer during sampling. 

Comparison of our data with the results 

of surveys for Labyrinthomyxa in more 

northerly mid-Atlantic coastal areas 

(Andrews and Hewatt, 1957; Dunnington, 

1956; Anonymous, 1958) shows that the 

parasite was much more prevalent in Florida during the winter months and maintained high infections over a longer summer period. However, a comparison of our 

findings with data on parasitism in the 

northern Gulf of Mexico (Mackin, 1962; 

Mackin and Sparks, 1962; Ray, 1966a) 

reveals surprisingly small differences in 

both period and intensity of summer and 

winter infestations; incidence and intensity seemed to drop slightly more during 

Florida winters. 

Relationship of Labyrinthomyxa to 

Temperature: Our distribution data reveals a strong positive temperature-infection relationship (Figure 4) similar to 

those observed by numerous other 

workers (summarized by Mackin, 1962). 

Incidence of infections and W.I. increased 

concurrently and almost linearly with 

temperature throughout the thermal range 

sampled. Critical temperatures, such as 

those affecting the parasite's well-being or 

the oyster's defense mechanisms, were not 

apparent. 

Distribution of Labyrinthomyxa Intensities: The number of distribution study 

oysters assigned each infection intensity 

rating is greatest in the "negative" intensity and decreases as intensity increases 

through "very light," "light," etc. (Figure 

5). Thus, there are about 50% fewer oysters in each successively higher infection 

Intensity level even though the range 

between the minimum and maximum 

hypnospore concentration increases by 

about ten in each successively higher infection intensity level (Table 2). This progression is similar for infections rated in 

P+S and in M+C fortifications of the 

F.T.M. cultures. The most obvious break 
0 }.• Cr 

•=C 

CC 

CO 1.61 

LaINCIDENCE 

NTENSITY 

100 

90 

80 

70 

ag 

60 

Infect ion 

50 

40 

c.) 30 

C 

-a • c.) 

c20 

10 

7 - 

1.2 

0.8 2 

C 

0.6 g 

C) 

0.4 C 

0.2 

12 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

in the pattern of decreasing numbers with 

increasing infection is seen in the "medium heavy" and "heavy" groups (code 5 

and 6) where the numbers are almost 

equal. This anomaly is probably valid 

because these ratings are easily distinguished, and because the trend was apparent in both P+S and M+C treatments. 

Since oysters with "heavy" infections 

should experience a higher death rate than 

those with "medium heavy" infections, it 

would seem that numbers of heavily 

infected oysters would be substantially 

reduced, but this was not true. Possibly, 

the hosts were so debilitated by disease of 

this magnitude that the infections adFigure 3. Labyrinthomyxa marina infection by season. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 13 

vanced very rapidly through "medium 

heavy" into "heavy." Oysters seemed to 

be removed from these infection groups 

only by death (see Analysis of Infection 

in Gapers), and it is the balance between 

death rate and the rate of infection increase that ultimately determines the 

numbers of "medium heavy" and "heavy" 

oysters. 

R elationship of Labyrinthomyxa to 

Salinity: Many studies have shown Labyrinthomyxa to be inhibited by low salinFigure 4. Labyrinlhomyxa marina infection relative to temperature. 

100 — 2.0 

90 

F 

O 80 

cD 

a• 70 ,

0 

60 

UOIZ 391U I 

50 

40 z 

30 

20 

10 

0 

1.8 

z 

t6 

_

▪ 

 • 

1.4 3 

3 

1.2 0 

z 

_• 

1.0 

co 

tn 

0.8 .c 

0.6 fl 

0.4 

0.2 

0. 

0- 11- 16- 21- 26- 31- 36- 

10 15 20 25 30 35 40 

TEMPERATURE in °C 
Antibiotics in F.T.M. 

1000 

900 

800 

700 

P+S 

M+C A 

U) 

r, 600 

(;) 

500 

.041400 

E 

z 300 

200 

100 

0 

1 2 3 4 6 

Infection Intensity 

14 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

ities. Infection rates and mortalities due 

to this parasite are depressed in the lower 

salinity areas of Louisiana (Ray, Mackin 

and Boswell, 1953; Mackin, 1956). Similarly, the parasite is totally absent upstream of certain isohalines of Chesapeake 

Bay (Andrews and Hewatt, 1957). The 

inhibitory effect of low salinity has also 

been demonstrated in the laboratory 

(Ray, Mackin and Boswell, 1953). 

In most respects, our data is not superFigure 5. Labyrinthomyxa marina infection intensity frequency distribution. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 15 

ficially in agreement with the salinityparasite relationship mentioned above, 

but there is no disagreement between our 

findings and those of previous workers 

( concerning a positive salinity-parasite 

correlation) if the rationale we have devised is valid. It is also worth noting that 

the habitats we allude to as being "low 

salinity" do not meet the criterion required by one of us (Mackin) to be so 

styled. By this standard, in order for a 

habitat to qualify for low salinity status, 

the range must lie between ()V.'. s and 

10./ • •. This is a rigorous definition which 

none of our sampling stations could meet. 

The listing of all oysters by salinity 

taken at the time of collection (Figure 6) 

reveals an unusual trend in which infections decrease to either side of a peak in 

the 6-15•/•salinity grouping. Such a 

trend might be apparent if some circumstance had caused the measured salinities 

to be abnormally low, but several factors 

tend to discount this possibility. The large 

number of samples and the regularity of 

their collection should have caused measured hydrographic values to average out 

very near the true mean values. Although 

data from one-time samples or some 

minimal samplings may have been abnormal, this should have been offset by data 

from the many other samples so that the 

net effect would have been to widen 

modal peaks rather than to change their 

values. If any data prejudice was produced, it should have been in favor of 

higher, rather than lower, salinities because our hydrographic values were 

measured just above the bottom, the layer 

into which higher salinity waters move 

earliest and leave latest under most conditions. In addition, most samples were 

taken by boat so that favoritism was 

probably given to time of high tide, usually the period of highest salinity in an 

estuary. Consequently, our salinity data 

appears to be representative of the areas 

under study and some other reason must 

be found for this apparent lack of support 

to past findings. 

Several circumstances relating to Florida oysters may explain this apparent contradiction: 1) our seasonal data showed 

that Labyrinthomyxa infection drops, but 

does not disappear, during the winter and 

rises very early in the spring, showing a 

great increase prior to May; 2) southern 

oysters are much more restricted to low 

salinities (Ingle and Dawson, 1953b; 

Menzel and Hopkins, 1954; Galtsoff, 

1964) than their northern counterparts 

with commercial populations seldom 

found above 25°/ — mean salinity even in 

north Florida (Pearse and Wharton, 

19 3 8) — Florida oysters are, therefore, 

more restricted to the propinquity of 

fresh water sources; 3) Florida characteristically experiences a summer rainy season from June to September, and one 

exceptionally wet period occurred during 

our sampling; and 4) Mackin (1955, 1962) 

has shown that previously established 

Labyrinthomyxa infections fall only 

slowly, if at all, in the presence of reduced 

salinities. Consequently, the following 

cycle of events probably accounts for the 

relationship observed between salinity and 

infection. Young oysters become infected 

during or before the fall of their first year 

but fail to develop higher intensities due 

to winter. Infections in older oysters also 

drop during the winter, and most oysters 

start each new year with low infection 

levels. These established infections begin 

to increase and new infections start as 

water temperatures rise after the short 

Florida cold season. Substantial infections 

develop before the summer rainy season 

produces increased freshwater runoff and 

a resultant drop in estuarine salinities. 

During this lower salinity period, established infections tend to remain the same 

or even to increase slightly due to the high 

Florida water temperatures (Figures 8, 10, 

15, and 17). As a result, low salinities may 

exist throughout the months when heavy 

infections are recorded. 

Data may thus be misleading, since 

summer low salinity infections actually 

developed at higher salinities. Data from 

stations sampled continuously (Tampa 

Bay composite, Figures 13 and 14; Sarasota Bay, Figures 15 and 16) supported 

this hypothesis. Dawson (1955) also 

found little effect of salinity on Labyrinthomyxa in Apalachicola Bay, Florida. 

Mackin (1962) noted incompleteness in 

Dawson's data and used information of 
16 FLORIDA DEPARTMENT OF NATURAL RESOURCES uoi

loajui 

50 

Figure 6. Labyrinthomyxa marina infection relative to salinity. 

100 

90 

■ 

z 

0 80 

a 

al 

= 

n 

a) 70 

0 -I, 

60 

40 0.8 

2.0 

1.8 

1.6 

1.4 

uol

l

oalui 

0- 6- 16- 26- 31- 

5 15 25 30 40 

SALINITY in %o 

at 

30 

20 

10 

0.6 

0.4 

0.2 

0.0 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 17 

Figure 7. Parasitism by Labyrinthomyxa marina in Suwannee Sound (lower salinity station 15). 

Intensity as W.I. A-- — 

0 0 0 0 

C) 01 .

0 

I I I I I I I I I I I 

0 0 0 0 0 0 0 0 0 0 0 

0 O) CO l'- CD 0 "4'M C•1 

r- % Incidence 111--111 

2 Cr) 

uz 

0 

Ct 

 oa 

a 

u_ 

z 

0 

U) 

-, 

2 

u_ 

z 

0 

(1) tD 

a 60 
18 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Ingle and Dawson (1953b) to argue that 

salinity had been effective. Since north 

Florida also experiences a summer wet 

season, it would seem reasonable that the 

same events indicated by our central Florida stations also occurred in Apalachicola 

Bay. 

