Biological Report 82(11.75) 

TR EL-82-4 

August 1987 

Species Profiles: Life Histories and Environmental Requirements 

of Coastal Fishes and Invertebrates (South Atlantic) 

HARD CLAM 

Arnold G. Eversole 

Department of Aquaculture, Fisheries and Wildlife 

323 Long Hall 

Clemson University 

Clemson, SC 29634-0362 

Project Manager 

Carrol Cordes 

Project Officer 

David Moran 

National Wetlands Research Center 

U.S. Fish and Wildlife Service 

1010 Gause Boulevard 

Slidell, LA 70458 

Performed for 

Coastal Ecology Group 

U.S. Army Corps of Engineers 

Waterways Experiment Station 

Vicksburg, MS 39180 

and 

National Wetlands Research Center 

Research and Development 

Fish and Wildlife Service 

U.S. Department of the Interior 

Washington, DC 20240 : 
This series may be referenced as follows: 

U.S. Fish and Wildlife Service. 1983-19 . Species profiles: life histories 

and environmental requirements of coastfi fishes and invertebrates. U.S. Fish 

Wildl. Serv. Biol. Rep. 82(11). U.S. Army Corps of Engineers, TR EL-82-4. 

This profile may be cited as follows: 

Eversole, A.G. 1987. Species profiles: life histories and environmental 

requirements of coastal fishes and invertebrates (South Atlantic)--hard clam. 

U.S. Fish Wildl. Serv. Biol. Rep. 82(11.75). U.S. Army Corps of Engineers, TR 

EL-82-4. 33 pp. 
PREFACE 

This species profile is one of a series on coastal aquatic organisms, 

principally fish, of sport, commercial, or ecological importance. The profiles 

are designed to provide coastal managers, engineers, and biologists with a brief 

comprehensive sketch of the biological characteristics and environmental 

requirements of the species and to describe how populations of the species may be 

expected to react to environmental changes caused by coastal development. Each 

profile has sections on taxonomy, life history, ecological role, environmental 

requirements, and economic importance, if applicable. A three-ring binder is 

used for this series so that new profiles can be added as they are prepared. 

This project is jointly planned and financed by the U.S. Army Corps of Engineers 

and the U.S. Fish and Wildlife Service. 

Users of this species profile should note that a Habitat Suitability Index 

(HSI) model is available for the hard clam. HSI models are designed to produce 

a numerical index of the relative value of a given site as fish or wildlife 

habitat. Those interested in obtaining copies of the model report should 

contact the Service's National Wetlands Research Center and request: 

Mulholland, R. 1984. Habitat suitability index models: hard clam. U.S. Fish 

Wildl. Serv. FWS/OBS-82.77. 21 pp. 

Suggestions or questions regarding this report should be directed to one of 

the following addresses. 

Information Transfer Specialist 

National Coastal Ecosystems Team 

U.S. Fish and Wildlife Service 

NASA-Slide11 Computer Complex 

1010 Gause Boulevard 

Slidell, LA 70458 

or 

U.S. Army Engineer Waterways Experiment Station 

Attention: WESER-C 

Post Office Box 631 

Vicksburg, MS 39180 

iii 
CONVERSION TABLE 

Metric to U.S. Customary 

Multiply EJ! 

millimeters (mm) 0.03937 

centimeters (cm) 0.3937 

meters (m) 3.281 

meters (m) 0.5468 

kilometers (km) 0.6214 

kilometers (km) 0.5396 

inches 

inches 

feet 

fathoms 

statute miles 

nautical miles 

square meters (m*) 10.76 square feet 

square kilometers (km') 0.3861 square miles 

hectares (ha) 2.471 acres 

liters (1) 0.2642 gallons 

cubic meters (m") 35.31 cubic feet 

cubic meters (m3) o.oooai10 acre-feet 

milligrams (mg) 0.00003527 ounces 

grams (9) 0.03527 ounces 

kilograms (kg) 2.205 pounds 

metric tons (t) 2205.0 pounds 

metric tons (t) 1.102 short tons 

kilocalories (kcal) 3.968 British thermal units 

Celsius degrees ("C) 1.8(OC) + 32 Fahrenheit degrees 

inches 25.40 millimeters 

inches 2.54 centimeters 

feet (ft) 0.3048 meters 

fathoms 1.829 meters 

statute miles (mi) 1.609 kilometers 

nautical miles (nmi) 1.852 kilometers 

square feet (ft2) 0.0929 square meters 

square miles (mi2) 2.590 square kilometers 

acres 0.4047 hectares 

gallons (gal) 3.785 liters 

cubic feet (ft") 0.02831 cubic meters 

acre-feet 1233.0 cubic meters 

ounces (oz) 28350.0 milligrams 

ounces (oz) 28.35 grams 

pounds (lb) 0.4536 kilograms 

pounds (lb) 0.00045 metric tons 

short tons (ton) 0.9072 metric tons 

U.S. Customary to Metric 

British thermal units (Btu) 

Fahrenheit degrees (OF) 

0.2520 

0.5556 (OF 

i *J 

To Obtain 

kilocalories 

32) Celsius degrees 
CONTENTS 

Page 

PREFACE.................................. iii 

CONVERSION FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv 

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi 

,W 

NOMENCLATURE/TAXONOMY/RANGE ........................ 1 

MORPHOLOGY AND IDENTIFICATION AIDS .................... 3 

REASONS FOR INCLUSION IN SERIES ...................... 3 

LIFE HISTORY ............................... 4 

Spawning ................................ 4 

Fecundity and Eggs ........................... 6 

Larvae 

Plantigrade iD;ssocotkh)'Stages : : : : : : : : : ........................ 

7 

8 

Adults 

GROWTH CHARAiTiRkiIk' : : : : : : : : : : : : : : : : : : : : : : : : : : : 

COMMERCIAL/RECREATIONAL FISHERIES ..................... 10 

Fisheries ................................ 10 

Population Dynamics ........................... 12 

ECOLOGICAL ROLE .............................. 

Feeding ................................ :3 

Parasites and Disease .......................... 14 

Predators ................................ 14 

ENVIRONMENTAL REQUIREMENTS ........................ 

Temperature ............................... :i 

Salinity ................................ 

WaterQuality .............................. :; 

Water Current .............................. 19 

Substrate ................................ 20 

Ecosystem Alteration .......................... 20 

LITERATURE CITED ............................. 23 
ACKNOWLEDGMENTS 

I gratefully acknowledge the reviews and comments of Joy G. Goodsell and 

Richard S. Knaub, Clemson University, Clemson, SC, E. W. Rhodes, National Marine 

Fisheries Service, Milford, CT, Ronald Dugas, Louisiana Department of Wildlife 

and Fisheries, New Orleans, and William D. Anderson, South Carolina Wildlife and 

Marine Resources Department, Charleston. I also acknowledge Jean Richardson's 

patience and help in preparing this species profile. 

vi 
Figure 1. Hard clam, Mercenaria mercenaria. 

HARD CLAM 

NOMENCLATURE/TAXONOMY/RANGE 

Scientific name.............Mercenaria 

mercenaria (Linnaeus 1758). Initially known under the European genus 

Venus, it was recognized to be sufficiently different and reassigned 

to the generic name Mercenaria 

(Frizzell 1936; Wells 19576). 

mercenaria was incorrectly used for 

M. mercenaria up to the mid-1960s. 

Preferred common name.....Hard clam in 

the Southern United States (Figure 

1) and quahog in the Northern United 

States. 

Other common names . . ..Hard shell clam, 

hard-shelled clam, quohog, quahaug, 

cherrystone clam, little-neck clam, 

chowders, round clam 

Class . . . . . . . . . . ..Bivalvia (Pelecypoda) 

Order . . . . . . . . . . . . . . ..Eulamellibranchia 

Suborder . . . . . . . . . . . . . . . . . ..Heterodonta 

Family . . . . . . . . . . . . . . . . . . . . . ..Veneridae 

1 

Geographic range: Mercenaria mercenaria is distributed from the Gulf of 

St. Lawrence, Canada, to Texas 

(Abbott 1974). It has also been 

successfully transplanted to California (Loosanoff and Davis 1963; 

Crane et al. 1975) and Europe 

i;;;;;il 1956; Heppell 1961; Ansell 

It is abundant from Virginia io Massachusetts and supports 

isolated breeding communities above 

Cape Cod (Turner 1953). Hard clams 

occur throughout the South Atlantic 

region (Figure 2) in estuaries from 

the intertidal zone to a depth of 15 

m or more (Porter 1974; Fox and 

Ruppert 1985). M. mercenaria 

texana, the only sui%pecies recognized by Abbott (1974), extends 

south from Cape Canaveral, Florida, 

on the Atlantic coast and west from 

Northwest Florida, on the Gulf of 

Mexico coast to Texas and northern 
NORTH CAROLINA 

SOUTH CAROLINA 

A Tl ANTIC UCEA N 

:. 

‘1.. Coastal distribution 

MILES 

O+e==%+ 

KILOMETERS 

Melbourne/Grant 

Figure 2. Distribution of hard clams in the South Atlantic, showing major clam 

fishing ports in North Carolina, South Carolina, Georgia, and Florida. 

2 
Mexico (Abbott 1974). The M. 

mercenaria notata "form," which 

Abbott (1974) does not recognize as 

a subspecies, occurs sympatrically 

with hard clams. The more conservative classification of Abbott (1974) 

is followed here. The very similar 

species M. campechiensis extends 

from Cape-Mav. New Jersev (Merrill 

and Ropes 1967), south io 'Florida 

and Mexico (Abbott 1974). The distributions of M. mercenaria and 

M. 

M. 

campechiensis overlap; however, 

campechiensis is usually in 

deeper, more saline areas, 

offshore of barrier islands. rn't:rr 

qrades of the two soecies occur in 

shallow water south of Indian River, 

Florida. 

