DISTRIBUTION OF THE SEA GRASS,

THALllSSIA, IN THE UNITED STATES·

DONALD R. MOORE

Institute of Marine Science, University of Miami

ABSTRACT

The distribution of Thalassia testudinum in the United States is found

to depend on a number of ecological factors. Limiting factors are temperature, depth of water, turbidity, salinity and wave action. Gaps in distribution of Thalassia are due to one or more unfavorable conditions, such as

the greatly increased turbidity and lowered salinity along the Louisiana

coast. Thalassia beds are considered to be of great importance to shallow

water animal populations, and commercially important species derive

both food and shelter from grassy areas.

Thalassia testudinum Konig, the common turtle grass of the southeastern

United States, is one of half a dozen aquatic flowering plants that live in

the brackish or marine waters of the southern states bordering the Gulf

of Mexico and Atlantic Ocean. Although these plants superficially resemble

grasses, they are members of the Hydrocharitaceae, Zosteraceae, and Potamogetonaceae. Several of these plants, including T. testudinum, are truly

marine, and are not found where the salinity is appreciably diminished.

One species, however, Ruppia maritima Linnaeus, is usually confined to

more brackish waters, and is found in a zone that only partially overlaps

that of the more marine species. In addition to the two species mentioned

above, the following species are known from the coastal waters of the

southeastern United States: Diplanthera wrightii (Ascherson) Ascherson,

Syringodium filiforme Kiitzing, Halophila baillonis Ascherson and H. engelmannii Ascherson.

The sea grasses have been little studied, for they grow in situations rarely

visited by most land botanists. Much of the work on the distribution and

biology of these plants has been carried out by marine algologists who included all marine plants in their observations. Thus Taylor (1928) recorded the depths in which sea grasses were collected around the Dry

Tortugas, and Humm (1956) and Humm and Caylor (1957) recorded

the presence of several species on the Mississippi coast. Recently, Phillips

(1960) published an extensive paper on the ecology and distribution of

sea grasses on the Florida coast, but admitted that there was little information for both sides of the northern part of Florida. Many observations

have been made by zoologists who found the grass beds to be of great

importance to the fauna of the areas studied. Strawn (1954) discussed

the use of the pushnet on grass beds for collecting small fishes. He later

(1961) published part of his observations that dealt with the grass beds

lContribution No. 475 from The Marine Laboratory, Institute of Marine Science, University

of Miami. This study was supported by National Science Foundation grant G-14521.
330 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

at the area be studied, Cedar Key, Florida. Stephenson and Stephenson

(1950) found Thalassia to be an important constituent in their ecological

study of the Florida Keys. Voss and Voss (1955) found the Thalassia

zone to be prominent around Soldier Key, and they discussed the importance of the turtle grass beds on the Florida coast. They published a generalized distribution map showing where grass beds occur. It should be

pointed out that this map shows the general area in which Thalassia is

found living along the coast of Florida, and there are undoubtedly areas

where grass beds are reduced or absent. Phillips (1960) has already

pointed out that he found no beds offshore in the Tampa Bay area. Kornicker (1959) in an ostracod study, and Voss and Voss (1960) in a general

survey, discussed the extent and influence of the Thalassia beds of the

Bimini, Bahama Islands, area. Thomas, et al. (1961) made a somewhat

different type of survey as they studied the extent of the damage done to

grass beds in Biscayne Bay by hurricane Donna.

The vagaries of the distribution of Thalassia became of interest to the

writer during a survey of the animal communities living in the grass beds.

The principal reasons for the success or failure of grass beds were considered to be physical factors, such as light, temperature, and salinity, for

no sign of mass destruction by animals or disease has been noted. In the

following account, an attempt will be made to correlate physical conditions

with the distribution of Thalassia.