We also observed high infections and 

mortalities in two areas of continual low 

salinities. Station 15, near the mouth of 

the Suwannee River, had salinities ranging 

from 2.5 to 247.. with an average of 

12.67 — (Figures 7 and 8). Although no 

epizootics were observed in monthly sampling from 1965 to 1969, gapers and 

boxes were present on numerous occasions and assays showed potentially lethal 

infections. The usual Labyrinthomyxa 

seasonal cycle was also demonstrated at 

this location. A similar situation occurred 

at the mouth of the Crystal River (Station 

20) which receives a continuous freshwater supply from deep limestone springs 

(Figures 9 and 10). This station had a salinity range of 0 to 30°/ 0. with an average 

of 12.7 0 /0. although salinity varied rather 

widely with each tidal change, due to the 

proximity of the Gulf. The incidences of 

infection in the Crystal River ranged near 

those at the Suwannee River but infection 

intensities ran significantly higher. 

Factors known to promote Labyrinthomyxa infection were checked as possible causes of the high infections observed 

in low salinities at Suwannee and Crystal 

Rivers. Mackin (1961, 1962) and others 

have demonstrated that rapid and heavy 

infestations are promoted by reduced 

water circulation. Inadequate circulation, 

however, was not the case since both stations experience almost continuous, and 

often rapid, water flow. It does not appear that chemical agents were responsible 

since both sampling stations were devoid 

of detectable pollution, domestic or 

industrial, with the exception of very low 

pesticide residues (Table 5; Butler and 

Springer, 1963; Butler 1966a, b). Other 

factors such as turbidity (averages of k 

0.3 at Station 15, fc-- 9.3 at Station 20; 

Mackin and Hopkins, 1962), trace metal 

content (Table 6; Hammerstrom et al., 

1967; Kopfler and Mayer, 1969; Pringle, 

et al., 1968), and freshwater sources 

(swamp fed Suwannee River and deep 

limestone spring fed Crystal River) varied 

widely and thus probably had no effect 

on infection. 

Although it would appear that temperature must again be the controlling factor, 

Mackin classified these two stations as 

being high or medium salinity locations 

by his definitions. He classes locations as 

high, medium, or low, according to the 

highest salinity reached, rather than the 

mean, because it is this highest salinity at 

which infections will develop, and he has 

found that succeeding lower values will 

cause very little reduction in these established infections as mentioned previously. 

Infections, therefore, are enhanced by 

exposure to the highest salinities reached, 

even though the length of exposure necessary to increase infection is unknown, 

probably being less than three weeks. No 

known oyster areas in Florida would qualify as permanent low salinity locations 

under this definition because all are subject to occasional periods of higher salinity. 

The high temperatures at Stations 15 

and 20, as elsewhere in Florida, certainly 

contributed to maintaining or increasing 

established high infections. The maximum 

temperatures in most previous field and 

laboratory work have been 30 0C or less 

and the highest field temperature recorded in association with work on Labyrinthomyxa was 32"C (Ray, 1954). Summer water temperatures at our stations 

commonly reached 32°C and remained 

above 30 0C for four months or more each 

year. Twenty-four regular samples from 

central and south Florida came from waters of 35 and 39 0C. Thus, it may be that 

temperatures in excess of 30°C not only 

maintain infection but induce a slow increase, even in the presence of lowered 

salinity. A tendency of temperature to 

dominate was also demonstrated in the 

converse situation where infections failed 

to proliferate at low temperatures even in 

the presence of highly saline waters, in 

both our study and others (Mackin, 1961; 

Andrews and Hewatt, 1957). 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 19 

Figure 8. Salinity and temperature in Suwannee Sound (Station 15). 

r I 

0 

------ <1--- _ 

I I I I I I 1 

Lo 0 IA 0 in 0 Lc) 0 

C.) cl cv of ,-- 

Temperature in °C and Salinity in Voo 

0-0 41----4 
20 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Relationship of Labyrinthomyxa to 

Oyster Size: Analysis of the distribution 

data with respect to oyster height (Figure 

11) revealed that both small and large oysters escaped the heavier Labyrinthomyxa 

infestations. By relating these oyster 

height groupings to oyster age (Figure 20, 

scale C') it can be seen that uniformly 

high infection ratings were present in 

those height groupings containing substantial numbers of oysters in their second 

year. 

Mackin (1951) and Ray (1953) also 

found young oysters to experience low 

infections, and Ray even suggested that 

some form of immunity may exist. Laboratory studies by Andrews and Hewatt 

( 1957), however, did not verify this 

hypothesis and they felt, with Mackin's 

( 1962) concurrence, that smaller and 

younger oysters probably ingested fewer 

infective cells. Earlier work had indicated 

that the intensity of infection and its rate 

of development in adult oysters were diTABLE 5. PESTICIDE RESIDUES IN OYSTERS AT SELECTED 

LABYRINTHOMYXA MONITORING STATIONS* 

Pesticides (ppm) 

DDT DDD DDE Dieldrin 

Suwannee River 

(Station 15) 

High 0.011 0.0 0.012 

Low 0.0 0.0 0.0 

Mean 0.007 0.0 0.004 

Crystal River 

(Station 20) 

High 0.016 0.0 0.012 

Low 0.0 0.0 0.0 

Mean 0.008 0.0 0.007 

Manatee River 

(Station 75) 

High 0.040 0.088 0.042 

Low 0.010 0.014 0.013 

Mean 0.017 0.037 0.025 

Sarasota Bay 

(Station 78) 

High 0.049 0.043 0.049 

Low 0.0 0.010 0.010 

Mean 0.018 0.026 0.025 

Pirate Harbor 

(Station 80) 

High 3.700 0.760 1.200 0.039 

Low 0.0 0.0 0.0 0.011 

Mean 0.028 0.029 0.011 0.020 

Caloosahatchee River 

(Station 81) 

High 2.80 1.40 1.50 0.011 

Low 0.0 0.016 0.0 0.011 

Mean 0.58 0.26 0.37 0.007 

*Analyses provided by Bureau of Commercial Fisheries 

Field Laboratory, Gulf Breeze, Florida. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 21 

rectly proportional to the number of 

pathogen cells received by the host (Ray, 

1953; Mackin, 1962). However, it would 

seem that smaller animals would require 

proportionately fewer pathogen cells for 

infection. Consequently, we now feel that 

other reasons for this apparent low infection rate of small oysters must be found. 

The presence of relatively low parasite 

intensities, even though incidences are 

high, among small oysters (Figure 11) 

indicates that environmental circumstances may have protected them. These 

young Florida oysters generally showed 

rapid growth and many apparently grew 

to 4 cm or more before intense infections 

had time to develop. Our measurements 

(267 oysters from Stations 23 and 43) 

showed oysters to average 6.9 cm at their 

first spawning. Growth rates on commercial beds have usually been even greater 

with average lengths of 12 to 15 cm being 

measured in 19 month old oysters (Ingle 

and Dawson, 1953b) and edge growth of 

1.9 cm occurring in 42 days (Ingle 1967). 

In addition, several investigations have 

shown a period of near two months to be 

required for lethal infections to develop in 

the laboratory (Mackin, 1962; Ray, 

1954a, b; Ray and Chandler, 1955; Ray 

TABLE 6. AVER AGE TRACE METAL CONCENTRATIONS IN OYSTERS 

FROM SELECTED STATIONS* 

Cu Cd 

M e t a 

Cr Pb 

1 s (ppm) 

Hg Zn Fe 

Suwannee Sound 

(Station 15) 

High 4.84 0.75 1.85 0.00 285.0 75.70 

High 4.84 0.75 0.69 1.85 0.00 285.0 75.70 

High 4.84 0.75 0.69 1.85 0.00 285.0 75.70 

Low 2.50 0.31 0.24 0.67 0.00 122.9 40.00 

Mean 3.77 0.53 0.39 1.17 0.00 212.1 42.81 

Crystal River 

(Station 20) 

High 28.00 1.65 0.67 1.63 0.00 105.0 57.00 

Low 13.00 0.86 0.24 0.56 0.00 71.0 40.20 

Mean 17.20 1.19 0.39 1.13 0.00 86.8 46.40 

Manatee River 

(Station 75) 

High 68.00 2.00 0.64 2.10 0.20 1084.0 57.00 

Low 21.70 0.53 0.00 0.82 0.00 574.3 40.82 

Mean 37.14 1.23 0.36 1.46 0.02 718.86 48.82 

Pirate Harbor 

(Station 80) 

High 4.92 0.72 0.61 1.59 0.00 213.5 78.10 

Low 2.37 0.32 0.00 0.76 0.00 145.0 49.30 

Mean 3.73 0.50 0.33 1.12 0.00 173.4 65.00 

San Carlos Bay 

(Station 81) 

High 8.50 0.89 0.52 1.70 0.00 200.0 76.10 

Low 2.06 0.26 0.51 1.16 0.00 151.0 69.00 

Mean 5.28 0.58 0.52 1.43 0.00 176.0 72.55 

*Analyses provided by Gulf Coast Marine Health Sciences 

Laboratory, USDHEW, Dauphin Island, Alabama. 
22 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 9. Parasitism by Labyrinthomyxa marina in Crystal River (lower salinity station 20). 