MORPHOLOGY AND IDENTIFICATION A IDS 

Hard clams undergo larval development and a series of morph0 logical 

changes in their life cycle. Characteristics of early life history stages 

of hard clams are presented in the 

LIFE HISTORY section. The following 

information on the morphology of the 

adult was summarized primarily from 

Pierce (1950), Gosner (1971)) and 

Abbott (1974). 

The shell of the hard clam is 

composed of two equal-size valves with 

an ovate-trigonal shape. Valves are 

joined dorsally by a dark brown 

external ligament. Swellings or umbos 

occur on either side of the ligament. 

Conspicuous concentric lines of growth 

extend out from the umbo on each valve 

(Figure 1). Ventral and anterior to 

the umbo lies a heart-shaped configuration called a lunule. The exterior 

shell is fawn colored or off-white and 

has a very thin periostracum. Shells 

of the form notata are marked with 

brown, zigzag mottlings. Chanley 

(1959) reported the notata markings to 

be inherited in asimpte Mendelian 

manner. 

Interior shell color is white or 

pale yellow with noticeable purple 

markings near the shell margin. A 

prominent adductor muscle scar is near 

each end of each valve. The scars are 

connected by the distinct pallial 

line! which is parallel to the shell 

margin. The pallial line forms a 

triangular or pallial sinus at one 

end of the valve. Opposite the pallial line and anterior to the ligament 

are internal cardinal teeth. 

The hard clam has two short siphons fused at the base; the inhalent 

siphon is ventral to the exhalent siphon and fringed with small tentacles. 

Ends of both siphons are pigmented, 

but color varies from opaque white or 

cream color to dark brown or black. 

The foot is large and somewhat 

hatchet-shaped. Mantle lobes, which 

cover the soft parts of the clam, are 

separate along the anterior and ventral edges of the shell, where the 

lobe borders are thickened and attached along the pallial line. The 

lobes of the mantle edge are fused at 

two points to form the siphons. Dissection guides are available for the 

hard clam (e.g., Pierce 1950; Sherman 

and Sherman 1976). Shuster (1969) 

also developed a three-ply representation of the hard clam so that sections of the inner and outer surfaces 

of the shell, mantle, and visceral 

mass can be cut out of a pamphlet and 

assembled to form a sequence of 

structures encountered when the hard 

clam is dissected. 

REASONS FOR INCLUSION IN SERIES 

Hard clams support an important 

commercial fishery along the Atlantic 

coast of the United States. Among the 

species of clams harvested in the 

United States, hard clams yield the 

highest dollar value, and are exceeded 

only by surf clams, Spisula solidissima, and ocean quahogs, Arctica islandica, in kilograms of meats har- 

-(Table 1). Less than 10% of 
Table 1. Commercial clam landings in 1984 (U.S. Department of Commerce 1985). 

Species Landings (meats) Ex-vessel value 

1,000 kg % $1,000 0 

Hard clam 6,704 11.10 49,849 42.79 

Surf clam 31,929 52.85 34,334 29.47 

Softshell clam 3,600 5.95 19,842 17.04 

Ocean quahog 17,642 29.20 11,829 10.15 

Other 545 0.90 637 0.55 

Total 60,420 100.00 116,491 100.00 

dIL ’ 

c 

the total U.S. commercial harvest results from aquaculture, though the potential for aquaculture in the United 

States is high (Manzi 1985). Recreational harvests of hard clams are 

important; thousands of persons participate in the South Atlantic region. 

However, no scientifically derived 

estimates are currently available for 

recreational harvest of hard clams 

from this region. The estimated recreational harvest in Great South Bay, 

New York, was 4,796 bushels or 1.42% 

of the reported commercial summer harvest (Fox 1981). The hard clam is a 

delicacy of considerable nutritional 

value; it is low in calories but high 

in protein and essential minerals such 

as iodine and iron (Miller et al. 

1975). 

Hard clams occur extensively in 

estuarine systems throughout the 

region, and because of their distribution, they may be exposed to a myriad 

of environmental impacts. Because 

hard clams have a sensitive planktonic 

larval stage (Carriker 1961) and are 

long-lived sessile organisms as adults 

(Chestnut 1951; Lutz and Haskin 1984), 

they appear to be particularly vulnerable to the effects of pollution and 

coastal development. 

LIFE HISTORY 

Spawning 

Mercenaria mercenaria exhibits 

consecutive hermaphroditism, going & 

through a juvenile or preadult sexual 

phase when it is a few months old and 

6-7 mm in shell length (Loosanoff 

1936, 1937a). Although it functions 

mostly as a male during this juvenile 

sexual phase, close examination of the 

gonad reveals both male and female sex 

cells. These sex cells are differentiated, but because of continued proliferation of the spermatogonia, the 

gonad acquires a predominately male 

character. Hard clams in this phase 

can discharge sperm and function as 

males (Coe 1943a). Hard clams go 

through a sex change after the juvenile sexual phase and can function only 

as males or females. Loosanoff (1936, 

1937a) established that sex change in 

p1. mercenaria was protandrous, or male 

to female. Dalton and Menzel (1983) 

found that M. campechiensis and its M. . 

mercenaria -hybrids exhibited protandric sexual development similar to 

that of M. mercenaria. L 

Eversole (1986), using data from 

various literature sources, calculated 

* 

4 
an approximate shell length of 33 mm 

at which M. mercenaria reaches sexual 

maturity.- Size at maturity tends to 

be smaller in males than in females 

(Eversole et al. 1980). Growth rate 

of young hard clams appears to be an 

important factor in determining early 

sexual maturity; faster-growing clams 

attain sexual maturity at an age of 

only 1 year in some localities (Eversole et al. 1980) and 2 years in 

others (Bricelj and Malouf 1980). 

Evidence from studies on the hard clam 

corroborate the hypothesis advanced by 

Quayle and Bourne (1972) that sexual 

maturity in clam species appears to be 

more a function of size than of age. 

The gametogenic and spawning 

cycle of the hard clam in North America varies with latitude (Table 2). 

Populations in Connecticut (Loosanoff 

1937b), New York (Kassner and Malouf 

1982) and Delaware Bay (Keck et al. 

1975) have an annual gametogenic 

cycle. Gametogenic activity and the 

period of ripeness in populations 

occur earlier in the year in Delaware 

Bay than in Connecticut and New York 

(Table 2), Populations in more southTable 2. Spawning times and temperatures (at the first major spawning peak) of 

populations of Mercenaria mercenaria in North America, based on histological 

evidence of gametogenic activity and gamete release. The solid lines show 

periods of peak spawning. 

Location 

Temp. Months 

("C) J FM AM J J A S 0 N D Sources 

Charles Island, 

Conn. 

Gr. South Bay, N.Y.a 

Gr. South Bay, N.Y.a 

Delaware Bay, De. 

23-25 mm_ - we_ Loosanoff 1937b 

20 

20 

25-27 

Core Sound, N.C. 

N. Santee Bay, S.C. 

27-30 me - ____ - __ Porter 1964 

20 

Clark Sound, S.C. 20-23 

Wassaw Sound, Ga. 22-26 

Al;;;aior Harbor, 

. 

16-20 

___ - ___ 

-- - __ 

Kassner and 

Malouf 1982 

________ - Keck et al. 

1975 

-_ - ______ - Manzi et al. 

1985 

-_ - ______ - Eversole et 

al. 1980 

a__ - _ -- - ___ Pline 1984 

-_ - _____----_ - _ Dalton and 

Menzel 1983 

c 

aObservation at same locality in different years. 

bSpawning cycle of young male clams less than 2 years old. 

5 
erly latitudes continue the trend of 

earlier and extended periods of gametogenesis until a second developmental 

and spawning period becomes possible. 

Porter (1964) observed two periods of 

gametogenic activity in North Carolina: a major redevelopment period in 

early spring after the fall spawning 

peak and a second minor redevelopment 

period after a June spawning (Table 

2). Two separate gametogenic cycles 

were observed in hard clam populations 

from South Carolina (Eversole et al. 

1980; Manzi et al. 1985) and Georgia 

(Pline 1984). Hard clams transplanted 

from northern latitudes to southern 

latitudes had a gametogenic cycle 

similar to that observed in native 

clams (Dalton and Menzel 1983). The 

trend of altering gametogenic activity 

toward a bimodal pattern in the more 

southerly latitudes appears to be 

linked to water temperature (LOOSanOff 

1937b). 

The spawning cycle of the hard 

clam parallels its gametogenic cycle, 

varying with latitude. Like the gametogenic cycle, the time of spawning 

appears closely related to water temperature (Loosanoff 1937a; Carriker 

1961; Ansell et al. 1964; Keck et al. 

1975). Temperature has been considered the most important factor in spawning because a certain degree of gonad 

ripeness must be achieved before the 

clams can respond to a specific spawning stimuli. Hard clams can be artificially conditioned to develop ripe 

gonads for early spawning by gradually 

increasing water temperature to about 

20 "C, and providing food (Loosanoff 

and Davis 1950). Similarly, they can 

be induced to spawn by rapidly 

increasing water temperature from 

20-22 "C to 26-28 "C and then decreasing it to 20-22 "C, over a 30-min 

cycle (Loosanoff and Davis 1963; Castagna and Kraeuter 1981). Other factors such as the presence of food 

(Breese and Robinson 1981), sperm infusions (Loosanoff and Davis 1963), 

and weak injections of serotonin (Gibbons and Castagna 1985) trigger hard 

clams to spawn. 

Porter (1964) suggested that differences in spawning temperatures of 

hard clams were expressions of racial 

differences or phenotypic responses to 

environmental factors (Table 2). Keck 

et al. (1975) observed that gonad 

developmental patterns for clams in 

Delaware were intermediate between 

those for clams in Long Island Sound 

and in North Carolina, and thereby 

provided evidence that different 

physiological races exist in these 

three areas. 