The distribution of Thalassia testudinum to the north appears to be

limited by temperatures. It is typically a tropical West Indian form, but

it extends well into warm temperate waters on the coasts of northwest

Florida, Alabama and Mississippi. It is not, however, known from the

east coast of Florida north of Cape Canaveral. Phillips (1960) compared

winter temperatures at Pensacola on the gulf coast and at Fernandina on

the northeast Florida coast. The Fernandina minima, however, were slightly

higher than those at Pensacola. In this case, it seems apparent that other

limiting factors have come to exclude Thalassia from this area. It is a

coast where winter storm waves from the broad Atlantic would tear loose

any attached vegetation attempting to establish itself offshore. There are

long, narrow lagoons in the New Smyrna Beach-Daytona Beach area where

turbid waters and lowered salinities exclude Thalassia. There are only four

inlets between the mouth of the St. Johns River and Sebastian Inlet, a

distance of 185 miles. It is only in the neighborhood of the inlets that there

is any chance that turtle grass would be able to live. There are no published

records for the area north of Sebastian, and it appears that Thalassia has

not been able to gain a foothold in this area. Thus, while temperature

limits the geographical extremes of Thalassia, depth of water, wave action,

turbidity and salinity are all important factors in local distribution.

In recent years, Thalassia has come to be thought to extend entirely
1963] Moore: Distribution of Thalassia 331

around the northern gulf. Humm (1956) states "There is reason to believe

that the distribution of Thalassia ... is essentially continuous around the

entire gulf coast." This is true for the area east of the mouth of the Mississippi, but the entire coast of Louisiana west of there appears to be

lacking sea grass. The reason for this is that the turbid, sediment-laden

waters from the Mississippi River drift to the westward, and are not dissipated until far down the Texas coast. These waters are reinforced by the

outflow of several bayous and especially by the Atchafalaya River. Indeed,

so much fresh water is poured out that live oysters are found on reefs

several miles offshore in the area where the Atchafalaya flows into the

Gulf. In western Louisiana the Calcasieu and the Sabine Rivers contribute

their brown flow to what is left of the Mississippi and Atchafalaya waters.

This mass of dirty water begins to dissipate on the Texas coast, for river

runoff drops off rapidly in the more southerly part of Texas. Ruppia

maritima, with its ability to live in almost fresh water, is the only one of

the sea grasses that is very successful along the central and western part of

Louisiana.

The Texas coast varies from sediment laden waters brought in by the

Sabine and Neches Rivers at the Louisiana border, to virtually no runoff

in the Laguna Madre area. Turbid water is still found along the shore,

but it becomes a narrow band during calm summer weather, and may even

disappear for short periods on the southern part of the coast. The southern

part of Texas is thus the best area west of the Mississippi for the growth

of sea grass. Thalassia is not the dominant sea grass, for much of this area

has salinities which are considerably higher than the upper limit for turtle

grass. A long, shallow lagoon, the Laguna Madre, extends from Corpus

Christi to Port Isabel, a distance of roughly 120 miles. Fresh water runoff

is less than evaporation in this area and salinities of over 100%0 have been

reported (Collier and Hedgpeth, 195'0). In Aransas Bay, however, good

stands of Thalassia can be found. Where it is not found in southern Texas,

due to high salinity or other factors, it is replaced by Diplanthera wrighti

(Simmons, 1957).

It must be borne in mind that Thalassia, in many places, is living in

areas where conditions may change rapidly in a very short time. Storms

and floods may bring in silt and turbid waters that can destroy large, flourishing beds. Turtle grass beds that are destroyed on fringe areas may take

many years to come back, and, where a chan~e has been brought about

in the habitat, may never return. The work of man, too, may accomplish

the destruction of sea grass beds by dredging and filling in the bays and

waterways. This is beginning to cause concern in Florida, but is not as

important in the other gulf states where grass beds occupy a much smaller

area. A change of climate, however, could bring Thalassia in to areas where

the lack of runoff had raised salinities above its limit of tolerance. The
332 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

Laguna Madre is such an area, and Thalassia has probably been forced

:)Utof the Laguna by the present period of extremely dry climate in southern

Texas (Price and Gunter, 1942).

Distribution and occurrence of Thalassia in this paper is based on records published in the literature and on personal observations of the author

and other interested persons. The most extensive locality records were

published by Phillips (1960). He gives a number of localities for the east

coast of Florida from Sebastian Inlet southward. The author and Robert

Work visited the Sebastian Inlet area on September 19, 1961 without

finding any trace of Thalassia. However, their time was limited, and they

did not get to the area where Tabb found it growing in 1954. Diplanthera

wrightii, however, was found growing abundantly in shallow beds close

to shore. Tabb also found sparse amounts near Ft. Pierce Inlet. Other

localities given by Phillipps (1960) are Lake Worth, Boca Raton, and

Ft. Lauderdale.