Intensity as W.I. A— — 

0 

C11 N T 0 

❑ 

z 

0 

U) 

co 

rn 

Uz 

0 

tt) 

0 0 0 0 0 0 0 0 0 0 0 

0 0) c0 t0 LO CN1 

r- 

% Incidence 0---4 
o 

z 

0 

41-____ - IA -- --- 

"1. 

■ 

- a 

41 _ n („, 

41- _ n Cr) T' 

. .

... 0'... < 

- 

1 0. - LL. 

-------- :ki _ n . - 

— ■••• 1,MP ,•• ..= 

Mb 

-------- 

r 1 I I 

---- . o-__-- 

4:.11= ami •Ml. OM& 

..... 

=m1. MED ME =I. ft. ...M1 

•■■ 

1 

- 

-------- .7;70.1 

<?.;:". - 

• ..X1 

• ------- -4-.. 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 23 

Figure 10. Salinity and temperature in Crystal River (Station 20). 

c:= :: -- -

-- •=::::::ft 

__a' 

- In - 0 

.. .%1 - ct 

r.... 

------- 

- - ,

..„4............ 

- 1 

...ft...4% 

- < 

%<1....... - 1 

..:X1 - 

4;:"...". _ -) 

ZJI - in 4' 

..... - z CD 

. 

 .. - 0 0

cy) 

4........ 

- In 1- 

• **NI... - a 

...... • I 1 I I I I I 1 ,,, 

'_ " 

In 0 to 0 LO 0 10 0 0101N01 

Temperature in °C and Salinity in 0/00 Li----d 

..... 

• 

• 

,- 
24 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

and Mackin, 1954), and infections have 

been found to develop even more slowly 

in the field (Mackin, 1962), thus further 

reducing the parasite intensity apparent in 

small oysters even though incidence may 

be high. 

Low winter temperatures may be a necessary integrant to this juvenile disease 

phenomenon, serving to reduce infection 

intensity in young oysters, since even oysters set during the first spring peak (May) 

must survive only four months before 

water temperatures begin to fall. Data 

from Station 43 showed an average age of 

8.2 months at first spawning in April and 

May. Spat set occurs throughout the summer months; therefore, most of the 8.2 

month prespawning juvenile period occurred during the winter. This allowed 

only a few summer months for both the 

contraction and intensification of the disease before proliferation was attenuated 

or arrested during the colder months. 

Thus, small oysters probably contract 

Labyrinthomyxa, but a combination of 

rapid growth rates and cold weather precludes the development of substantial 

infection intensities. Observations indicated that, once established, infections 

increased nearly as rapidly in young oysters as in older ones. 

Lower infection rates and intensities 

were also observed in larger, generally 

older oysters (Figure 11). Further analysis 

of the 113 oysters in the 13-21 cm group 

revealed that only a single heavy infection 

(0.9%) was assayed in contrast to 42 

heavy infections (2.8%) among the 1,486 

oysters in the 4 through 12 cm groups. 

Only 7 heavy infections (1.8%) were 

found among 377 oysters in the 11 

through 12 cm length grouping. It would 

appear that oysters have some form of 

acquired or innate resistance to Labyrinthomyxa infection after their first year as 

adults. 

Relationship of Labyrinthomyxa to 

Oyster Depth: Analysis of Labyrinthomyxa distribution data with respect to 

the depth relative to mean low water at 

which each sample oyster was living (Figure 12) revealed that disease incidence 

was similar at all depths, being only slightly higher in the upper littoral zone. The 

average infection intensities, however, 

increased with decreasing depth. The 

harsh conditions of intertidal life, such as 

temperature extremes and shortened feeding periods, may have been responsible for 

this increased parasitism. In a later experiment, temperatures of the tissues of exposed oysters were measured by placing 

minute thermistor probes inside the shells 

of living animals. These temperatures 

were found to vary from 10°C above the 

air temperature in oysters exposed to direct sunlight to about 2°C below air temperature in oysters exposed on a clear 

night. Diseased intertidal oysters may also 

become autoinfective during long periods 

of closure if the parasite cells contained in 

leucocytes voided by diapedesis are still 

viable and infective. Therefore, intertidal 

oysters experience severe environmental 

extremes which seem to be reflected not 

only in their small size but in their high 

incidence of Labyrinthomyxa infection. 

Relationship of Labyrinthomyxa to 

Oyster Condition: Distribution data indicated that disease incidence and intensity 

increased steadily as oyster condition 

worsened (Figure 13). However, since 

condition index is closely correlated with 

seasonal changes, this observation might 

be coincidental. In fact, oysters in very 

good condition (code 9) have occasionally 

been found harboring very high intensities. Nevertheless, most of these oysters 

with highest intensities were in poorer 

condition than the contemporary average. 

This lower quality may he due to Labyrinthomyxa. 

Annual Labyrinthomyxa Parasitism at 

Selected Sites: Figures 14, 15, 16, and 17 

display systematic hydrological and parasite data taken at two different locales 

during the distribution study. As in Figures 7, 8, 9, and 10, the typical summer 

infection highs and winter low are obvious. These two graphs are significant because heavy mortalities were experienced 

at both stations during epizootics in September 1968 and are discussed later. The 

salinities and other environmental parameters are more typical than, and provide 

good contrast to, the low salinity 

oyster areas illustrated in Figures 7, 8, 9, 

and 10. 
Figure 11. Labyrinthomyxa marina infection relative to oyster height (length). — 2.0 

1.8 

1.1 

1.6 -,, 

CD 

0 

p4 

a 1.4 3 

0 rpi 

0.6 

0.4 

0.2 

0.0 

0- 

3 

4- 

5 

6- 

8 

9- 

10 

11- 

12 

13- 

21 

--Juveniles One Year Old Adults—Two Years & Up— 

OYSTER LENGTH in cm. 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 25 
100 

90 — 

80 — 

70 —

60--

50— 

40 

530 

at 

111 20 

10 

0 

uolpajui Joaouappui 

a) 

co 

0.8 co 

co' 

8. 

0.6 a 

c.) 

0.4 ao 

iii: 

3 

0 

M 

26 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 12. Labyrinthomyxa marina infection relative to depth. 

Above MW- 0.4- 2.0- 

MW 0.3 m 1.9m 3.0m 

DEPTH 
2.0 

1.8 

0.6 

0.4 

0.2 

0.0 

20 

10 

0 

100 

90 

80 

70 

60 

50 

340 

30 

aouap

pul 

0 uol

loalu 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 27 

Figure 13. Oyster condition and associated infection by L. marina 

1 2-5 6-7 8-9 

CONDITION INDEX 
g 

28 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 14. Parasitism by Labyrinthomyxa marina in Tampa Bay oysters (medium salinity stations 

18-48). Note infection peaks associated with general epizootic of late summer, 1968. 

Incidence of Infection in 

0 81. 

F) (.•3 A (Ti Cr) 0 0 0 0 0 

DECEMBER _ I I I I I I I 

ok ...„....0 

-' ...I CO t 

0 0 0 0 

I I I I 

AUGUST -

SEPTEMBER 

JANUARY -

FEBRUARY 

MARCH -

APRIL -

MAY - 

—L. JUNE co 

C) JULY - 

a) AUGUST -

SEPTEMBER -

OCTOBER -

NOVEMBER -

DECEMBER -

JANUARY -

FEBRUARY 

MARCH -

APRIL 

MAY —L. 

CD JUNE 

Cr) V JULY 

AUGUST -

SEPTEMBER 

OCTOBER 

NOVEMBER - 

DECEMBER -

JANUARY 

FEBRUARY -

MARCH -

APRIL - 

1 MAY - 

CD JUNE - 

O IULY - O 

AUGUST -

SEPTEMBER 

OCTOBER 

NOVEMBER 

DECEMBER -

JANUARY -

FEBRUARY -

MARCH -

APRIL 

MAY - _L. 

CD JUNE - 

O IULY - O 

I I I I 1 I l 

2 o ..I. - na n.) P 503 A o (Ti 6 Ln 0 - al o a, o 

Infection Intensify as W. I. Ar------ 
I 

=MI.. co 

(0 

CD 

1— 

_. 

M 

c:C 

M 

LiN-- 

to 

C) 1— 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 

Figure 15. Salinity and temperature in Tampa Bay (Stations 28-48). 

29 

L I I I I I I 1 1 

o ul o us) o us) o in 0 

Tr el C) 01 Cd /-- r_ 

Temperature in °C and Salinity in 0/00 

O

A- - - -A 

---o 
—L _A. K.) N W W 42. 

Infection 

c.71 

Intensity as W. I. 

30 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 16. Parasitism by Labyrinthomyxa marina in Sarasota Bay (higher salinity station 78). A 

severe epizootic occurred here in September 1968. 

Incidence of Infection in oio rs......04 

_L IQ CA.) .0. al a) ".1 CO tO 

0 0 0 0 0 0 0 0 0 O 

—L 

O 

O 

—A. JUNE 

CD JULY 

O) AUGUST 

SEPTEMBER 

OCTOBER 

NOVEMBER 

DECEMBER 

JANUARY 

FEBRUARY 

MARCH 

APRIL 

MAY 

• JUNE 

O JULY 

AUGUST 

SEPTEMBER 

OCTOBER 

NOVEMBER 

DECEMBER 

JANUARY 

FEBRUARY 

MARCH 

APRIL 

MAY 

—a. 

co JUNE 

• JULY 

oa 

AUGUST 

SEPTEMBER 

OCTOBER 

NOVEMBER 

DECEMBER 

JANUARY 

cn FEBRUARY 

CD MARCH 
...... a* 

.-41 ..... 