Fecundity and Eggs 

Estimates of fecundity of 

hard clam vary. Belding (1931) 

ported that hard clams averaging 

the 

65:; 

mm in shell length produced about 2 

million eggs in a spawning season. 

Bricelj and Malouf (1980) observed 

induced spawning in hard clams of 

similar sizes to produce an average of 

6.3 million eggs over a spawning 

season. These fecundity estimates are 

lower than the average fecundities 

(about 25 million eggs) reported by & 

Davis and Chanley (1956). Some of the 

discrepancies in these estimates may 

be explained by differences in clam 

size and condition. Ansell and 

Loosmore (1963) detected a direct relationship between condition and 

spawning potential in the hard clam. 

Bricelj and Malouf (1980) found a general trend of increasing fecundity as 

shell size increased, some 15%-25% of 

the variance in total fecundity being 

accounted for by differences in shell 

length. Total fecundity varied 

significantly among the three commercial sizes of hard clams; number of 

eggs spawned were about equal in clams 

36.5-41.3 and ~41.3 mm shell length, 

and greater than the number spawned by 

clams 25.4-36.5 mm long (Bricelj and 

Malouf 1980). Other measures of relative fecundity (e.g., gonadal-somatic 

indices) varied significantly with 

shell length (Peterson 1983; Eversole 

et al. 1984). 

Spawning clams release eggs 

through the exhalent siphon. The 

6 

u 
. 

average diameter of the newly discharged eggs is 70-73 urn (Loosanoff 

and Davis 1963). The eggs are surrounded by a gelatinous envelope about 

25 urn thick which swells after contact 

with water to a thickness of 95 urn, 

resulting in a total diameter of about 

270 urn (Carriker 1961). Bricelj and 

Malouf (1980) observed that clams induced to spawn 

P 

reduced a wide range 

of egg sizes 50-97 pm), and that 

small eggs were most abundant late in 

the spawning season. Survival was 

much higher in large eggs cultured in 

the laboratory than smaller ones 

(Kraeuter et al. 1982). The gelatinous envelope provides buoyancy and 

enables water currents to carry eggs. 

Fertilization occurs in the water 

after actively swimming sperm come 

into contact with and penetrate the 

gelatinous envelope. To insure fertilization, numerous sperm must be 

available. Bricelj and Malouf (1980) 

calculated an optimum ratio of 1,800 

sperm to 1 egg for successful fertilization under culture conditions and it 

is likely that higher gamete ratios 

are required in nature. 

Fertilized eggs develop rapidly; 

cleavage begins within 30 min at 27-30 

"C and after about 10 h a ciliated 

gastrula can be seen spinning within 

the gelatinous envelope (Belding 

1931). Continuous beating of the cilia 

tears the surrounding envelope, permitting the embryo to escape. The 

embryo immediately changes from a 

sphere to a pear-shaped form called a 

trochophore, which is a nonshelled 

planktonic stage (Carriker 1961). 

Larvae 

The hard clam has two nonshelled 

larval stages, trochophore and early 

veliger (Carriker 1961). During these 

two larval stages, the shell gland and 

mouth develop and feeding begins. 

Both nonshelled larval stages propel 

themselves with a ciliated velum 

(Loosanoff and Davis 1950). At about 

1 day of age, the organism enters the 

first shelled larval stage, called a 

straight-hinged veliger, or sometimes 

the prodissoconch I stage; it has a 

smooth shell secreted by the shell 

gland (Eyster and Morse 1984). Valves 

of this shell range from 90 to 140 urn 

in length and are slightly asymmetric 

(Carriker 1961). After about 3 days, 

the second shelled larval stage - the 

umboned veliger or prodissoconch II 

stage - begins. Minute striae appear 

on the shell surface, and a smooth arc 

appears near the hinge line at the 

point where the umbo forms (Carriker 

1961). Age and shell length of umboned veligers are 3-20 days and 140- 

220 urn (Carriker 1961). The velum is 

well developed in both stages, enabling the veligers to swim well 

enough to move 7-8 cm/min (Mileikovsky 

1973) and maintain themselves in the 

water column. Vertical distribution 

of the larvae in the water column is 

uniform at night and concentrated 

about 1 m below the surface during the 

day - possibly to keep larvae away 

from bottom-dwelling predators (Carriker 1952). Larvae may be stimulated 

to rise in the water column by turbulence resulting from water currents 

and waves (Carriker 1961). The horizontal distribution of larvae is 

patchy and is influenced by water 

movement and the temporal aspect of 

spawning (Carriker 1961). During the 

spawning seasons, larval hard clams 

are important members of the zooplankton community, reaching densities 

that exceed 500 larvae/l (Carriker 

1961). 

The next stage, near the end of 

planktonic life, is the pediveliger. 

At sometime after they are 6-20 days 

old and 170-220 pm long, clams develop 

a strong ciliated foot, but maintain 

use of the velum (Carriker 1954, 

1961). The presence of both of these 

locomotor organs allows the clam to 

crawl, examine the substrate, and swim 

to another area. The pediveliger 

stage terminates when the velum is 

lost and the clam takes up a benthic 

existence; locomotion is then limited 

to crawling. 

7 
Plantigrade (Dissoconch) Stages 

Initially, the pediveliger metamorphoses into a byssal plantigrade 

that attaches to the substrate by 

means of a byssus (Carriker 1961). 

When larvae settle, they are commonly 

called spat and the stage is referred 

to as the setting or spatting stage. 

Byssal plantigrades are active during 

this period, breaking byssal attachments and crawling on or very close to 

the substrate surface to other attachment sites. Characteristics of this 

stage are a well-developed foot with a 

byssus gland (Belding 1912), the 

active development of siphons and 

fusion of the mantle, and the deposition of conspicuous concentric shell 

ridges (Carriker 1961). The byssal 

plantigrade develops into the juvenile 

plantigrade stage at a shell length of 

about 7-9 mm (Belding 1912; Carriker 

1961). The foot shortens and the 

byssus gland becomes nonfunctional. 

Juvenile plantigrades maintain contact 

with water by means of two completely 

developed siphons, and hold their position in the substrate with a stout 

hatchet-shaped foot. Movement decreases as the clam grows, its siphons 

elongate, and it burrows deeper 

in the substrate. 

The hard clam shows gregarious 

setting behavior (Keck et al. 1972). 

Clams often set along edges of sandbars or channels where differentials 

water 

Z59). 

current occur (Carriker 

Concentrations of clams in 

these areas may be more of a mechanical sorting process than the selection 

of a site (Moulton and Coffin 1954; 

Carriker 1959). Keck et al. (1974) 

demonstrated in laboratory studies 

that clams preferred sand as a setting 

substrate rather than mud. They 

hypothesized that the organic matter 

(with its associated bacteria) was 

responsible for reduced setting in the 

mud substrate. Sand substrates 

treated with clam liquor resulted in 

higher sets than sand did without 

liquor (Keck et al. 1974). Presence 

of a pheromone on an appropriate substrate, possibly released by early 

setting larvae, may provide the necessary cue for metamorphosing larva to 

set nearby and lead to the aggregated 

distribution of clams. 

Predation is an important factor 

in influencing the density and distribution of clams. Heavy sets of clams 

(e.g., 125/m*) are often reduced to 

negligible quantities in areas without 

protection from predators (Carriker 

1959, 1961). Bottoms with shell and 

subtidal grasses appear to have better 

conditions for spat survival than unstructured bottoms - probably because 

these bottoms offer some protection 

from predators, or because fewer 

predators are present (Kerswill 1941; 

Wells 1957b; MacKenzie 1977; Peterson 

1982). Experimental areas with calico 

scallop shells had significantly 

greater numbers of juvenile hard clams 

than control areas (Parker 1975). Experimental exclusion of predators by 

caging illustrated that in unvegetated 

areas survival was higher in the 

absence of predation (Peterson 1982). 

The use of crushed stone, pens, and 

traps to protect clams from predators 

and reduce their impact on clam mariculture has been successfully demonstrated by Castagna ar?d Kraeuter 

[iiN;{ and Kraeuter and Castagna 

. 

&. 

Adults 

Hard clams live in the substrate 

with the long shell axis 25-45" from 

vertical (Stanley 1970). Mean depths 

of clams average 2 cm in sand and 1 cm 

in mud (Pratt and Campbell 1956); 

smaller clams burrow deeper than large 

clams (Stanley 1970). I have noticed 

that very large clams lie on their 

side at the surface of firm substrates 

such as oyster bars. Horizontal move- . 

ment of adult clams is limited and the 

distance traveled is generally correlated with the size of clams, the & 

smaller clams being the more active 

(Chestnut 1952). Juvenile clams seemingly are able to change their habitat 

U 

8 
or correct for displacement caused by 

disturbances (e.g., wave action). The 

hard clam is a moderately rapid 

burrower (Stanley 1970) and can generally escape from a 15-50 cm burial 

with native sediment (Kranz 1974). 

Vertical movement of approximately 44 

cm/h has been recorded (Kranz 1974). 

Burial in sediment different from 

native sediment (e.g., spoilage from 

dredging) radically diminishes the 

clam's ability to escape. 

The spatial distribution of hard 

clams is aggregated and explained by a 

negative binomial distribution (Saila 

and Gaucher 1966; Anderson et al. 

1978). Clams also occur at higher 

densities in certain bottom types. 

Densities are highest in sandy bottoms 

containing shell (Pratt 1953; Wells 

1957b; Anderson et al. 1978; Walker et 

al. 1980; Walker and Rawson 1985). 

The clams are usually found in the 

intertidal and subtidal zones of estuaries and protected bays at depths 

less than 4 m (Godwin 1967, 1968b; 

Anderson et al. 1978; Walker and 

Rawson 1985). Hard clams in the South 

Atlantic region occur in beds along 

the outer edges of highly saline 

bodies of water (Godwin 1968b; Walker 

et al. 1980). Unlike hard clam populations in northern locations, clams 

are abundant (densities up to 101/m*) 

small tidal creeks 

IEose with a prominent 

especially 

dyster bar at 

the mouth (Walker et al. 1980). 