Voss and Voss (1955) reported that Thalassia grew in almost unbroken,

dense, matted beds from Biscayne Bay throughout the Keys to the outer

reef and westwards to the Dry Tortugas. There is indeed a great deal of

sea grass in this area, but there are considerable areas, particularly where

the water is more than six meters deep, where sand or mud bottoms prevail. Most of these places occur in the outer half of Hawk Channel, where

the bottom is much more varied than it is closer to shore. Where Thalassia

occurs in the deeper areas, five to ten meters, it is usually mixed with Syringodium. A considerable area west of Key West also apparently has turtle

grass beds, but a stretch of some twenty miles between the quicksands and

Dry Tortugas is much too deep for Thalassia. From the Keys northward

along the west coast of Florida, the distribution map given by Voss and Voss

(1955) extends, in places, out to the ten fathom line. This is again probably too deep, for 11 meters appears to be about the greatest depth in which

turtle grass will live. The shallow waters of Florida Bay have extensive

beds of Thalassia, but the coast from the Cape Romano area to Tampa

Bay appears to have too much wave action and turbidity to permit turtle

grass to exist offshore. There are grass beds in the inshore waters of bays

and sounds on this coast, however. Woodburn, et al. (1957) state that

Thalassia was the dominant sea grass on the inshore bait shrimping areas

along the west coast of Florida.

There· is apparently no offshore growth in the Tampa-St. Petersburg

area (Phillips, 1960), but grass beds occupy a great deal of the shallow

offshore waters from Clearwater north to Apalachee Bay. According to

the writer's colleague, Robert C. Work, fishermen maintain that a bare,

sandy band extends from the mouth of the Suwannee River in a southwesterly direction through the grass beds to deeper water offshore. This

may well be true, for the Suwannee drains a large area of northern Florida
1963] Moore: Distribution of Thalassia 333

and extends into southern Georgia. At any rate, Thalassia does not appear

to be well established offshore in this area.

The author made three dredge hauls in the summer of 1954 in the area

between Cedar Key and the mouth of the Suwannee River with the following results. First dredge haul, five miles offshore, abundant Syringodium;

second, 10 miles out, Caulerpa sertularioides (alga); third, 15 miles out,

sand, shell, and sea urchins. The shallower water closer to shore, however,

supported dense beds of turtle grass. There are few records from this area,

but grass beds are probably almost continuous in the shallow coastal waters

from Clearwater to Alligator Harbor. It is reported from offshore waters

in the Alligator Harbor-Dog Island area (Menzel, 1956), but is apparently

not found offshore anywhere to the westward as far as the Rio Grande.

Apalachicola Bay, from the author's observations in 1954, is much too

turbid to support the growth of sea grass, but it is found again in St. Joseph

Bay, St. Andrews Bay and in Santa Rosa Sound. The author has found

it to be abundant along the mainland side of Santa Rosa Sound between

Gulf Breeze and Navarre. Thalassia was not seen growing in Pensacola

Bay, but considerable quantities were found washed in on the beach at

Yniestra about six miles north of Pensacola.

Mobile Bay appears to have salinities too low to enable sea grasses to

survive. The writer collected on the west side of the bay about five miles

north of Cedar Point and found beds of Ruppia maritima along with

the low-salinity mollusks Neritina reelivata and Rangia euneata. Mississippi

Sound, however, has salinities that are generally better than 25 %0 along

the north side of the barrier islands. Narrow beds of sea grass are found

in these protected saline waters from Dauphin Island to Cat Island. The

inlets, or passes, between the islands do not have vegetation. Humm (1956)

has reported on the sea grasses of this area. He later (1960, with R. M.

Darnell) mentioned that Thalassia is found on the protected lee side of

the Chandeleur Islands flanking the Mississippi Delta.