• 

• 

. AI .

....... 

a 

z 

0 

Cr) 

a 

-, 

-, 

2 

41 

UOYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 31 

Figure 17. Salinity and temperature in Sarasota Bay (Station 78). 

L I I I I I 1 I I 

0 LO 0 U) 0 in 0 U) 0 

Tr C) cl or es] .1- .- 

Temperature in °C and Salinity in %o 

A----A (:)■•=.■0 
Station 43 

4 

ty 

.c 

3 

a 

co 

a 

N 

1 

32 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 18. Spatfall at two stations during 1969 expressed as macroscopic spat per adult oyster shell. 

Station 23 

4 

MA M J J ASOND 

1969 

MAMJ J ASOND 

1969 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 33 

Part II: Studies of 

Oyster Mortalities 

YEAR CLASS ANALYSIS AND ANNUAL 

MORTALITY 

Mackin (1961) successfully used shell 

heights to establish year classes of oysters 

and, from these (assuming a constant 

annual spatfall), calculated past annual 

mortalities. No such year class pattern 

could be determined in the Florida oysters tested. This lack of distinct year class 

height differential is probably due to a 

combination of rapid growth rates discussed previously and an extended setting 

period of over seven months each year 

with peaks in spring and fall. Ingle and 

Dawson (1953b), in extensive studies in 

Apalachicola Bay, found setting commonly from April or June through October or 

November each year. Ingle and Dawson 

(1953a) observed oyster larvae in the 

plankton during every month except January, February and March in Cedar Keys 

and cited year-round spatfall during some 

years in this more southerly location. We 

made limited spatfall observations at the 

Tampa Bay area stations 23 and 43, still 

further south, by simply counting the 

macroscopic spat on each oyster in the 

regular monthly sample. Since larger spat 

counted in any sample may have actually 

set more than one month prior, and because microscopic spat were not counted, 

the data obtained is not comparable to 

the more precise methods of Ingle and 

Dawson (1953a,b). Nevertheless, our studies did show significant setting for nine 

months of the year and some set throughout the year (Figure 18), thus effectively 

masking the size differential between year 

classes. This, coupled with very rapid 

growth rates, undoubtedly prevents any 

year class determinations based on length 

(height) frequencies. 

Determination of Oyster Age by Shell 

Contours: Since year classes could not be 

separated by length frequency, another 

method had to be developed. Analyses of 

shell growth rings indicated that they could 

be utilized with some degree of accuracy 

to separate year classes. As oysters grow, 

minute edges are formed approximately 

parallel to the generating curve and perpendicular to the direction of growth. 

These "micro-annuli" are the distal margins of prismatic layers of shell (Figure 

19a, b, c) and are approximately 0.1 mm 

thick. 

Counts of micro-annuli in oysters of 

known age, and even oysters of the same 

age growing adjacently, indicated that 

there was no set rate or regularity of formation. In addition, the counts varied 

along different radii of each oyster because individual micro-annuli were not 

continuous around the shell circumference. Nevertheless,. the average distance 

between micro-annuli increased with increasing growth rate. Conversely, the distance between micro-annuli decreased 

during periods of slow growth and 

individual annuli were sometimes even 

repressed under the edge of overlying shell 

layers during periods in which no growth 

occurred. The thickness of individual 

micro-annuli became greater during warm 

seasons, especially during periods of 

spawning. During rapid growth, the 

increased width plus the decreased 

micro-annulus thickness results in wide, 

relatively smooth expanses of shell. 

Slower summer shell growth is 

characterized by rough areas composed of 

numerous, closely packed, irregular, 

thickened micro-annuli. Thickening of the 

prismatic shell layers in spawning oysters 

accompanied by slowing of growth 

usually produces a "spawning shelf" along 

the generating curve (Figures 2 and 

19e,f,). This distinctive shell feature was 

found to be a reliable indicator of 

spawning in central Florida oysters. 

Aging Technique: It was found that 

oyster shells could be aged with 80% 

accuracy to within four months by using 

observations of micro-annuli and shell 

layer trends along with general 

information on growth rates and spat set 

intensities. A slide rule-like device was 

constructed as an aid to age estimation 

(Figure 20). Its stationary arms carry five 

scales as follows: A, a scale of time 

calibrated in months and years from left 

to right which serves both for calculations 

and as the horizontal time axis for all 
34 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

graphic scales; B, a graphic representation 

of typical micro-annuli configurations as 

they would have been laid down in adult 

oysters over each year represented; C, a 

graphic representation of generalized 

growth rate for reference; D, a graph of 

the average oyster death rate for 

reference; and E, a graph of the average 

spat set rate for reference purposes. The 

movable slide carries two scales: F, a scale 

of time identical in spacing to Scale A, 

but calibrated in consecutively numbered 

months, years, and fractions of years from 

right to left; and C,' a scale of 

measurements giving the average oyster 

size at various ages. 

In use, the slide is placed so that 0 on 

the F scale is directly opposite the date on 

the A scale on which the test oyster was 

killed or on the current date if the oyster 

is alive. The appropriate date on the A 

scale closest to the right end is selected. 

The hairline is then moved from 0 on the 

F scale over all micro-annuli configurations present on the test oyster as illustrated on the B scale. Mirco-annulus configurations are matched in order from the 

growing posteroventral margin toward the 

beak of the test oyster. When the hairline 

reaches the point on the B scale comparable to the growth the test oyster produced as a postlarval spat, the age is read 

from the F scale under the hairline in 

months and years. This instrument and 

system were designed to use with the right 

(upper) valve since this one is usually 

flatter and its more linear growth allows 

more detail to be seen. The left valve was 

sometimes used but curvature, oblique 

growth, and substrate influences had to be 

considered. 

Experience and familiarity with oysters 

are of great assistance in using this system. 

Scales B and C, for example, are repetitions of 12-month cycles based on first 

year adults but, as seen on scale C', juveniles grow much faster and do not produce a spawning shelf, while older oysters 

grow more slowly. In addition to changing 

with age, individual oysters vary widely 

and the investigator must recognize each 

oyster as an individual problem and use 

care and discretion in matching microannulus configurations and growth rates 

with the generalized scales. Even when 

oysters are carefully aged, the results must 

be considered accurate only to the values 

stipulated previously (80% of the oysters 

to within four months). 

This method of year class separation 

for the calculation of annual oyster mortality was initially field tested in conjunction with experiments which included 

mortality determination by counts. Average annual mortalities obtained by comparison of year classes varied only 4.3% 

from those calculated from periodic 

counts of the same oysters over three 

one-year periods at Stations 23 and 43. 

These results also agreed well with those 

of another method of mortality estimation based on gapers. 

General Results of Year Class Analysis: 

The results of mortality calculations from 

year classes established by shell contours 

demonstrated that the annual mortality of 

Florida oysters varied greatly between 

years and between different locations during the same year. The mortality variation 

at Station 43 is typical, with annual 

deaths near 52% in 1965, 87% in 1966, 

70% in 1967, 98% in 1968, and only 49% 

in 1969. The rates at Station 23, however, 

were 61% in 1967, 91% in 1968, and 

100% in 1969. These rates were extremely 

high, averaging 71% at Station 43 which 

seemed to be typical of the central Florida area tested. The fact that medium to 

large oysters were always present in abundance at Station 43 in spite of these substantial death rates is further evidence of 

the rapid and continuing recruitment and 

growth in Florida. 

The validity of this type of annual mortality calculation in Florida is hampered 

by the extreme variability in abundance 

of spatfall and the high rate of mortality 

causing few oysters to survive past two to 

three years. These difficulties were minimized by sampling each area annually, 

preferably between October and February. This allowed field determination of 

annual spatfall and mortality from only 

one sample and was much simpler than 

the tray and plot methods of mortality 

determinations. 

Shell contour analysis also provided 
Figure 19. Oyster shell configurations. A - photomicrograph of a prismatic shell layer forming by 

growth of calcium carbonate crystals on a conchiolin membrane; B - a completed prismatic 

layer at the growing edge of an oyster shell (phase contrast); C - the lip of an oyster showing 

the intact prismatic layer edges or micro-annuli stranded on the surface of the shell by 

progression of the growing edge; D - micro-annuli as they appear after weathering has 

destroyed the delicate edges; E - two oysters collected in October at ages of 14 and 12 

months, upper and lower, showing spawning shelves (scale in cm); F - an 18-month old 

oyster showing a particularly prominent overhanging spawning shelf on the right (upper) 

valve. 

100 p 100 p 

A 
36 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

D E 

F 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 37 

Figure 20. The oyster age calculator. A - calculator; B - working scales of calculator set for a 26-month 

oyster which set during May and was aged or died in July. 

-1 

A 

Closed •---* 

Plot ■...-• 

Open 0-0 

Plot o ci 

• 

• 

.0--,-A 

••*".• • .. • 

Er •• •, 

.• 

'4ir.....a"' II_ I I I I a I 1 

M J J A S 0 N D 

1967 

• • 

• 

0% 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 39 

Figure 23. Unadjusted oyster numbers and mortality during Station 23 plot test. 