GROWTH CHARACTERISTICS 

Relation of shell height 

(dorso-ventral axis), shell width 

(lateral axis), and the cube root of 

clam weight to shell length 

(anterior-posterior axis) is linear; 

thus, there are no changes in proportions with growth (Haskin 1950). 

Shell growth of hard clams is greater 

in the first year after metamorphosis 

than in succeeding years (Haskin 1950, 

1952; Menzel 1963). The average 

length of hard clam seed, 3 mm long 

when planted in Alligator Harbor, 

Florida, was 28.1 mm after the first 

year of growth, 49.6 mm after the 

second, and 61.5 mm after the third 

(Menzel 1963). Monthly shell length 

increments averaged 2.47, 1.42 and 

1.08 mm for the first, second, and 

third years of growth, respectively 

(Menzel 1963). In South Carolina, 

incremental growth declined similarly 

in hard clam seed planted at a greater 

shell length (13 mm) over a 3-year 

experimental period (Eldridge et al. 

1979). Decreased incremental growth 

with increased shell length has been 

observed in the hard clam throughout 

its range (e.g., Belding 1931; 

Chestnut 1952; Gustafson 1955; Pratt 

and Campbell 1956; Crane et al. 1975). 

Growth of hard clams also appears to 

decrease with increasing age (Eversole 

et al. 1986). Older and slower growing clams thicken at the margin of the 

shell and become blunt as progressively more calcium is deposited (Belding 

1931). Little incremental shell 

growth can be detected in these older 

clams, which are referred to as 

blunts. When two clams about 3 years 

old and 49 and 58 mm long were marked 

with a file between the years 1947 and 

1951 and collected alive in 1980, they 

were only 87 and 99 mm long (Lutz and 

Haskin 1984). Annual growth in length 

had thus averaged about 17.8 mm for 

the first 3 years of life and 1.3 mm 

thereafter. Estimated ages of 36 and 

33 years for these two clams are the 

oldest reported for the hard clam 

(Lutz and Haskin 1984). 

Hard clams longer than 80 mm are 

common in unexploited populations such 

as that in Wassaw Sound, Georgia, 

where about 50% of the clams were reported to be >80 mm (Walker et al. 

1980). Walford plots for hard clams 

grown in the York River, Virginia, and 

Clark Sound, South Carolina, predicted 

that clams would cease growing at 

lengths of 80 and 70 mm, respectively 

(Loesch and Haven 1973; Ng et al. 

1982). These estimates of asymptotic 

sizes are not valid everywhere because 

9 
hard clams >lOO mm long have been reported in South Carolina and Georgia 

(Anderson et al. 1978; Walker et al. 

1980). 

Shell growth in hard clams is a 

dynamic process that is continually 

influenced by environmental, physiological, and genetic factors. Growth 

of veliger larvae of the hard clam is 

fastest at temperatures of 22.5- 

36.5 "C and salinities of 21.5-30.0 

ppt (Lough 1975). The temperaturesalinity conditions at which maximum 

growth occurs are somewhat higher than 

those at which survival is the highest 

(Lough 1975). Ansell (1968), who 

studied growth of adult hard clams 

throughout their geographical range 

from published literature, concluded 

that shell growth was fastest at 20 "C 

and stopped below 9 "C and above 31 

"C. Annual growth increases progressively from the northern portions to 

the southern extreme of the range 

distribution (Ansell 1968). The number 

of years required to reach a commercial length of 45 mm (average minimum 

legal size calculated from data in 

Ansell, 1968) is about 5.5 in Canada; 

4 in Maine, 3 in Massachusetts, Rhode 

Island and New York, 2 in South Carolina and Georgia, and less than 2 in 

Florida (Ansell 1968; Godwin 1968a; 

Menzel 1963; Eldridge et al. 1979). 

Ansell (1968) attributed these latitudinal differences in growth rate to 

differences in the length of the 

growing season, but stressed the 

importance of food availability within 

any range of water temperatures. 

Growth correlates well with available 

food (Pratt and Campbell 1956). Growth 

of hard clams in northern areas such 

as Canada and Maine is restricted 

principally to the summer months 

(Kerswill 1941; Gustafson 1955; Ansell 

1968). The growing season is longer 

in southern areas, and extends 

throughout the year in South Carolina 

(Eldridge et al. 1976, 1979). However, 

growth was fastest during spring and 

fall when water temperatures 

approached 20 "C (Menzel 1963; 

Eldridge et al. 1979). 

Hard clams store little food 

reserves (Ansell and Loosmore 1963). 

Growth and gonadal development in the 

hard clam therefore require continuous and substantial energy input 

(Ansell and Loosmore 1963; Eversole 

1986). When gonadal development is 

most active (e.g., during oocyte formation), competition for food reserves 

may intensify and decrease growth 

rate. Data on interrelationships 

between growth and the physiological 

state of hard clams are rare. 

Some variation in growth among 

hard clams may be attributed to genetic factors. In one generation, hard 

clams selected for fast growth were 

60% larger than wild stock after 15 

months (Chanley 1959). Natural hybridization of p1. mercenaria and M. campechiensis appears to occur where the 

suecies overlao (e.a.. Indian River. 

Florida). Growth "of hybrids in 

Gloucester Point, Virginia, and Beaufort. North Carolina, was considerably 

better than that of M. mercenaria 

(Chestnut et al. 195z Haven and 

Andrews 1957). M. campechiensis suffered high wintermortalities at these 

two locations, suggesting inability to 

withstand low temperatures. In more 

southerly areas, M. campechiensis survives the winter-and outperforms M. 

mercenaria; growth of hybrids is 

intermediate (Menzel 1964). Recommendations for hybridization programs 

provided by Menzel (1977) appear suitable to environmental conditions in 

the South Atlantic region. 

COMMERCIAL/RECREATIONAL FISHERIES 

Fisheries 

Three of four commercial grades 

of hard clams are harvested legally in 

the United States (Table 3). Littlenecks or "necks" are sold as part of 

the live shell trade and are the most 

expensive. Price per bushel of clams 

varies, but usually that of littlenecks averages 4.5 times and 2.7 times 

that of a bushel of chowders and 

10 
Table 3. Commercial hard clam categories (Anderson et al. 1978). 

Commercial Shell length 

grade (mm) 

Seeda (50 

Littlenecks 56-65 

Cherrystones 66-79 

Chowders >80 

aMinimum legal length varies by State 

along the Atlantic seaboard, but averages about 45 mm (Ansell 1968). 

cherrystones, respectively (Ritchie 

1977). Chowders, the largest and 

least valuable clams marketed, are 

frequently processed or made into 

chowder. 

Commercial harvesting of clams in 

the United States is dominated by four 

species (Table 1): hard clams, ocean 

quahogs (Arctica islandica), softshell 

clams (ruiya arenaXZ)?K!i surf clams 

(Spisula solidissima). Hard clams 

yield the highest dollar value of the 

commercial catch, producing 11.1% of 

the clam meats landed in United States 

(Table 1). The Middle Atlantic region 

(New York, New Jersey, and Delaware) 

has long been the leading producer of 

hard clam meats (U.S. Department of 

Commerce 1984). Production of meats 

in the South Atlantic region (North 

Carolina, South Carolina, Georgia, and 

Florida east coast) was less than that 

in the New England region, similar to 

that in the Chesapeake region, but 

more than that in the Pacific region 

(U.S. Department of Commerce 1984). In 

the South Atlantic region, North 

Carolina is the leading producer. 

Production throughout the South Atlantic region has increased over the last 

6-10 years (Table 4), partly in 

response to greater demand, higher 

value, and the evolution and use of 

mechanized 

(Rhodes et al. 

harvesting equipment 

1977; Guthrie and Lewis 

1982). Official harvest statistics of 

hard clams are underestimated, probably because the industry is diffuse, 

Table 4. Mean annual commercial landings of hard clams (1,000 kg meats) and the 

percentage of the harvest in the South Atlantic region (calculations based on 

data from volumes of the U.S. Department of the Interior's and the U.S. Department of Commerce's Fishery Statistics of the United States, various years). --~ 

1928-32 1950-54 1973-77 1979-83a 

State 1,000 kg % 1,000 kg % 1,000 kg % 1,000 kg % 

North Carolina 146.8 92.7 280.5 97.5 181.5 61.1 681.7 78.8 

South Carolina 0.9 0.6 4.9 1.7 67.5 22.7 145.5 16.8 

Georgiab 0.9 0.6 0.0 0.0 1.5 0.5 1.6 0.2 

Florida 9.7 6.1 2.3 0.8 46.8 15.7 36.5 4.2 

Total 158.3 287.7 297.3 865.3 

$nofficial statistics. 

East coast of Florida only. 

11 
and recreational harvests are not included in the landings. 

There are no comprehensive 

records of recreational harvesting. A 

creel-census-type survey of the recreational hard clam fishery in Great 

South Bay, New York, indicated that 

about 2,000 persons harvested 2,200 kg 

(4,800 bushels) of clam meats in 1977 

(Fox 1981). Estimates of recreational 

harvest range from less than 1% to 

about 25% of the commercial harvest 

(Conrad 1979; Fox 1981). The impact 

of the recreational harvest on hard 

clam resources is undetermined, and is 

probably less significant in the South 

Atlantic region than in the New 

England and Middle Atlantic regions, 

where recreational clamming is very 

popular. Recreational clamming is 

administrated by state governments in 

the South Atlantic region and is less 

restricted than the administration of 

shellfish grounds in either the New 

England or Middle Atlantic regions. 

For example, for recreational harvesting in South Carolina, no license is 

required, and clams may be harvested 

in season from state and public shellfish grounds throughout the state 

(Moore 1979). 