The coast line from the mouth of the Mississippi River to Galveston

Bay, a distance of about 330 miles, is an area of very gentle slope of the

bottom to seaward and of brackish turbid waters. Vast quantities of fresh,

silt-laden water carried to sea by the Mississippi drift to the westward

along the coast. Before this water mass dissipates it is joined by the Atchafalaya ~nd other, smaller streams. Gunter (1951) found the commercial

oyster, Crassostrea virginiea Gmelin, living on oil platforms several miles

offshore in the open Gulf. Larval C. virgimea settle where the water is

brackish, and the presence of these oysters is an indication of hydrographic

conditions. Geyer (1950) reported that salinities of the upper layer of

water on the Louisiana coast, 6 to 8 miles offshore, ranged seasonally

from 15 %0 to 35 %0' Low salinity periods are also periods of very high

turbidity, and it is almost impossible for sea grasses to establish themselves
334 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

under these conditions. Ruppia maritima, however, is abundant In very

shallow water in the brackish water marshes.

The writer has visited three areas along this coast, and has collected

around Grand Isle on several occasions from 1954 through 1959. Each

time, he found turbid low salinity waters and many brackish water organisms. Grand Isle has a sand beach. This merely indicates that wave action

is strong enough to maintain a beach, for the bottom for many miles to

sea is soupy mud. Cypremort Point, another locality visited by the writer,

is a more protected locality, and the marsh grasses grow along the water's

edge. There is no beach.

There was a mud beach at Cameron when the author visited that area,

but sand an inch or so thick overlaid the mud out from the beach.Although

this is part of the open coast, the extremely gentle slope of the sea bottom

keeps wave action at a minimum. The water was turbid and no attached

vegetation was observed.

The Texas coast near Sabine Pass is similar, but it soon changes to a

wave-washed sand beach to the west. The water in the Galveston area is

also turbid, but there is one report (Hoese, 1960) of Thalassia living in

West Bay near San Luis Pass. Farther down the coast is Matagorda Bay,

and Thalassia is possibly living in the bay along the Matagorda Peninsula

near Pass Cavallo. San Antonio Bay, the next bay to the south, is somewhat more enclosed than Matagorda Bay, and may not have a suitable

environment for the growth of sea grass. The writer has collected in the

neighborhood of Port O'Connor on Matagorda Bay, and near Seadrift and

Austwell around San Antonio Bay without seeing a trace of Thalassia.

The best developed turtle grass bed on the Texas coast appears to be

the one in the Redfish Bay area adjacent to Aransas Bay. In the summers

of 1954 and 1955, the writer found a lush growth of turtle grass on the

east side of Ransom Island near the town of Aransas Pass. The grass close

to the beach was all Diplanthera wrightii, and Thalassia occupied the

deeper water a little offshore. This is the typical zonation in many localities

where the two grasses are found together. On the west side of Ransom

Island the bottom was muddy and Thalassia was scarce. Diplanthera was

the dominant vegetation. Copano Bay, an inner bay that is connected with

Aransas Bay, has beds of Diplanthera but no Thalassia.

The Laguna Madre is a long narrow coastal lagoon extending from

Corpus Christi Bay on the north to within three miles of the Rio Grande

on the south. The lagoon is about 115 miles long and averages 5 or 6

miles in width. This part of Texas is extremely dry, and the lagoon is

slowly being filled in by windblown sands from Padre Island on its eastern

flank. The only connections with the sea are two narrow openings into

Corpus Christi Bay, and Brazos Santiago, the pass near the south end of

the lagoon. No rivers, except for creeks which flow only after rain, empty
1963] Moore: Distribution of Thalassia 335

into the Laguna, and evaporation is usually considerably higher than fresh

water runoff. Salinities of 5'0 /{o to 60 % c are not unusual in the north

central area, and higher salinities have been recorded, Simmons (1957).

Salinities closer to that of sea water are found in the lower end of the

lagoon near Brazos Santiago. Thalassia is not found in most of the Laguna,

apparently due to the hypersaline conditions. Simmons (1957) found

Diplanthera wrightii to be abundant in the upper Laguna along with Ruppia

maritima and sparse amounts of Syringodium filiforme. The last two species

were found in the lower range of salinity.

The writer made observations and collected in the vicinity of Port Mansfield, about thirty miles north of Brazos Santiago. He dredged in the deeper

part of the Laguna near the mainland at two localities, and found the

muddy bottom to be covered with a thick growth of Diplanthera. The water

was quite shallow three to four miles east of the mainland and the bottom

was sandy, with Diplanthera in thin scattered patches. The writer also

investigated an extensive grass bed a mile west of Port Isabel and almost

opposite Brazos Santiago. Here, too, he found a lush growth of Diplanthera.