600 

500 

IT 

6400 

cn 

'05 

(3' 300 

46 

6200 

100 

0 

100 

90 

80 

11114 

70i 

io 

60 a E 

50 0 

40 ::1 - 1

TO 

30 ° E 

20 ; -

:z... 

10 

0 
40 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

preliminary information on first spawning 

times and sizes, number of seasons in 

which spawning occurred, survival, and 

growth data. Although the spring setting 

peak resulted in more spat than the fall 

peak, the majority of yearling oysters surviving until the following summer were 

members of the fall set. About 50% of the 

Florida oysters which set in April or May 

spawn in September or October of the 

same year (4 to 5 cm height). About 5% 

of the oysters setting in summer or fall do 

not spawn the following year even though 

they are of adult size. Oysters which 

spawn generally grow less than nonspawning oysters of the same size. 

ETIOLOGY OF NATURAL OYSTER 

MORTALITIES 

Plot tests were used to determine the 

oyster mortality caused by Labyrinthomyxa and the effect of other parasites, 

predators, and physical or chemical conditions on oysters. 

First Year of Plot Studies: Data was 

taken from paired plots maintained at Station 23 from early March to early September 1967 (Figures 21, 22, and 23). Two 

later plot tests at this location were tampered with and had to be terminated. 

Basically these oysters showed the 

usual warm weather increase in Labyrinthomyxa parasitism, even during lowered 

salinities caused by an unusually wet central Florida summer. The greatest mortalities closely paralleled rises in the weighted 

incidence of the fungus and amounted to 

57% in the open plot and 44% in the 

closed plot during the six-month period. 

Data from other stations cited above indicated that these high death rates were 

representative of or lower than usual 

summer mortalities among adult oysters 

in Florida. 

Mortality due to predation would normally be represented as the difference 

between the percentages of death in the 

open and closed plots, but certain factors 

prevent this from being completely accurate. For example, small predators such as 

Thais were occasionally found in the enclosure. Other organisms, such as Stylochus, usually classified as scavengers, are 

possibly significant as predators of weak 

oysters and could also have entered the 

closed plot. In longer plot tests, predators 

may also have entered as larvae or juveniles and grown to destructive size. Independent observations in Louisiana by 

Mackin indicated that the zinc in new 

galvanized oyster trays (and thus possibly 

the plot fencing) caused slight oyster kills; 

however, the majority of our fencing was 

not new. 

Observations indicated that predation 

of adult oysters in the enclosed plot was 

negligible, although one adult blue crab, 

Callinectes sapidus, and seven small Thais 

were found in the plot. Both plots harbored the usual smaller fauna associated 

with oysters. ExaMination of empty shells 

provided direct evidence that substantial 

predation did occur in the open plot. 

'Total adjusted mortality was 13% 

greater in the open plot during the entire 

test, but the month-to-month difference 

between the plots varied widely (Figure 

23) and mortality was slightly greater in 

the enclosed plot during June and July. 

Stone crabs were a significant predator 

in this study, accounting for 4.1% of the 

verified mortalities in the open plot. This 

crab probably also caused a portion of the 

unclassified loss (shells not located) by 

crushing shells beyond recognition or by 

carrying them off, as reported by Menzel 

and Hopkins (1954), since local currents 

were not sufficient to remove them. The 

area appeared to be relatively free of 

other predators of adult oysters because a 

0.7% loss to conchs was the only other 

evidence of attack. A slight mortality 

occurred in June, but no marks were left 

on shells and infection intensities were 

very light; no cause could be found. 

Second Year of Plot Studies: A fourth 

set of plots was established at Station 43 

the following year (1968). The summer of 

1968 was unusually sunny, hot, and dry 

and estuarine temperatures and salinities 

climbed to high levels. These conditions 

apparently contributed to an epizootic in 

this test plot (Figures 24, 25, 26) which 

killed so many oysters that work had to 

be discontinued for lack of survivors. Although the mortality was high (80%), it 
4 . 0 

3 . 0 1 

2 . 0 

In 

0 

I 4 

o 

o 0 3 0 

c — • 2 0 

— 10 c 

— cu ar 

ts) 

- 0 `1) 

J J A 

1968 

OYSTER PARASITISM BY LABY RINTHOMYXA MARINA IN FLORIDA 41 

Figure 24. Labyrinlhomyxa marina infection during Station 43 plot test. 
I I 

.1. 

I 

Closed •---0 

Plot •—• 

Open 0-0 

Plot D 13 

•M 

• 

• -. 

• 

• 

i 

I 

ii 4:1 .... ... 43... .• ..0 ii  . 

' -• • '4-- . ....".0%.u.:,••• I 

1 I i I i 

100 

90 

80 

.4 I; 

i : 70 • 

• El 

a 

6 0 E 

L 

4,) 

5 0 a 

I. 

40 

co 

1... 

0 

30 E 

0"--- 

20 

10 

42 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 26. Unadjusted oyster numbers and mortalities during Station 43 plot test. 

700 

600 

500 

9 I 1 400 

a 

0 

a) 4-,3 0 0 

o 

0 

6 

200 

a 

z 

100 

0 

M A M J J A SON 

1968 
Figure 27. Incidence of each Labyrinthomyxa marina infection intensity detected by P+S F.T.M. 

assay among gapers in the field. Incidence in living oysters included for comparison. 

0 

LO 

II° 

OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 43 

in Tr co CI T 

Infection Intensities 
44 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

occurred over a period of almost two 

months. 

These new plot tests substantiated 

some of the previous results. The stone 

crab was again shown to be the major 

predator, but there was more evidence of 

predation by gastropods with Busycon 

contrarium and Melongena corona often 

observed in the open plot. Blue crabs, 

Callinectes sapidus, and other smaller 

crabs and fishes seem to attack only 

young oysters under 3 cm in height; this 

may be beneficial in many areas of Florida where the heavy spatfall often produces serious overcrowding. 

The oyster "leech," Stylochus frontalis 

(Pearse and Wharton, 1938; Galtsoff, 

1964) was observed in the plots and elsewhere preying on very young oysters and 

barnacles. This polyclad worm was also 

found feeding upon many adult oysters, 

but in all Labyrinthomyxa assays of the 

host oyster, a potentially lethal infection 

was found. Thus, this leech appears to be 

a predator of young oysters but only a 

scavenger of adults. 

A slight mortality of unknown origin 

occurred in both plots in early June, as 

observed the previous year at Station 23. 

It is possible that these light June mortalities may have been caused by an undescribed parasite, currently being investigated by Mackin. 

Another unexplained series of events 

occurred in the plot tests at Stations 23 

and 43. Around September 1967 and 

1968 (at the different sites) mortalities in 

the closed plots exceeded those in the 

open plots. This unusual occurrence was 

coincidental with highest Labyrinthorny xa mortalities (reaching epizootic 

proportions at Station 43). Other workers 

have observed similar occurrences in mortality studies using oysters maintained in 

closed trays (Owen, 1955; Andrews and 

Hewatt, 1957). Possible contributing factors are: 1) reduced current, such as may 

have resulted from summer fouling of the 

enclosures, has been shown to increase 

infection (Mackin, 1962); and 2) dying 

oysters which cannot be scavenged may 

liberate greater numbers of infective 

spores as the tissues decay (Valiulis and 

Mackin, 1969). 

Plot tests indicated that predation is a 

relatively minor cause of adult oyster 

mortalities in central Florida. Labyrinthomyxa, however, causes considerable 

mortality. 

DETERMINATION OF MORTALITY RATES 

OVER SHORT PERIODS 

Observations indicated that gapers were 

present in larger numbers during periods 

of higher mortality and attempts were 

made to quantitate the relationship to 

provide another method of determining 

oyster death rates. Gaper counts were 

made at the Station 23 plot test as a pilot 

study and results were encouraging. Box 

counts made for comparison varied widely 

as noted by Mackin (1961). Since the preliminary tests indicated that this method 

had potential, further and more detailed 

tests were conducted at Station 43. The 

average numbers of gapers (animals unable 

to maintain closure of shells but with 

meat present) per square meter in 10 

meter square areas were compared with 

the average number of living oysters per 

square meter in the same area. The ratio 

of gapers to living oysters was then 

compared with the known death rates 

from plot tests and a conversion factor 

was calculated. The following relationship 

resulted: 

CG>K= 

where G = gapers/m2, L = living oysters/m2, K = a conversion constant, and M 

is the -tokareowt oyster mortality/week. The 

value of K was calculated to be 6.2 in this 

test. 

Further applications of this technique 

were made at several locations where mortalities were high. Back calculations of 

mortalities using estimates of original populations and the counts of oysters and 

gapers per square meter indicated that 

gaper counts provided accurate evaluation 

of current mortality rates if these exceeded 10% per month. The value of the 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 45 

conversion factor, K, seemed to be somewhat temperature dependent, decreasing 

with temperature, but the average value at 

20 to 31 °C was 5.5. 

The value of K is related to the length 

of time a gaper may exist. Since colder 

temperatures slow bacterial decay and 

usually reduce the activities of scavengers, 

gapers (which by definition must still contain oyster tissue) can persist longer under 

cooler conditions. Calculations of gaper 

"life" indicate an average of about 17 

hours from the time the oyster ceases to 

maintain shell closure until no more tissues remain in the shell. This figure is 

based on a daytime temperature of about 

30°C. However, boxes (shells still hinged 

but with no tissues remaining) were found 

to persist from one hour to over 60 days 

and maybe much longer. 