Traditionally, most clams have 

been harvested with rakes or tongs. 

In North Carolina, a unique method of 

legally harvesting clams called "kicking" is practiced. Guthrie and Lewis 

(1982) outlined the evolution of this 

technique. Essentially, in clam kicking, the wash from the boat's propeller dislodges clams, shells, and other 

objects from the substrate. An otter 

trawl, or similar net, is pulled 

behind the vessel to collect the 

clams. Clam kicking is a very effective fishing method and a topic of 

some controversy in North Carolina 

(Hart 1982). Clams harvested by 

kickinq are listed as taken by trawl 

in Fiihery Statistics of J&- United 

States (U.S. Department of Commerce). 

The use of hydraulic escalator 

harvesters has significantly increased 

hard clam production in South 

Carolina. Average annual meat yield 

increased from 37,050 kg in 1971-73, 

before the escalator harvester was 

introduced, to 95,400 kg in 1974-76 

after its use became legal in South 

Carolina (Rhodes et al. 1977). Mechanization of clam harvesting equipment 

appears more acceptable to administrative and legislative bodies in the 

South Atlantic region than in other 

fishery regions. 

Population Dynamics 

In South Carolina, the hard clam 

begins spawning during the second year 

of life at a length of about 25-30 mm 

(Eversole et al. 1980). Females can 

spawn millions of eggs twice a year 

for approximately 2 years before they 

reach commercial size. The fecundity 

of hard clams is tremendous, but so is 

preadult mortality. Larval hard clams 

are abundant in the plankton, reaching 

maximum population densities of 672/l 

in Little Egg Harbor, New Jersey 

(Carriker 1961). Larvae survive in 

sufficient numbers to provide thousands of spat per square meter, yet 

the highest density of spat recorded 

by Carriker (1961) in the Little Egg 

Harbor was 125/m*. Hard clam larvae 

are carried and concentrated by water 

currents and in some areas set at 

densities up to 270,000/m2 (Dow and 

Wallace 1955). However, many larvae 

are lost from the system, eaten by 

filter feeders and predatory zooplankton, or set in unsuitable substrate. 

Mortality of spat and seed clams 

is often many times higher than that 

of adults. Mortality reached 100% in 

experimental plantings of hard clam 

seed in unprotected plots in Florida 

and Georgia (Menzel and Sims 1964; 

Godwin 1968a). Mortalities among hard 

clams averaging 20.0 mm shell length 

were significantly lower than among 

those averaging 10.5 mm length when 

they were exposed to crab predation 

(Whetstone and Eversole 1981). Clams 

that survive and grow become less vulnerable to predation, probably because 

12 
the shell thickens and predators are 

less able to open, crush, or bore into 

larger clams (Carriker 1959; MacKenzie 

1977; Whetstone and Eversole 1978). 

Whetstone and Eversole (1978) observed 

an inverse relationship between percentage mortality and shell length of 

hard clams grown in experimental culture units in South Carolina. Data 

also indicated that predators selected 

smaller clams (Whetstone and Eversole 

1978). The degree of predation on 

small clams ( (20 mm long) often 

determines their relative abundance in 

a habitat (MacKenzie 1977). Seed 

clams appear to suffer significant 

mortalities in summer when predators 

are active and abundant (Whetstone ard 

Eversole 1.978). 

Mortality of hard clams in the 

absence of predation appears low. Annual mortality rates of 1.43% and 

4.06% have been calculated for hard 

clams grown in trays, protected 

against predation, in South Carolina 

(Eldridge and Eversole 1982; Eversole 

et al. 1986). Predation on large hard 

clams >50 mm long is very low 

(MacKenzie l.977). The natural mortality of the hard clam is expected to 

decrease after it becomes 50 mm long; 

however, fishing mortality may be 

extensive beyond this size. Crane et 

al. (1975) observed hard clam densities of 53/m2 to decrease to 1 to 2/m* 

after only 1 year of clamming. Some 

mortality among the smaller sublegal 

clams is associated with exploitation 

(e.g., breakage and burial). Dow 

(1953) estimated that these mortalities may reach 30% under some circumstances. The generalized survivorship 

curve of hard clams approximates the 

classical Type IV concave curve 

(Slobodkin 1962), where mortality is 

extremely high in the early life 

stages (larvae and spat) until the 

clams reach a certain shell size, and 

then mortality greatly decreases. 

Calculated estimates of the bioenergetics of a hard clam population 

on an intertidal mudflat in Southampton, England, indicated that of the 

annual food intake of 1,292 kcal/m2, 

71% was deposited as feces and pseudofeces (59%) or excreted (12%), and 29% 

was assimilated (Hibbert 1977). The 

largest proportion of the 374 kcal 

assimilated was used for respiration 

(65%) and approximately equal amounts 

were allocated to tissue growth (19%) 

and gamete production (16%). Hard 

clams contribute annually 884 kcal/m* 

to other trophic levels in Southampton 

waters (Hibbert 1977). The largest 

amount of energy leaving the hard clam 

population passed in the form of biodeposits to the decomposers. Net 

annual production of the Southampton, 

England, population was comparable to 

production values (3-8 g/m*, ash-free 

dry weight) reported for sites in 

Wassaw Sound, Georgia (Walker and 

Tenore 1984). 

ECOLOGICAL ROLE 

Feeding 

Suspension-feeding bivalves such 

as the hard clam obtain food by 

filtering suspended particulate matter 

and absorbing dissolved organics from 

the water. Water enters through the 

ventral inhalent siphon, passes 

through the gills to an exhalent cavity and out the dorsal exhalent siphon. Food particles suspended on the 

inhalent surface of the gills are 

sorted and passed to the gill edges 

and then moved anteriorly to the 

labial palps. Large particles are 

rejected from the gill and palp surfaces and periodically voided from the 

mantle cavity into the water. This 

rejected material is usually called 

pseudofeces. Spasmodic contractions 

of the adductor muscles act to force 

pseudofeces out through the inhalent 

siphon (Kellogg 1903). Food becomes 

entangled in a mucus strand and is 

passed to the mouth by cilia on the 

palps. Food particles passed through 

the digestive system of the hard clam 

are expelled by feeding currents from 

the exhalent siphon into the water as 

11 
compact rod-shaped fecal pellets, 1.4- 

4.2 mm long (Haven and Morales-Alamo 

1972). 

Larval and adult forms of hard 

clams are capable of selective feeding, regulating the quality and quantity of food ingested. Larvae of the 

hard clam offered a mixture of algal 

cells, selected the relatively larger 

cells of Chlamydomonas and rejected 

the cells of Por h ridium (Loosanoff 

and Davis 1963 )+ concentration of food cells exceeded an optimum 

level, larval clams rejected cells and 

their stomachs contained less food 

than did those of larvae kept at lower 

cell concentrations (Loosanoff and 

Davis 1963). Mortality was considerable among larvae exposed to high concentrations of food for long periods, 

apparently from clogged feeding apparatus and bacterial fouling (Guillard 

1959; Loosanoff and Davis 1963). In 

adult hard clams, feeding rate 

increases with increasing food concentrations to a maximum, followed by 

decreased feeding when food is further 

;;;;~ntrat~rowt(hTenore and Dunstan 

I . of hard clams 

correlates better with the presence or 

absence of particular algal species 

than with gross chemical or amino acid 

compositions of algal diets (Epifanio 

1979). Feeding in the hard clam appears well adapted to changing food 

levels and is sensitive to the algal 

species composition. Hard clams also 

eat suspended detritus and its associated bacteria, and absorb dissolved 

organic matter directly from the water 

to help meet their energy requirements 

(DiDomenico and Iverson 1977). 

Parasites and Disease 

Literature on clam parasites and 

disease is sparse, particularly in 

comparison with the literature available the American 

CrassostOrnea virginica. 

oyster, 

Few diseases 

of M. mercenaria have been identified. 

TubTash et al. (1965) described a 

bacillary necrosis in larvae of the 

hard clam that was probably caused by 

a species of Vibrio and Davis et al. 

(1954) described a fungal invasion of 

larval clams as that of Sirolpidium 

zoopthorum. In culture conditions, S. 

zoopthorum sometimes causes signifTcant mortality to both larvae and juveniles. 

Mercenaria mercenaria are infested by few fauna1 parasites. 

Uzmann (1955) reported the trematode 

Aimasthala quissetensis, pathogenic to 

humans, in hard clams in New York. 

Cake (1977) found heavv infestations 

of the larval cestode Tylocephalum 

sp., which reduces the quality of 

meats, in clams in the Gulf of Mexico. 

Styloccus ellipticus, a free-living 

turbellarian flatworm, has been found 

in hard clams in Virginia and may be 

parasitic (Andrews 1970). Other parasites include nemerteans, Malacobdella 

grossa (Coe 1943b); mudworms, Polydora 

websterii (Davis 1969); and parasitic 

copepods, 

cola spinosa, and M. 

1947; Humes 1954; -Humes and Cressey 

1960). Diagnosis and control of parasites and diseases in shellfish were 

reviewed by Sindermann (1974). 

Predators 

Predators of larval hard clams 

have not been identified; however, 

many zooplanktivores, as well as bottom dwelling filter feeders including 

adult clams, have been suggested as 

consumers of larvae (Belding 1931; 

Carriker 1961). The list of predators 

of the bottom-living hard clam over 

its geographical range is long (Table 

5). Abundance and importance of any 

particular predator varies among locations with the time of year. 

Crabs appear to be the major 

predators of the hard clam in the 

South Atlantic region. The blue crab. 