No Thalassia was found, but it might have been living farther out from

the beach. No dredging was done at this locality. A recent publication by

Breuer (1962, p. 165) states that vast meadows of turtle grass occurred

in South Bay both north and south of Queen Isabella Causeway. This is

the area closest to Brazos Santiago Pass, and is where salinities and temperatures would be close to that of the open gulf.

The Mexican Laguna Madre is even more hypersaline than its Texas

counterpart. It seems that Thalassia must be very sparse or non-existent

all the way down the coast to the vicinity of Tampico. However, that area

is very poorly known, and is also beyond the scope of this paper.

DISCUSSION

From the above account, it seems that the occurrence of turtle grass

is the result of a complex of environmental conditions which favor the

establishment and growth of grass beds. The northern limit of distribution

is apparently limited by low temperatures, yet turtle grass would probably

be found much farther up the east coast of Florida if other conditions were

favorable. Phillips (1960) gives a table of winter temperatures for Fernandina on the east coast, and for Pensacola on the west coast of Florida.

Winter minima were consistently slightly higher at Fernandina. However,

Thalassia appears to occur mainly in the clear, high-salinity waters of Santa

Rosa Sound in the Pensacola area; there is nothing to compare with this

in the Fernandina region. Thus, the reason that Thalassia is not found in

the Fernandina area is that no suitable habitat is present. The offshore

area is churned up by huge storm waves during the winter and no sea grass

could survive there. The inshore area has considerable fresh water runoff
336 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

with ensuing low salinities and high turbidity. Diplanthera wrightii may

possibly live here in very shallow water, but it seems unlikely that any

other sea grass could survive in the area. In this situation the problem of

minimum winter temperatures has little bearing on the presence or absence

of Thalassia.

While it appears impossible to draw a line north of which the temperatures are too low to enable turtle grass to survive, it seems that there must

be sufficient change in climate between Florida and North Carolina to

make it virtually impossible for Thalassia to live in the coastal lagoons of

the North Carolina coast. The only sea grasses found in these waters are

Diplanthera wrightii, Ruppia maritima, and Zostera marina (Humm, personal communication).

The virtual absence of powerful waves generated far out to sea makes

it possible for turtle grass to live in shallow, offshore coastal waters. In

the area under consideration this happens in the Florida Keys, the outer

part of Florida Bay, and along the northwestern part of the Florida peninsula. Moore (1959) has discussed the effect of the Bahama Islands and

associated shallow banks on the waves striking the Florida Keys. This area

is also protected by Cuba from storm waves coming from the south. In

addition, much of the shallow platform off the Florida Keys is further

protected by a line of coral reefs along the edge of the Florida Current.

The grass beds in the outer portion of Florida Bay and along the northwestern edge of the peninsula are not protected from the west by islands or

reefs, but by the long and gentle slope of the bottom out to the edge of

the continental shelf. Large, powerful, open sea waves lose force when

traveling over shallow areas, and this is the mechanism that damps the

big sea waves in these areas. Sizeable waves are built up near shore during

local storms, but they are short waves with little distance between crests.

These waves do not generate the force necessary to tear grass beds loose

from the bottom. Even hurricane winds may not do a great deal of damage

(other than breaking off blades) in shallow water areas (Thomas, et al.,

1961 ). The Louisiana coast west of the Mississippi is another area where

wave action is reduced to a minimum. However, extreme turbidity in this

area prevents the establishment and growth of Thalassia.

Reference has been made to the exclusion of turtle grass due to low

or unusually high salinities. As yet, no one has experimented to discover

the salinity tolerance of Thalassia. Because inshore waters fluctuate widely

in their physical components, one reading or even a series of readings may

be misleading. It appears to the writer, however, that salinities consistently

around 20 %0 or lower will have a deleterious effect on turtle grass beds.

The lowest reading reported where Thalassia was found living is that of

Phillips (1960) who found it to be 10 %0 at the south end of Crystal Bay.