ANALYSIS INFECTION IN GAPERS 

Data of infected gapers (Figure 27) 

shows that the lethal level of Labyrinthomyxa varies among individuals but was 

usually "medium heavy" or above. Data 

also indicated that lower infection intensities may have resulted in mortality in 

the early fall, apparently because of oyster debilitation by spawning. In addition, 

many gapers assayed for Labyrinthomyxa 

showed little or no infection. These 

oysters were undoubtedly dying from 

other causes as yet undetermined. 

DESCRIPTION OF A LABYRINTHOMYXA 

EPIZOOTIC 

A very short but severe oyster epizootic 

occurred in late September 1968 at Station 78. A sharp rise in infection rate 

accompanied by the appearance of a few 

gapers in July 1968 prompted us to increase the sampling frequency, first to 

biweekly and then weekly, continuing 

into the following winter. Determinations 

of gapers per square meter were also 

begun, 

The incidence of Labyrinthomyxa rose 

rapidly from 18% in May to 100% in July 

(Figure 28) and continued near that level 

until after the mortality. The infection 

intensity lagged behind the incidence, 

increasing slowly from 0.8 in June to 2.0 

in September, and then advanced rapidly 

to over 4.0 in two weeks and remained 

near that extremely high level into the 

epizootic (about four weeks). Death rates 

calculated from gaper counts varied but 

showed a distinct rise during the month 

just prior to the massive oyster kill. 

During the oyster epizootic, incidence 

of infection dropped rapidly and then 

continued to fall slowly until the last sampling of adult oysters in November. The 

new young oysters on the bed then took 

up the normal winter cycle of low incidence starting at 20%. The intensity of 

infection in living oysters also dropped 

rapidly during the kill but appeared to be 

rising again by the November sampling. 

These declines are undoubtedly caused by 

the death and removal of oysters from the 

living population. 

Hypnospore enlargement in F.T.M. just 

prior to and during the oyster epizootic 

was very meager, particularly in tissues 

from gapers. Small 5-15 ht hypnospores, 

which stained slowly and seldom got 

darker than a translucent blue, resulted. 

Small hypnospores were often mixed with 

large in varying proportions with intermediate sizes and colors also being observed 

occasionally. Although small, these 

hypnospores were extremely numerous 

and produced infections many times as 

intense as any previously rated. 

One possible cause of these unusual 

minute hypnospores was the infection by 

an organism other than Labyrinthomyxa 

marina or by another organism concurrently. Culture studies supported this 

hypothesis indicating the presence of a 

Labyrinthomyxa very similar to L. marina 

but with a distinct sporulation pattern. It 

is possible that a substantial number of 

the oysters killed at the height of the 

epizootic succumbed to this undescribed 

organism. As mentioned earlier, Mackin 

has isolated several other closely related 

Labyrinthula oyster parasites. One or 

more of these forms may have been responsible for disease and death during this 

epizootic. Several of these do not respond 
46 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

to the F.T.M. assay we used but can be 

found only by histological preparation or 

culture. 

The peak of this massive epizootic 

occurred just prior to sampling on September 11, 1968. At that sampling the 

odor of rotting flesh was strong at the 

beds and strings of bacterial scum were 

draped over the exposed oysters. The oyster bar was littered with fresh shells, 

boxes, and gapers in various stages of 

death and decay. Small mud snails, particularly Nassarius, were numerous, especially in and on the many fresh boxes. 

Small crabs, primarily young blue crabs, 

were unusually numerous and were feeding on gaping oysters. Schools of small 

fish were prevalent in the water around 

the oyster bar. 

The following week (week 10), the bar 

was apparently beginning to return to 

normal but several large and small blue 

crabs were still feeding on dying oysters. 

The mortality rate, however, had dropped 

only slightly from the previous week's 

sampling and fresh boxes far outnumbered living oysters. The small remnant 

population continued to suffer significant 

mortality and from the second week of 

October on, several hours were required 

to find enough oysters for a sample. Two 

persons searched the bar four hours and 

were able to find only ten adult oysters 

for the November sample. The mortality 

was essentially complete. However, a 

heavy spatfall had begun just prior to the 

epizootic and continued into October; the 

spat grew rapidly and provided material 

for subsequent samples. By the following 

spring, the oyster population had essentially fully recovered. 

Additional Oyster Epizootics: Several 

other oyster epizootics were visited and 

sampled during late August and September of 1968, and observations indicated 

that from 80 to 97% of the oysters in 

stricken areas were killed. The death rates 

varied widely, however, even between different locations along a single oyster reef. 

We would estimate that over 60% of the 

Gulf coast from Apalachee Bay southward 

to the Florida Keys was ravaged by epizootics. Mortalities were also reported 

from the Atlantic coast but were not confirmed. 

Of all the oyster kills observed, that at 

Station 78 was the most rapid, the majority of deaths occurring in less than two 

weeks, and most complete with essentially 

the entire adult oyster population being 

killed over a wide area. The observed characteristics of other kills were similar, with 

oysters of less than about three months 

old or 3 cm long seemingly unaffected. 

This combined with a heavy fall spat set 

to prepare the beds for rapid rejuvenation 

Damage to Oysters from Labyrinthomyxa: The data gathered in the several 

studies described has repeatedly incriminated Labyrinthomyxa as the cause of 

major oyster mortality in Florida. The 

parasite alone is generally responsible for 

a 50 to 60% annual mortality among the 

adult population and is, therefore, a major 

factor in the limitation of oyster populations in Florida growing areas. In addition 

to this normal yearly enzootic destructiveness, this pathogen is capable of causing massive epizootics. Also, the synergistic effect of this fungus probably causes 

many nonlethal situations to result in 

death and may accelerate other lethal conditions. For example, most of the earliest 

oysters which died at Station 23 after a 

salinity drop to 07 o o (1969) showed L. 

marina of "medium" to "heavy." Unusually high parsitism was also encountered in the severely polluted Pensacola 

Bay area (Hopkins, 1969; Butler, 1969) 

during January and February of 1968 and 

1969. 

The parasite greatly affects the oyster 

industry in Florida, as well as in other 

states (Mackin, 1962; Ray, 1954b; Hewatt 

and Andrews, 1954; and others). Labyrinthornyxa not only selectively and severely 

reduces the numbers of marketable oysters but prevents mass production of 

larger, older oysters. In addition, the destruction of oysters after their first spawning hinders natural development of resistance because deaths before the second 

spawning are so numerous that disease -

selected breeding stock have little or no 

chance of competing reproductively with 

the first year unselected breeding stock. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 47 

Figure 28. Labyrinthomyxa infection and oyster mortality during a severe epizootic in Sarasota Bay 

(Station 78). 

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48 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Fortunately, the effect of this parasite on 

the oyster industry is restricted to the 

oyster itself. Labyrinthomyxa marina is 

no direct threat to man from eating infected oysters (Hoese, 1964). Various project personnel consumed the raw oysters 

remaining after sampling on numerous 

occasions with no ill effects, although 

analysis often showed both high incidence 

and intensity of L. marina. 

THE REDUCTION OF LABYRINTHOMYXA 

DAMAGE TO COMMERCIAL OYSTERS 

Many methods of protecting oysters 

from Labyrinthomyxa have been proposed, but most depend on the inhibition 

of the parasite by continuously low salinities and would apparently not work in 

Florida. Juvenility and low temperature 

were the only factors we found to limit 

Labyrinthomyxa parasitism of oysters. 

Juvenility obviously provides only a short 

reprieve from the fungus. Avoiding dangerously high temperatures for extended 

times is, unfortunately, also impossible in 

Florida. Even in Apalachicola Bay in the 

northern part of the state, temperatures 

regularly remain above 25°C from midApril until mid-October and commonly 

exceed 30°C from mid-July until September (Ingle, 1951). In Cedar Keys, temperatures averaged over 25°C from mid-March 

to late October and often exceeded 30°C 

from late May until early September during 30 years of monitoring (Ingle and 

Dawson, 1953a). Afternoon water temperatures in Tampa Bay in central Florida 

average above 25°C from late May until 

mid-November and average above 30°C 

from mid-April until mid-October, even 

exceeding 34°C during June through September. Temperature further south could 

be expected to surpass these values. Additionally, minimum temperatures are insufficient to cause disappearance or even serious attenuation of established infections 

since the coldest month averaged 13.5°C 

in Apalachicola (Ingle and Dawson, 

1953b), 15.1°C in Cedar Keys (Ingle and 

Dawson, 1953a) and 15.4°C at Tampa 

Bay Station 43. 

A plan for growing oysters to avoid 

extensive mortalities from Labyrinthomyxa marina and other causes was 

formulated based on the following factors: 1) most Florida oyster areas receive 

spatfall throughout the summer; 2) since 

several months are required for young 

oysters to develop their first infection, 

these oysters are temporarily safe, even 

when exposed to summer temperatures; 

3) several months are required for infections to intensify to lethal levels in 

both old and young oysters after the 

winter low; and 4) Florida oysters are capable of extremely rapid growth under 

favorable conditions. Consequently, 

oysters spawned in midsummer would experience little mortality before winter and 

little more until midsummer of the following year, thus providing a year of 

growth with low mortality. The only 

other problems were obtaining an adequate set at the proper time and inducing 

growth rates sufficient to produce Marketable oysters in the necessary 12-month 

period. 