Callinectes sapidus, is probably the 

most destructive predator amona crabs: 

mud crabs and stone crabs prey-less on 

hard clams (M enzel and Sims 1964; 

Godwin 1968a; Whetstone and Eversole 

1978). Crabs, especially the mud 

14 
Table 5. Predators of hard clams. 

Comnon name Scientific name Sourcea 

Horseshoe crab Limulus polyphemus 

Snapping shrimp Alpheus heterochaelis, A. normanni 

American lobster Homarus americanus 

Blue crab Callinectes sapidus 

Stone crab 

Green crab 

Rock crab 

Mud crabs 

Calico crab 

Moon snails 

Oyster drills 

Whelks 

Atlantic murex 

Banded tulip 

Sea star 

Rays 

Flounders 

Tautog 

Puffer 

Herring gull 

Waterfowl 

Menippe mercenaria 

Carcinus maenas 

Cancer irroratus 

Eurypanopeus depressus, Neopanope sa_yJ, 

1. texana, Rhithrapanopeus harrissi, 

Panopeus herbstii 

Ovalipes ocellatus 

Polinices duplicatus, Lunatia heros 

Urosalpinx cinerea, Eupleura caudata 

Busycon carica, _ B. canaliculatum, 

B. contrarium 

Murex fulvescens 

Fasciolaria hunteria 

Asterias forbesi 

Dasyatis centrura, Gymnura micrura, 

Rhinoptera bonasus 

Paralichtys dentatus, 

Pseudopleuronectes americanus 

Tautoga onitis 

Sphaeroides maculatus 

Larus argentatus 

not identified 

1 ,u 

a 

V 

c,d,f,g,h, 

1 ,vw 

Y 

e,l ,t 

m 

e,h,k,l,m, 

Y 

h 

b,c,e,l,m,u 

c,e,g,h,j, 

1 ,m,u 

b,h,m,n,r 

W 

X 

b,m,s,u 

fmw 

h,m 

aSources: a-Beal 1983; b-Belding 1931; c, d, e-Carri ker 1951, 1959, 1961; 

f-Castagna and Kraeuter 1977; g-Chestnut 1951; h-Flagg and Malouf 1983; i-Hibbert 

1977; j-Kellogg 1903; k, l-Landers 1954, 1955; m-MacKenzie 1977; n-Megalhaes 

1948; o-Menzel and Sims 1964; p-Menzel et al. 1976; q-Nelson 1947; r-Peterson 

1982; s-Pratt and Campbell 1956; t-Ropes 1968; u-Ropes and Martin 1960; v-Saila 

and Pratt 1973; w, x-Wells 1958a,b; y-Whetstone and Eversole 1978. 

15 
crabs, usualTyteedn themhard clams (Whetstone and Eversole 

1978). Crab attacks on hard clams 

>50-60 mm long are less successful 

than those on smaller clams (MacKenzie 

1977; Whetstone and Eversole 1978; 

Walker et al. 1980). Whetstone and 

Eversole (1981) observed that the percentage of mud crabs, Panopeus herbstii, that opened clams increased as 