This observation was made after a period of thundershowers, and the grass
1963] Moore: Distribution of Thalassia 337

found was very sparse. Moore (1961) reported a reading of 11.5 %0 near

the north side of Ship Island, Mississippi, where Thalassia is locally abundant. This observation was made during an unusually wet summer, however, for salinities there are usually well over 20 %0' The writer and Robert

C. Work have sampled a turtle grass bed on the west side of Biscayne

Bay during the course of a year. One very low reading of 13.1 %0 was

made, but all others were around 20 %0' or more. The maximum measured

at this station was 34.2 %0'

The upper range of salinity is found under quite different conditions.

Here evaporation exceeds runoff in a shallow bay or lagoon, and this

condition may be fairly stable for months at a time. Tabb, et al. (1959)

reported grass beds with salinities as high as 48 %0 in Florida Bay. This

maximum was found after a long period of dry weather and before the

summer rains. Presumably, Thalassia will not do well where salinities

remain much over 40 %0 for long periods of time. The Laguna Madre of

Texas is such a place. Simmons (1957) found no turtle grass in the

upper Laguna Madre where salinities ranged from 27.3 %0 to 79.2 %0'

Most of his salinities ranged from 40 %0 to 50 %0 or higher. In this area,

Diplanthera wrightii was abundant, Syringodium filiforme was sparsely

scattered, and Ruppia maritima formed dense mats in sheltered areas where

the salinity remained under 45 %0' Salinities in this area remain above

4'0 %0 for long periods of time, and this appears to be the single limiting

factor that excludes Thalassia. Turtle grass appears to be a rather stenohaline marine organism in which optimum growth takes place in waters

of 25 %0 to 40 %0' Some interesting physiological problems are presented

for this is a flowering plant that has apparently become isotonic with sea

water.

Depth of water and turbidity go hand in hand in marine phototropic

organisms. Where turbidity is low, plants may live in fairly deep water.

Where turbidity is high, plants are forced to live in quite shallow areas

or die from lack of energy-giving light. Thalassia appears to need large

amounts of sunlight for it is rare in depths of more than eight meters even

where the water is unusually clear. Phillips (1960) reported a depth of

30.5 meters near Molasses Reef. This is based on a sight record, however,

and could possibly have been Halophila. H. baillonis was collected in a

depth of 29.3 meters by Taylor (1928) in the Dry Tortugas area. He

found H. engelmanii in even deeper water in the same area. Any definite

record, however, of attached Thalassia living in depths of more than 12

meters would be of great interest.

Turtle grass is able to tolerate the full light of the blazing noonday sun

without any apparent damage. It has been noticed by the writer, however,

that Thalassia in very shallow areas has short blades, and does not appear

to have nearly as lush a growth as does grass in water just half a meter
338 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

deeper. Desiccation is apparently fatal, for no plants have been observed

to be completely out of water at low tide. This is in contrast to Diplanthem

wrightii which the author has often seen high and dry at low tide.

Some Thalassia might be out of water during unusual conditions such

as a wind-blown low tide, but this would be a very unusual circumstance.

Although turtle grass is often the dominant sea grass in an area, it is

usually associated with one or more of the other grasses. Phillips (1960)

gave a schematic drawing of sea grass zonation in shallow water. He found

Diplanthem abundant in the intertidal zone, with Ruppia also very abundant at a slightly lower level. Thalassia and Syringodium were found below

the normal low tide line, but Syringodium prefers slightly deeper water

than does Thalassia. Apparently, Syringodium will not withstand any

exposure to air, and is confined to a level below the lowest spring tide.

Although Diplanthem is most abundant in very shallow water, it has been

dredged in quantity in a depth of ten feet in the Laguna Madre by the

author. At this locality, however, it was the only sea grass found, and was

doing well in the absence of competition. Ruppia is often absent where the

other sea grasses flourish, and is apparently never found offshore.

Deeper water associations below the level of the lowest tides to the

lower limit of Thalassia growth are usually between Thalassia and Syringodium. The writer has noticed that almost all of the grass patches that

he has observed in depths of six meters or more were mixtures of both

species. There is even less known about the maximum depth for Syringodium than there is known for Thalassia. Another species sometimes found

with turtle grass in deeper water is Halophila engelmannii. It is often

found within beds of Thalassia or mixed Thalassia and Syringodium. It is

nearly always overlooked because of the much taller leaves of its associates,

and apparently thrives in the reduced light of the shade of the larger plants.