This method was utilized in the Indian 

River in 1968-69 and proved extremely 

successful. Spat were collected in midsummer on subtidal cultch in an area with relatively high salinity and good current. 

After two to three months the oysters 

were 3 to 5 cm in height (length) and 

were moved to a carefully selected growing area with a narrow range of salinity 

(Ingle and Dawson, 1950) near 257.0(private lease) where they were evenly spread 

over the gravel and shell bottom. Harvesting was initiated in May, prior to heavy 

spawning, when the oysters had good 

shape, little fouling, excellent quality and 

averaged 12 cm in height (Figure 29). 

Additional harvesting was done in July 

when the oysters were of slightly reduced 

quality but averaged 15 cm in height. 

The apparent success of this method in 

one isolated incidence is indicative, but 

further testing is needed to verify its usefulness. In addition, the application of 

other knowledge of oyster biology could 

further improve the method. Current data 

indicates that successful application of 

these methods could yield a mortality 

from planting to harvest of only 20%. 

Excellent quality and substantial production are also indicated. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 49 

SUMMARY 

A new parasite of the American oyster, 

Crassostrea virginica, was described in 

1950 by Mackin, Owen, and Collier and 

named Dermocystidium marinum. As a 

result of extensive studies, this parasitic 

fungus was assigned to another genus and 

became Labyrinthomyxa marina (Mackin, 

Owen, and Collier) Mackin and Ray 

(1966). Several authors demonstrated 

extensive oyster mortalities caused by this 

parasite in many areas, notably Louisiana 

and Texas. A simple, quantitative assay 

for L. marina in the host was developed 

by Ray (1952b), and an adaptation of this 

fluid thioglycollate medium (F.T.M.) 

culture technique has been used extensively in testing Florida oysters since 1965. 

Antibiotics were originally used in 

F.T.M. assays to control contamination 

but they also affected the essential enlargement of the parasite cells. Ray's 

(1966b) findings that mycostatin induced 

enlargement of the individual parasite 

cells and caused a greater proportion of 

the cells to enlarge were confirmed by our 

studies. Duplicate tissue cultures showed 

parasite cells (hypnospores) averaged 21% 

greater in diameter when cultured with 

mycostatin plus chloramphenical (M+C) 

than when cultured with penicillin plus 

streptomycin (P+S). We also found that 

about 30% more cells enlarged in M+C 

than in P+S cultures at low infection intensities but this percentage decreased as 

infection intensities increased. 

Although M+C improves the accuracy 

of the assay, it has some deficiencies 

which, under certain conditions, make 

P+S fortification preferable. 

Extensive sampling of oysters from 

over 100 stations demonstrated that L. 

marina was present in oysters throughout 

both the Atlantic and Gulf coastal areas 

of Florida. 

The results of sampling over several 

years revealed that L. marina goes through 

annual cycles of low winter and high 

summer incidences and intensities similar 

to those observed in more northerly Gulf 

states. Infection rates and intensities in 

Florida, however, remained substantial 

throughout the winter and began a rapid 

rise toward summer peaks much earlier 

(February). 

A strong positive correlation was demonstrated between temperature and infection in nature. The relationship applied to 

infection rates and intensities and conformed with the findings of many workers 

(Mackin, 1962) for field and laboratory 

studies. Infection intensities generally 

increased gradually through the "medium" rating but increased rapidly through 

"medium-heavy" and into "heavy." The 

reasons for this occurrence are not known 

but it perhaps indicates a decline in the 

efficiency of the host's defense mechanisms. 

It was demonstrated that after infections have developed, high temperatures 

can maintain or increase these intensities, 

even in the presence of low salinity. In 

areas of relatively low salinity, short periods of higher salinity may combine to 

promote infection increases. These findings establish the overriding importance of 

temperature rather than salinity as an 

infection promoting mechanism. 

Juvenile oysters were not found to have 

special immunity to L. marina. The failure 

of juvenile or yearling oysters to suffer 

loss from this parasite is probably because 

the time between setting and fall cooling 

is insufficient for lethal infections to 

initiate and develop. In addition, oyster 

growth rates are so rapid in many areas of 

Florida that spat can grow to adult-like 

sizes of 4-5 cm height during the several 

months required for lethal infection to 

develop. 

The analysis of infection among oysters 

of various age groups indicated that individuals of two years or older, although 

very few in number, possess some form of 

special resistance to L. marina. These 

oysters had fewer and lighter infections. 

Infections were found to be greatest in 
50 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

Figure 29. Oysters grown by the mortality minimizing technique. A, B - ten month old oysters harvested in May for best meat quality (scale in cm); C, D - oysters harvested at larger size but 

lower meat quality in July after 13 months of growth; E, F - survivors harvested at the end 

of the summer with severe fouling and boring damage after 18 months. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 51 

Figure 29. (Cont.) 
52 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

both incidence and intensity in high intertidal oyster, diminishing with increasing 

depth. Although data indicated that more 

heavily infected oysters had poorer condition, the correlation was probably false, 

being produced by the coincidental debilitation of oysters by summer spawning 

rather than by Labyrinthomyxa. Severe 

infections ("medium heavy" and above), 

however, do seem to cause sporadic, and 

sometimes extreme, lowering of host condition. 

Mackin (1961) used size frequency 

analysis to establish year classes from 

which he determined annual mortalities 

for Louisiana oysters. Attempts to determine year classes of Florida oysters by 

this method were unsuccessful because no 

dependable size modes were present. This 

lack of size modes seemed to result from 

the long Florida spawning season and 

rapid growth rates. 

Further efforts to delimit year classes 

for mortality calculations led to the development and testing of shell contours as a 

method of aging oysters. The system was 

simplified for routine use. Calculations 

demonstrated that oyster mortalities vary 

greatly from year to year. The analysis of 

shell configurations was also used to 

determine the season each oyster set and 

the number of times it spawned. This data 

indicated that the survival rate of oysters 

setting in fall was significantly greater 

than of those setting in spring. 

Field tests indicated that the normal 

annual mortality from L. marina parasitism is about 50% of the adult population. Deaths from other causes averaged 

about 15% of the adult population. The 

stone crab. Menippe mercenaria, was the 

most active predator in the areas studied, 

accounting for a four to eight percent 

annual adult oyster kill. Significant spring 

mortalities of unknown origin occurred. 

All adult oysters found to contain the 

oyster "leech," Stylochus frontalis, were 

in advanced stages of L. marina infection. 

The ratio of gapers to living oysters 

present at any time in an area was found 

to be quantitatively related to the short 

term oyster mortality rate. The method of 

determining the gaper/living oyster ratio 

was standardized and a conversion formula to mortality rate is presented. Field 

testing indicated that the conversion factor was temperature dependent because 

individual gapers were present for longer 

periods at lower temperatures. 

Several epizootics caused by L. marina 

were observed and one kill was studied 

extensively. The oyster mortality from 

these infrequent occurrences was severe 

and rapid. Spat and very small oysters, 

however, were almost unaffected and oyster beds destroyed by disease displayed 

essentially complete recovery in nine 

months. 

The appearance of unusual minute 

hypnospores at the height of the oyster 

epizootics suggested the presence of a second lethal oyster parasite, and culture 

studies demonstrated a second species of 

Labyrinthomyxa with a distinct sporulation. In other investigations, Mackin has 

isolated and studied several other closely 

related oyster parasites, some of which 

may have been significant in the epizootics or other studies since they cannot be 

detected by the F.T.M. assay we used. 

Analysis of gaper infection intensities 

confirmed that infections of L. marina 

as low as "medium" may result in death 

of the host. However, most animals do 

not succumb until heavy or medium 

heavy parasite intensity is reached. The 

existence of other pathogenic or unfavorable conditions often combine to cause 

death at abnormally low levels of L. 

marina. 

An oyster culture schedule was devised 

to minimize L. marina mortalities while 

also lessening other oyster losses and 

producing the best product in the shortest 

time. A field evaluation of this schedule 

under optimum conditions showed that 

the time from set to harvest was 10 to 13 

months, average harvest oyster height was 

12 to 15 cm, mortality from L. marina 

was about 10%, and the overall post spat 

loss was approximately 20%. Oysters were 

moved once and planted at medium to 

high density. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 53 

ACKNOWLEDGMENTS 

We wish to thank Mrs. Violet Stewart 

and Messrs. Dale McCall, Robert Cissel, 

Don Lockard, and Andrew Kaloski, who 

worked directly on the project, and many 

other members of the laboratory staff 

who have assisted in numerous ways. Our 

thanks also to Mr. R. M. Ingle, Chief of 

the Bureau of Marine Science and Technology, for assistance and direction 

throughout the study and to Mr. E. A. 

Joyce, Jr., Supervisor of the Marine 

Research Laboratory, who helped in preparation of the manuscript. 

BUTLER, P. A. 

1969. Monitoring pesticide pollution. 

BioScience, 19(10): 889-891. 

BUTLER, P. A. and P. F. SPRINGER 

1963. Pesticides a new factor in 

coastal environments. Trans. 

28th N. Amer. Wildl. Manage. 

Res. Conf.: 378-390. 

DAWSON, C. E. 

1955. Observations on the incidence 

of Dermocystidium marinum 

infection in oysters of Apalachicola Bay, Florida. Tex. J. Sci., 

7(1): 47-56. 