crab size increased. Large crabs also 

were more successful in opening larger 

clams. Hard clams >35 mm long were 

not opened by any size of mud crab 

tested (lo-40 mm carapace width). Mud 

crabs are an important source of mortality to hard clams because they are 

ubiquitous and abundant throughout the 

of the clam (Carriker 1961; 

~~~~~tone and Eversole 1978). 

Gastropods of the genus Busycon 

can open hard clams 20-75 mm long at 

the rate of about one per week 

(Carriker 1951). Busycon preys preferentially on the larger size classes 

of hard clams (Peterson 1982). Knobbed 

whelks, (!B. carica) and .lightning 

. contrarlum) have similar 

feeding behaviors. These whelks grasp 

the clam shell with their muscular 

foot in such a way that when the columellar muscle contracts, the whelk 

shell crashes against the ventral edge 

of the clam shell. When enough of the 

clam shell is chipped away, the whelk 

either wedges open the shell or 

inserts its proboscis into the shell 

and the clam's soft parts (Megalhaes 

1948; Carriker 1951). The chipping 

behavior characteristic of these 

species appears to be poorly developed 

in B. canaliculatum,' which usuaily 

inserts its shell between the valves 

of an unsuspecting clam and wedges it 

open (Carriker 1951). Other carnivorous gastropods (Urosal inx, Eupleura 

Polinices,~ and * Lunatia use -the-?; 

radulae to rasp holes in the shell to 

gain entrance to the soft parts of the 

clam (Carriker 1981). 

It is not known how detrimental 

other species of predators are to hard 

clams in the South Atlantic region. 

Starfish appear to be tiportant preda-p 

tors in the Northeast (MacKenzie 1977) 

and rays cause significant mortality 

to clams in the Chesapeake Bay area 

(Castagna and Kraeuter 1977). 

Predation can substantially reduce clam abundance. <- MorX?Tities 

reached 100% in experimental plantings 

of hard clam seed in Florida and 

Georgia (Menzel and Sims 1964; Godwin 

1968a). Over 90% of the losses at 

these two locations were due to predation by blue crabs. An increase in 

recruitment of hard clams in Great 

South Bay, New York, was linked to the 

decline of biue crabs after a severe 

winter (Greene and Becker 1978). 

Densities of hard clams increased 

seven- to eight-fold after predator 

numbers were reduced by a single 

application of a pesticide (MacKenzie 

1977). Peterson (1982) demonstrated 

by exclusion experiments that hard 

clam survival is high in the absence 

of predators. Predation appears to be 

the most important biotic factor in 

limiting hard clam populations 

(Virstein 1977; MacKenzie 1979). 

ENVIRONMENTAL tx@mmENTs 

Temperature 

Temperature has been considered 

the most important factor in determining time of spawning, because a 

certain degree of gonad ripeness or 

maturation must be attained before 

hard clams can respond to specific 

spawning stimuli (Loosanoff 1937b: 

Ansell et al. 1964). Once an appropriate level of ripeness is reached, a 

critical temperature level or increase 

may trigger spawning (Loosanoff and 

Davis ~1963). Gonadal_ develo_pment 

appears to begin at 10 "C and spawning 

occurs between 16 and 30 "C (Table 2). 

Because water temperatures in the 

South Atlantic region are relatively 

high, gametogenesis can occur year 

around and spawning occurs from early 

spring to late fall (Porter 1964; 

Q& 
Eversole et al. 1980; Manzi et al. 

1984; Pline 1984). 

Hard clams were cultured from the 

egg to the spat stage at constant 

temperatures ranging from 18 to 30 OC 

(Loosanoff et al. 1951). Larvae 

ingested food at temperatures as low 

as 10 OC, but did not grow (Davis and 

Calabrese 1964); minimum temperature 

for larval growth was 12.5 OC. Growth 

was generally more rapid at higher 

temperatures, peaking at 25-30 

"C (Davis and Calabrese 1964). 

Abnormal larval development and heavy 

mortality occurred at temperatures 

above 33 "C (Loosanoff and Davis 

1963). Response surface estimations 

(the response surface technique is a 

statistical method for determining the 

maximum biological response to more 

than one independent variable) of the 

temperatures and salinities yielding 

maximum larval growth and survival 

were 21.5-33.0 "C and 22-31 ppt (Lough 

1975). In Little Egg Harbor, New 

Jersey, larval hard clams grew to 

setting size in about 8 days at water 

temperatures of .23.4-26.2 "C (Carriker 

1961). High temperatures had more 

effect on the growth and survival of 

developing embryos than those of 

straight-hinged larvae (Kennedy et al. 

1974; Lough 1975). 

Ansell (1968), who summarized the 

growth rates of the hard clam over its 

geographical range, found that growth 

was optimal at 20 "C and ceased at 9 

and 31 "C. Loosanoff (1939) demonstrated that hard clams became progressively less active as temperature 

decreased to 9 "C and became inactive 

at about 4 "C. Water temperatures 

rarely reach 4 "C and almost never remain at this low temperature for long 

periods in the South Atlantic region - 

one reason why hard clams grow there 

throughout the year (Menzel 1963, 

1964; Godwin 1968a; Eldridge et al. 

1976, 1979). Shell growth (length) is 

fastest in spring and fall, slower in 

winter, and the slowest in summer, 

when water temperatures often exceed 

30-33 "C (Menzel 1963, 1964). Kennish 

and Olsson (1975) observed decreased 

growth in hard clams when thermal effluent was discharged in the summer 

from a power plant in Barnegat Bay, 

New Jersey. Hatchery-reared hard 

clams raised in warm-water upwelling 

systems in tropical St. Croix, Virgin 

Islands, grew little and nearly all 

died (Sunderlin et al. 1975). The 

upper lethal temperature of the hard 

clam is 45.2 'C (Henderson 1929); however, temperatures above 30 "C adversely affect its behavior and physiology (Hamwi 1968; Savage 1976; Van 

Winkle et al. 1976). Adult hard clams 

survive below-freezing temperatures to 

-6 Y and succumb to low temperatures 

only when a majority (64%) of the 

tissue freezes (Williams 1970). Hard 

clams covered by flowing water or 

sediment survive low temperatures 

better than those exposed in intertidal areas (Dow and Wallace 1951). 

Salinity 

The hard clam is an osmoconformer 

and moderately euryhaline. It has 

been found growing in waters of 4 to 

over 35 ppt salinity (Chestnut 1951; 

Wells 1957b, 1961; Godwin 1968b; 

Anderson et al. 1978), but growth is 

optimal at 24-28 ppt (Turner 1953; 

Chanley 1958). Native clam beds are 

known to occur at salinities of lo-28 

ppt in North Carolina, 4-35 ppt in 

South Carolina, and 18-35 ppt in 

Georgia (Chestnut 1951; Godwin 1968b; 

Anderson et al. 1978). Minimum 

salinity conducive to favorable hard 

clam growth and survival is 12.5 ppt 

(Castagna and Chanley 1973). Hard 

clams can close their shells tightly 

during periods of stress, as when 

freshets occur, and respire anaerobically (Lutz and Rhoads 1977). Mortality was less than 5% in hard clams in 

the Santee River system, South Carolina, exposed to low salinities ((10 

ppt,) during 2- and 3-week periods 

while mortality in oysters averaged 

about 50% (Burrell 1977). The lower 

hard clam mortalities were attributed 

to their ability to remain closed 

longer than oysters. Hard clams cease 

17 
pumping at salinities below 15 ppt and 

above 40 ppt (Hamwi 1968). 

Eggs of hard clams developed into 

normal straight-hinged larvae at 

salinities of 20.0-32.5 ppt (Davis 

1958), but larvae did not metamorphose 

to byssal 

salinities 

plantigra$s5 $p;t) at 

below 

(Castagna and Chanley 1973): 

PPt 

Larvae 

appeared less tolerant of low salinities than adults: minimum salinity 

for survival was about 12.5 ppt for 

adults and 15.0-17.5 ppt for larvae 

(Davis 1958; Castagna and Chanley 

1973). Similarly, eggs require higher 

salinities than larvae for development: no eggs developed at 17.5 ppt 

salinity and only 16%-21% at 20 ppt 

(Davis 1958). Optimum salinity for 

egg development and larval growth and 

survival is 26-27 ppt (Davis 1958; 

Davis and Calabrese 1964; Castagna and 

Chanley 1973). 

The range of temperatures 

tolerated by larval hard clams was 

reduced considerably when salinity was 

reduced. Lough (1975) reevaluated the 

combined effects of temperature and 

salinity, using response surface tech- 

. and noted that some of the 

$~fqf~r?nces between studies may be due 

to the temperature-salinity interaction. 

Water Quality 

Dissolved oxygen (DO) concentrations of 6.8-7.4 mg/l are recommended 

for successful culture of the hard 

clam (Hartman et al. 1974). Morrison 

(1971) reported that eggs developed at 

0.5 mg/l DO, but mortality was high 

and growth was nonexistent at levels 

less than 0.3 mg/l. He observed that 

prolonged exposure to low DO lengthened the larval life stage and 

decreased the probability of survival. 

Growth rate of larvae returned to 

normal and metamorphosis proceeded 

when larvae were returned to higher 

concentrations of DO. Larval growth 

and metamorphosis appeared normal at 

DO levels higher than 4.1 mg/l 

(Morrison 1971). Hourly oxygen consumption by larvae was about 4.5-4.8 

ml/g of dry weight (Marinucci 1975). 

Adult hard clams encounter a wide 

range of DO concentrations and have 

apparently evolved several metabolic 

mechanisms to handle conditions such 

as anoxia (Greenfield and Crenshaw 

1981). Hamwi (1969) observed that 

hard clams decreased oxygen consumption as DO concentrations decreased 

below 5 mg/l. Hard clams incurred an 

oxygen debt when deprived of oxygen 

(Hamwi 1969). Little correlation was 

observed between the growth of hard 

clams and DO concentrations in Narragansett Bay, Rhode Island (Pratt and 

Campbell 1956). Hard clams exposed to 

oxygen-impoverished conditions (~1 

mg/l) for up to 3 weeks maintained the 

ability to burrow (Savage 1976). They 

appear not to be severely affected by 

low DO, and associated stresses apparently had no long-lasting effect 

(Savage 1976). 

The hard clam usually lives in 

well-buffered areas; however, pH may 

decrease below 7.0 in tide pools and 

estuaries with poor circulation, heavy 

siltation, pollution, and hydrogen 

sulfide production. In laboratory 

experiments, Calabrese and Davis 

(1966) demonstrated that embryos of 

hard clams developed normally over a 

pH range of 7.0-8.75. The range for' 

normal larval growth and survival was 

6.75-8.50 and 7.25-8.75, respectively 

(Calabrese and Davis 1966). Hard clams 

appear to require that the pH not be 

below 7.0 nor above 9.0 for successful 

recruitment (Calabrese and Davis 1966; 

Calabrese 1972). 

In addition to the effect of reduced pH associated with high concentrations of silt, silt itself appears 

to be directly detrimental at high 

concentrations to eggs and larvae of 

the hard clam. Egg development was 

adversely affected by silt concentrations above 0.75 g/l, and no eggs 

developed normally at concentrations 

of 3 g/l or higher (Davis 1960). 

18 
Growth of larvae was retarded at l-2 

g/l and stopped at 3-4 g/l (Davis 

1960). Larvae are affected differently by various suspended-solid producing substances; e.g., larvae were 

more adversely affected by kaolin than 

by silicon dioxide (sand) of the same 

particle size (Davis and Hidu 1969). 

The particle size of silt is also an 

important factor in the survival of 

larvae. Hard clam larvae in the presence of a high concentration of 

smaller particles of kaolin eventually 

exhausted their sorting apparatus; as 

a result their stomachs became packed 

and they died (Davis and Hidu 1969). 

Eggs appear to tolerate higher levels 

of suspended solids than do larvae, 

which can tolerate higher levels than 

those normally encountered in natural 

waters (Davis and Hidu 1969). Although 

several hypotheses have been postulated, no clear relationship has been 

demonstrated between levels of 

suspended solids and the growth and 

survival of adults (Pratt and Campbell 

1956; Rhoads et al. 1975). Cabelli 

and Heffernan (1971) noted a marked 

reduction in the number of coliform 

bacteria in hard clams at higher 

levels of suspended solids. 

Hard clams tolerate wide ranges 

in different water quality variables 

such as ammonia, nitrite, nitrate, 

phosphates, and sulfur compounds. 

Epifanio and Srna (1975) reported that 

the 96-h median tolerance limit of the 

hard clam was 110-172 mg/l for ammonia 

and 1,863-1,955 mg/l for nitrite ion. 

Tolerance to nitrate and orthophosphate was so high that even clams cultured in effluents of secondarily 

treated domestic sewage would rarely 

be exposed to these levels. They 

concluded that hard clams are unlikely 

to be exposed to acute or chronic 

levels of ammonia, nitrite ion, 

nitrate ion, or orthophosphate in the 

natural environment. However, Calabrese (1972) showed that hydrogen 

sulfide production and its potential 

to reduce pH levels in some systems 

may have a negative effect on hard 

clams. 

Hard clam embryos and larvae 

exposed to a variety of pollutants and 

toxicants have a wide range of responses; however, among almost all the 

52 compounds tested, slowed shell 

growth was the first symptom of toxi- 