H. engelmannii is also found in water much deeper than both Thalassia

and Syringodium can exist. Taylor (1928) found it to a depth of 73.2

meters and one estimated depth of 91 meters at Dry Tortugas.

It is difficult to compute the importance of the turtle grass beds as a

habitat for marine animals. Few animals eat the grass, but the attached

algae, diatoms, and bacterial film are utilized by countless numbers of

small animals. These animals are eaten in turn by shrim1J, fish, and other

larger animals. The grass blades not onlv greatly increase the surface area

available for plants and animals, but they provide a suitable substrate

for a great many organisms that would not be able to utilize a bare sand

or mud bottom.

The importance of Thalassia as a habitat for animal communities can

be seen when quantitative sampling is done. In 1960 and 1961, the

writer, with Robert C. Work and Lowell P. Thomas, sampled several

stations in Biscayne Bay near Miami, Florida. Two methods of sampling
1963] Moore: Distribution of Thalassia 339

were used. One was a "plug" sampler invented by Gilbert L. Voss. This

removed a plug of the bottom 1/ 10 square meter in surface area and

about 300 to 400 mm deep. The other piece of equipment used was a

push net which was one meter wide. The plug sampler was excellent for

obtaining the total fauna of a small area and especially the microfauna.

The push net was excellent for collecting the larger free moving animals,

but was poor for sessile animals.

The abundance of animal life in Thalassia can be seen by some of

the figures obtained by quantitative sampling. A station on the Atlantic

side of Virginia Key produced 280 live, burrowing pelecypods per square

meter. In length, these clams ranged from 18 mm to 55 mm, and the

average size was 26.7 mm. There were also 140 non-burrowing pelecypods

in the same sample. A push net sample at this station produced 49 shrimp

per square meter. Nearly all of these were carideans, and were too small to

be fished commercially. They are, however, certainly very important as

food for fish.

At Matheson Hammock, another Thalassia station on the mainland side

of Biscayne Bay, physical conditions were quite different, - and larger

numbers of fewer species were present. An average of six push-net

samples gave 72 caridean shrimp per square meter. Small mollusks were

also few in species, but several of these were extremely abundant. A list of

the more important species from one sample gives the following results:

Gastropods

Bittium varium

Rissoina chesneli

Caecum pulchellum

Mitrella lunata

Number per square meter

2170

5440

13220

130

794'0

230

Pe~ecypods

Brachidontes exustus

Amygdalum papyria

The least abundant gastropod, Mitrella lunata, is a carnivore. All the

others listed are herbivores or filter feeders.

The importance of these mollusks is difficult for the layman to comprehend. All are too small to be of any commercial value, even as bait.

However, the six species listed, at the density given above, number approximately 95 billion per square mile. The importance of these animals as

food for young fish and penaeid shrimp must be tremendous. Caridean

shrimp, at a density of 72 per square meter would number 235 million

per square mile, and must be of great value as food for fish.

A third station at Soldier Key on the outer edge of Biscayne Bay was

a much more stable area in regard to salinity and temperature. Many
340 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

species aIe present, but no single species is very abundant. Caridean

shrimp averaged only 20 per square meter, and the mollusks were represented by many dead shells for each living specimen.

However, the transparency of the water is considerably greater at Soldier

Key than at Matheson Hammock, and many species were probably

inhibited from living on the shallow bank where the samples were taken.

This area is probably more important than the meager figures so far

obtained.

Also of great importance is that Thalassia beds provide protective cover

for young shrimp and fish. The writer has often found young Florida

crawfish, Panulirus argus, in Thalassia, and he believes that this important

commercial species would be hard hit by the destruction of the Florida

turtle grass beds. Tabb and Manning (1962) have completed a survey

of the shallow marine waters of the Everglades National ·Park at the

southern extremity of the Florida Peninsula. They found large areas to

be covered with Thalassia, and were impressed by the numbers and

variety of animals living in the grass beds. Hoese (1960) stated that

juvenile Penaeus duorarum were found only in the Thalassia flats in the

Galveston Bay area, and that the absence of P. duorarum in some areas

was apparently due to the absence of grass beds. This is the pink shrimp

which is of great importance to the Florida fisheries. Adult pink shrimp

appear to be quite able to fend for themselves, and seem to prefer a muddy

substrate. However, without a favorable habitat for the juveniles there

would not be any adults.