LITERATURE CITED 

ANDREWS, J. D. and W. G. HEWATT 

1957. Oyster mortality studies in Virginia. II. The fungus disease 

caused by Dermocystidium 

marinum in oysters of Chesapeake Bay. Ecol. Monogr., 27: 

1-25. 

ANONYMOUS 

1958. Summary report on the fungus 

disease of oysters Dermocystidium in Delaware Bay. 

New Jersey Oyster Research 

Laboratory, Bivalve, N. J. Mimeo. 

BUTLER, P. A. 

1966a. The problems of pesticides in 

estuaries. Amer. Fish. Soc., 

Spec. Publ. No. 3: 110-115. 

BUTLER, P. A. 

1966b. Pesticides in the marine environment. J. Appl. Ecol., 3 

(Suppl.): 253-259. 

BUTLER, P. A. 

1966c. Fixation of DDT in estuaries. 

Trans. 31st N. Amer. Wildl. 

Nat. Res, Conf.: 184-189. 

DUNNINGTON, E. A. 

1956. Oyster parasite distribution 

studies in Maryland waters. 

Maryland Tidewater News, 

12(9): 1-3. 

GALTSOFF, P. S. 

1964. The American oyster Crassostrea virginica(Gmelin). U.S. 

Bur. Comm. Fish., 64: 1-480. 

HAMMERSTROM, R. J., V. L. 

CAPSER, E. A. ROBERTSON, JR., 

J. C. BUGG, JR., and J. L. GAINES 

1967. Studies of pesticides in shellfish 

and estuarine areas of Louisiana 

and Alabama. Proc. Gulf S. 

Atlantic States Shellf. Sanitat. 

Res. Conf.: 85-112. 

HEWATT, W. G. and J. D. ANDREWS 

1954. Oyster mortality studies in Virginia. I. Mortalities of oysters in 

trays at Gloucester Point, York 

River. Tex. J. Sci., 6: 121-133. 

HOESE, H. D. 

1964. Studies on oyster scavengers 

and their relation to the fungus 

Dermocystidiurn marinum. 

Proc. Nat. Shellf. Assoc., 53: 

161-174. 
54 FLORIDA DEPARTMENT OF NATURAL RESOURCES 

HOPKINS, T. L. 

1969. The Escambia River and Escambia Bay during summer, 

1969. 1. Physical/chemical 

studies on the Escambia River 

complex. II. Physical/ chemical 

and biological studies on Escambia Bay. Mimeographed. 

INGLE, R. M. 

1951. Spawning and setting of oysters 

in relation to seasonal environmental changes. Bull. Mar. Sci. 

Gulf Carib., 1(2): 111-135. 

INGLE, R. M. 

1967. Artificial food for oysters. Sea 

Frontiers, 13(5): 296-303. 

1NGLE, R. M. and C. E. DAWSON 

1950. Variations in salinity and its relation to the Florida oyster. Salinity variations in Apalachicola 

Bay. Proc. Gulf Carib. Fish. 

Inst. 3rd Ann. Sess.: 35-42. 

1NGLE, R. M. and C. E. DAWSON 

1952. Growth of the American oyster 

Crassostrea virginica(Gmelin) in 

Florida waters. Bull. Mar. Sci. 

Gulf Carib., 2(2): 393-401. 

INGLE, R. M. and C. E. DAWSON 

1953a. A survey of the Cedar Key area. 

Fla. Bd. Conserv., Tech. Ser. 

No. 9. 

INGLE, R. M. and C. E. DAWSON 

1953b. A survey of Apalachicola Bay, 

Florida. Fla. Bd. Conserv., 

Tech. Ser. No. 10: 39 pp. 

KOPFLER, F. C. and J. MAYER 

1969. The occurrence of trace metals 

in oysters (Crassostrea virginica) 

from the south Atlantic and 

Gulf of Mexico. Abstract. Proc. 

Nat. Shellf. Assoc., 59: 5. 

MACKIN, J. G. 

1951. Diseases of oysters and their relation to the Gulf coast oyster 

industry. Abstract. Proc. Gulf 

Carib. Fish. Inst. 3rd Ann. Sess. 

MACKIN, J. G. 

1955. Dermocystidium marinum and 

salinity. Proc. Nat. Shellf. 

Assoc., 46: 116-128. 

MACKIN, J. G. 

1961. A method of estimation of mortality rates in oysters. Proc. 

Nat. Shellf. Assoc., 50: 41-51. 

MACKIN, J. G. 

1962. Oyster disease caused by Dermocystidium marinum and 

other microorganisms in Louisiana. Publ. Inst. Mar. Sci. Univ. 

Tex., 7: 132-229. 

MACKIN, J. G. and S. H. HOPKINS 

1962. Studies on oyster mortality in 

relation to natural environments and to oil fields in Louisiana. Publ. Inst. Mar. Sci. Univ. 

Tex., 7: 1-131. 

MACKIN, J. G., H. M. OWEN 

and A. COLLIER 

1950. Preliminary note on the occurrence of a new protistan parasite, Dermocystidium marinum 

n. sp. in Crassostrea virginica 

(Gmelin). Science, 111(2883): 

328-329. 

MACKIN, J. G. and S. M. RAY 

1966. The taxonomic relationship of 

Dermocystidium marinum, 

Mackin, Owen and Collier. J. 

Invert. Pathol., 8(4): 544-545. 

MACKIN, J. G. and A. K. SPARKS 

1962. A study of the effect on oysters 

of crude oil loss from a wild 

well. Publ. Inst. Mar. Sci. Univ. 

Tex., 7: 230-261. 

MACKIN, J. G. and D. A. WRAY 

1952. Report on the second study of 

mortality of oysters in Barataria 

Bay, La. and adjacent areas. 

Part II. Disease caused by Dermocystidium marinum. Texas 

A. and M. Res. Found., Project 

9 Rep.: 1-40. 
OYSTER PARASITISM BY LABYRINTHOMYXA MARINA IN FLORIDA 55 

MENZEL, R. W. and S. H. HOPKINS 

1954. Studies on oyster predators in 

Terrebonne Parish, La. Texas A. 

and M. Res. Found., Project 9 

Rep.: 140 pp. 

OWEN, H. M. 

1955. The oyster industry of Louisiana, p. 1-387. (Ms. in files of 

Div. of Oysters and Waterbottoms of Louisiana.) 

PEARSE, A. S. and G. W. WHARTON 

1938. The oyster "leech," Stylochus 

in im icus Palombi, associated 

with oysters on the coasts of 

Florida. E col. Monogr., 8: 

605-655. 

PRINGLE, B. H., D. E. HISSONG, 

E. L. KATZ and S. T. MULAWKA 

1968. Trace metal accumulation by 

estuarine mollusks. J. Sanit. 

E ngng. Div., 94(SA3), Proc. 

Pap. 5970: 455-475. 

RAY, S. M. 

1952a. Cultural studies of Dermocystidium marinum with special reference to diagnosis of this parasite in oysters. Masters Thesis, 

Rice Institute, Houston, Texas. 

RAY, S. M. 

1952b. A culture technique for the diagnosis of infection with Dermocystidium marinum in oysters. Nat. Shellf. Assoc. Conven. 

Addr.: 9-13. 

RAY, S. M. 

1953. Studies on the occurrence of 

Dermocystidium marinum in 

young oysters. Proc. Nat. 

Shellf. Assoc., 44: 80-88. 

RAY, S. M. 

1954a. Biological studies of Dermocystidium marinum, a fungus 

parasite of oysters. Rice Inst. 

Pamphl., Spec. Issue: 114 pp. 

RAY, S. M. 

1954b. Experimental studies on the 

transmission and pathogenicity 

of Dermocystidium marinum, a 

fungous parasite of oysters. J. 

Parasit., 40: 235. 

RAY, S. M. 

1966a. Notes on the occurrence of Dermocystidium marinum on the 

Gulf of Mexico coast during 

1961 and 1962. Proc. Nat. 

Shellf. Assoc., 54: 45-54. 

RAY, S. M. 

1966b. A review of the culture method 

for detecting Dermocystidium 

marinum, with suggested modifications and precautions. Proc. 

Nat. Shellf. Assoc., 54: 55-69. 

RAY, S. M. and A. C. CHANDLER 

1955. Dermocystidium marinum, a 

parasite of oysters. Exper. Parasitol., 4(2): 172-200. 

RAY, S. M. and J. G. MACKIN 

1954. Studies on the transmission and 

pathogenicity of Dermocystidium marinum, I. Texas A. and 

M. Res. Found., Project 23, 

Tech, Rep. No. 11: 11 pp. 

RAY, S. M., J. G. MACKIN 

and J. L. BOSWELL 

1953. Quantitative measurement of 

the effect of oyster disease 

caused by D. marinum. Bull. 

Mar. Sci. Gulf. Garth., 3(1): 

6-33. 

STEWART, V. N. 

1966. Dermocystidium marinum, parasite of oysters. Fla. Bd. Conseru. Mar. Lab., Salt Wat. Fish. 

Leaf. No. 5: 4 pp. 

VALIULIS, G. A. and J. G. MACKIN 

1969. Formation of sporangia and 

zoospores by Labyrinthomyxa 

sp. parasitic in the clam Macoma 

balthica, J. Invert. Path., 14: 

268-270.