;ii& (Davis 1961; Davis and Hidu 

It was also apparent that the 

life 'stages of hard clams responded 

differently to toxicants; at some 

concentrations, larval growth was 

reduced significantly but embryonic 

development was little affected (Davis 

and Hidu 1969). Sublethal concentrations of many pollutants such as 

petroleum products have detrimental 

effects; hard clams exposed to only 

0.6 mg/l of the water-soluble fraction 

of Nigerian crude oil exhibited 

decreased feeding rates, use of food 

consumed, and growth (Keck et al. 

1978). Once exposed to a pollutant 

(e.g., petroleum hydrocarbons), hard 

clams retain a fraction of pollutant 

until it can be depurated. Persistence of the pollutant is related to 

duration of exposure and chemical 

qualities of the pollutant (Boehm and 

Quinn 1977). Ansell (1964b) observed 

that hard clams are relatively tolerant of pollution; however, organic 

pollution may include some microbes 

pathogenic to humans and toxicants 

that limit the commercial harvest of 

clams without extensive depuration. 

Water Current 

Water current that provides adequate circulation is essential for 

good growth and recruitment of hard 

clams. Water current performs several 

services: provision of food; maintenance of acceptable water quality; 

removal of biodeposits; and transportation of eggs and larvae (Belding 

1931). Kerswill (1949) observed that 

hard clams grew more rapidly in areas 

with substantial flow (7.5 cm/set) 

than in areas with little water circulation. He attributed this better 

growth to the increased water circulation and hence increased food availability. One kg of seed clams in an 

upflow nursery system utilizing a 

19 
vertical flow of water increased its 

biomass 126% during the fall at a flow 

rate of 15 l/min, compared with 213% 

increase at a flow of 29 l/min (Manzi 

et al. 1986). Growth was positively 

correlated with flow rate in this culture system during most of the year. 

Strong tidal currents, however, may 

scour the bottom and displace clams 

(Wells 1957b). 

Saila et al. (1967), after studying several environmental factors in 

an attempt to explain the distribution 

of hard clams, concluded that current, 

vegetation, predation, and sediment 

properties all affect clam distribution. Abundance of the larvae is 

uneven; densities are highest in the 

central basin of Little Egg Harbor, 

New Jersey, and away from the inlet, 

where tidal exchanges diluted larval 

density (Carriker 1961). In Orr's 

Cove, Maine, the abundance of hard 

clam larvae was highest 3 h after high 

tide; lowest abundance was near low 

tide (Moulton and Coffin 1954). Hard 

clams appear to set where differentials in water current exist; larval 

densities may be high there because of 

mechanical sorting and concentration 

by water currents (Moulton and Coffin 

1954; Carriker 1961). 

Substrate 

Substrate type and the degree of 

sorting among the sediments are determined in part by water current (Newell 

1970). Obviously, the interaction 

confounds any conclusions as to clam 

distribution, growth, and survival 

being related to substrate type. 

Sediment appears to be an important factor influencing setting of 

hard clam larvae. Keck et al. (1974) 

observed in the laboratory that hard 

clams preferred to set in sand rather 

than in mud; 2,083 clams set in 500~urn 

sand compared to only 781 in 50-pm 

mud. Carriker (1959) recommended firm 

substrate free of organic mud for 

optimal setting of larval hard clams 

in culture. Adult hard clams occur at 

highest densities in sandy bottoms 

with shell (Pratt 1953; Wells 1957b; 

Anderson et al. 1978; Walker et al. 

1980). In Georgia, averages of 22 hard 

clams/m2 were found in sandy bottoms 

with shell, 12/m2 in sandy bottoms, 

and 3/m2 in mud bottoms (Walker et al. 

1980). Although sandy bottom with 

shell is not the dominant substrate 

type in South Carolina, it was the 

source of 68% of the total hard clams 

collected and supported the highest 

density of clams recorded during a 

statewide survey of the resource 

(Anderson et al. 1978). When 

hatchery-reared seed clams were 

planted in South Carolina, survival 

was highest in substrates with the 

greatest fraction of shell (Eldridge 

et al. 1976). Pratt and Campbell 

(1956) observed less shell growth of 

hard clams in sediments with a high 

silt-clay content. Hard clams planted 

in sand grew faster than clams planted 

in mud (Pratt 1953) and a significant 

correlation existed between clam shell 

size and particle size of the substrate (Johnson 1977). 

Ecosystem Alteration 

Major alterations of the ecosystem, such as the rediversion of 80% of 

the flow from the Cooper River to the 

Santee River, South Carolina, are 

expected to affect the hard clam 

resource (Kjerfve and Greer 1978). 

About 80% of the commercial clam harvest in South Carolina came from the 

Santee River (Rhodes et al. 1977). 

Rediversion is expected to reduce 

salinity to such low levels that it 

may destroy this clam resource 

(Kjerfve and Greer 1978). Water is 

being rediverted from the Cooper River 

to help alleviate the problem of 

shoaling in Charleston Harbor and the 

consequent need for continued dredging. Dredging itself may reduce clam 

numbers in Charleston Harbor and the 

Atlantic Intercoastal Waterway. 

Average densities decreased from 7.5- 

12.1 to 0.3-2.9 clams/m2 after a navigation channel was dredged through a 

lagoon on Long Island, New York 

711 
(Kaplan et al. 1974). Hard clams not 

exposed to the mechanical disturbance 

of dredging exhibited little immediate 

mortality; however, some delayed mortality, possibly due to dredging, 

occurred in adjacent clam beds (Kaplan 

et al. 1974). Burial in sediment different from the clam's native sediment, such as some dredged materials, 

radically reduces a clam's ability to 

escape (Kranz 1974). Disposal areas 

resulting from dredging may also contribute to the loss of valuable habitat for hard clams. 

An annotated bibliography with 

over 2,200 cross-referenced titles on 

the hard clam was prepared by McHugh 

et al. (1982). This bibliography will 

help save time in searching past 

literature and in checking details of 

publications cited in this species 

profile. 

21 

LITERATURE CITED 

Abbott, R.T. 1974. American seashells. Van Nostrand Reinhold Co., 

New York. 663 pp. 

Anderson, W.D., W.J. Keith, F.H. 

Mills, M.E. Bailey, and J.L. 

Steimeyer. 1978. A survey of South 

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S.C. Wildl. Mar. Resour. Dep., Mar. 

Resour. Cent., Tech. Rep. 32. 17 pp. 

Andrews, J.D. 1970. The mollusc 

fisheries of Chesapeake Bay (USA). 

Proc. Symposium Mollusca, Mar. Biol. 

Assoc. India, 111:847-856. 

Ansell, A.D. 1964a. Venus mercenaria oyster spat in Wellfleet Bay. 

in Southampton water. Ecology 44: Wright and Potter Printing Co., 

396-397. Boston, Mass. 134 pp. 

Ansell, A.D. 1964b. Experiments in 

mollusc husbandry. Fish. News Int. 

3:216-219. 

Ansell, A.D. 1968. The rate of 

growth of the hard clam Mercenaria 

mercenaria (L.) throughout the geographical range. J. Cons. Perm. 

Int. Explor. Mer 31:364-409. 

Ansell, A.D., and F.A. Loosmore. 

1963. Preliminary observations on 

the relationship between growth, 

spawning and condition in experimental colonies of Venus mercenaria L. 

J'. Cons. Perm. Int. Explor. Mer 28: 

285-294. 

Ansell, A.D., K.F. Lander, J. Bricelj, V.M., and R.E. Malouf. 1980. 

Coughlan, and F.A. Loosmore. 1964. Aspects of reproduction of hard 

Studies on the hard-shell clam, clams (Mercenaria mercenaria) in 

Venus mercenaria, in British waters. Great South Bay, New York. Proc. 

I. Growth and reproduction in Natl. Shellfish. Assoc. 70:216-229. 

natural and experimental colonies. 

J. Appl. Ecol. 1:63-82. 

Beal, B.F. 1983. Predation of 

juveniles of the hard clam Mercenaria mercenaria (Linne) by the 

snapping shrimp Al heus 

chaelis Say and + pheus normanni =: 

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Belding, D.L. 1912. A report upon 

the quahaug and oyster fisheries of 

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histor.y, growth and cultivation of 

the quahaug (Venus mercenaria), and 

observations on the set of the 

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Mass. Dep. Conserv., Div. Fish. 

Game, Mar. Serv. 2. 41 pp. 

Boehm, P.D., and J.G. Quinn. 1977. 

The persistence of chronically accumulated hydrocarbons in the hard 

shell clam Mercenaria mercenaria. 

Mar. Biol. (Berl.) 44:227-233. 

Breese, W.P., and A. Robinson. 1981. 

Razor clams, Siliqua patula (Dixon): 

gonadal development, induced spawning and larval rearing. Aquaculture 

22:27-33. 

23 
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spring of 1975. Proc. Natl. 

Shellfish. Assoc. 67:35-43. 

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Calabrese, A., and H.C. Davis. 1966. 

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69-74. 

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24 
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27 
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28 
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33 
SO272 -101 

REPORT DOCUMENTATION 1. “ECOIIT ~0. 2 3. mwJi.nrs Accnslon No. 

PAGE Biological Report 82(11.75)* P 

~ 4. lit* l nd 8ubutl~ 5. Rwoon Da1a 

I Species Profiles: Life Histories and Environmental Requirements of August 1987 

i Coastal Fishes and Invertebrates (South Atlantic) -- Hard Clam & 

I 

7. Authww a hr(om*n~ o~~nizBtion ~.pt. Np 

Arnold G. Eversole 

i. hr(ormlq Oqm~ix~tion Nem, .nd Add-9 10. hol@/T~lk/Work Unit No. 

11. bmnct(C> OI Gr8nt(G) No. 

(Cl 

12 S~onsorinS Or#*nlzatlon Name l nd Addnss 

- m 

National Wetlands Research Center U.S. Army Corps of Engineers 

Fish and Wildlife Service Waterways Experiment Station 

U.S. Department of the Interior P. 0. Box 631 

Washington, DC 20240 Vicksburg, MS 39180 

11 Supplemwdaty Notes 

*U.S. Army Corps of Engineers Report No. TR EL-82-4 

-la asma (Limit: 200 words) 

Species profiles are literature summaries of the taxonomy, morphology, range, life history 

and environmental requirements of coastal species. They are designed to assist in 

environmental impact assessment. The hard clam, Mercenaria mercenaria, supports an 

important commercial fishery in the South Atlantic, averaging about 1 million kg of meats 

annually from 1979 to 1983. It also is an important constituent of estuarine systems 

throughout the region. Spawning occurs in the spring and the fall at 16 to 30 "C. Planktonic eggs and larvae are carried by water currents, and larvae set sometime after 6 days 

of age. Mortality is highest in egg and larval stages, the most sensitive part of the 

life cycle. Spat display gregarious setting behavior and appear to select sand over finer 

substrates. Highest densities of clams occur in sandy bottoms with shell. Crab predation 

is an important factor influencing the density and distribution of clams. Blue crabs and 

mud crabs appear to be the most important predators. Hard clams are infested by few parasites. Adult clams feed by filtering suspended particulate matter from the water. Growth 

of clams decreases with size and age. Growth occurs year-round with peaks in spring and 

fall. Growth of adult hard clams occurs at 9-31 "C and at 4-35 ppt (optima near 20 "C 

and 24-28 ppt). Hard clams mature in 2 years and reach commercial size in 3 years in the 

South Atlantic. Tight-fitting shells permit hard clams to survive poor water quality for 

short periods. 

I 

17. Docunwnt Analysis l . O*wxipton 

Shellfish Fisheries Life cycles 

Growth Suspended sediments Oxygen 

Salinity Sediments Contaminants 

Temperature Feeding habits 

b. Id.ntlfl~n/OpwMnd~d Terms 

Mercenaria. mercenaria 

Hard clam 

~ Quahog 

Habitat requirements 

C. COSATl Field/Group 

ia AvMl~bllity StJt*ment 19. Secunty Class (This Rtpm-0 21. No. of PsSes 

Release unlimited Unclassified 33 -- 

20. kcurity Class CThis Pace) 22. Price 

Unclassified 

[So. ANSl-ZS9.lSl OPTIONAL FORM 272 (4-77 

(Formrly NTIS-35) 

oopmtment of Comm*rce 
Species proiiies : 

Eversoie, Arnoia 6. 

L 
As the Nation’s principal conservation agency, the Department of 

the Interior has responsibility for most of our nationally owned 

public lands and natural resources. This includes fostering the 

wisest use of our land and water resources, protecting our fish 

and wildlife, preserving the environmental and cultural values of our 

national parks and historical places, and providing for the enjoyment of life through outdoor recreation. The Department assesses 

our energy and mineral resources and works to assure that their 

development is in the best interests of all our people. The Department also has a major responsibility for American Indian reservation 

communities and for people who live in island territories under U.S. 

administration. 

I 

U.S. DEPA;RTMENT OFTHE INTERIOR 

FISH .AND WILDLIFE SERVICE 

,.. 

TAKE PRIDE 

in America 

UNITED STATES 

DEPARTMENT OF THE INTERIOR 

FISH AND WILDLIFE SERVICE 

National Wetlands Research Center 

NASA-Slidell Computer Complex 

1010 Gause Boulevard 

Slidell, LA 70458