Thalassia and its distribution is not of interest only to the botanist; it

contains a vast assemblage of animals of great interest both to the zoologist

and to the sports and commercial interests of much of our southern

seaboard.

CONCLUSIONS

1. Thalassia testudinum Konig lives in tropical or subtropical waters.

Growth in the warmer part of warm temperate waters is possible if

other conditions are favorable.

2. Thalassia grows best where the salinity ranges between 25%0 and

40%0' It may survive for some time in salinities much lower or higher

than this range, but grass beds are not present where salinities are

consistently below 20%0 or above 45%0'

3. The range in depth depends on the clarity of the water. Yet, even in

the most transparent waters, Thalassia apparently does not live deeper than

about 11 meters.

4. The presence of grass beds in offshore waters depends on reduced

wave action. This occurs where reefs break up oceanic waves, or where
1963] Moore: Distribution of Thalassia 341

the configuration of the bottom and coastline reduces the strength of

waves while still some distance offshore.

5. Animal life is often extremely abundant in Thalassia beds. Thalassia

affords both food and shelter to small shrimp, fish, and other animals.

Some of these animals are the young of commercially important species,

while many others serve as food for commercial species.

SUMARIO

DISTRIBUCION DE LA HIERBA MARINA Thalassia EN LOS ESTADOS UNIDOS

La distribuci6n de Thalassia testudinum en los Estados Unidos se ha

encontrado que depende de un numero de factores ecol6gicos. Los factores

limitantes son: temperatura, profundidad del agua, turbulencia, salinidad

y acci6n de las olas. Los fallos en la distribuci6n de Thalassia se deben a

una 0 mas condiciones desfavorables tales como: aumento de la turbulencia y disminuci6n de la salinidad a 10 largo de la costa de Louisiana.

Los lechos de Thalassia son considerados de gran importancia para la poblaci6n animal de aguas bajas, y especies comercialmente importantes

encuentran alimento y refugio en las areas de esta hierba.

REFERENCES

BREUER, J. P.

1962. An ecological survey of the lower Laguna Madre of Texas, 1953-

1959. Pub!. lnst. Mar. Sci. Univ. Tex., 8: 153-183.

COLLIER, ALBERT AND JOEL HEDGPETH

1950. An introduction to the hydrography of the tidal waters of Texas.

Pub!. lnst. Mar. Sci. Univ. Tex., 1 (2): 125-194.

GEYER, RICHARD R.

1950. The occurrence of pronounced salinity variations in Louisiana coastal

waters. J. Mar. Res., 9 (2): 100-110.

GUNTER, GORDON

1951. The West Indian tree oyster on the Louisiana coast, and notes on

growth of the three gulf coast oysters. Science, 113 (2940): 516-517.

HOESE, H. D.

1960. Juvenile penaeid shrimp in the shallow Gulf of Mexico. Ecology, 41

(3): 592-593.

HUMM, HAROLD

1956. Sea grasses of the northern gulf coast. Bull. Mar. Sci. Gulf & Carib.,

6 (4): 305-308.

HUMM, HAROLD AND R. L. CAYLOR

1957. The summer marine flora of Mississippi Sound. Pub!. Inst. Mar. Sci.

Univ. Tex., 4 (2): 228-264.

HUMM, HAROLD AND REZNEAT M. DARNELL

1960. A collection of marine algae from the Chandeleur Islands. Pub!. lnst.

Mar. Sci. Univ. Tex., 6: 265-276.

KORNICKER, LOUIS S.

1959. Ecology and taxonomy of recent marine ostracodes in the Bimini

area. Great Bahama Bank. Pub!. lnst. Mar. Sci. Univ. Tex., 5:

194-300.
342 Bulletin of Marine Science of the Gulf and Caribbean [13(2)

MENZEL, R. WINSTON, EDITOR

1956. Annotated check list of the marine fauna and flora of the $t. George's

Sound-Apalachee Bay region, Florida Gulf Coast. Oceanog. lnst.,

Fla. State Univ. 1-78 (Mimeo.).

MOORE, DONALD R.

1959. Notes on Blanquilla Reef, the most northerly coral formation in the

western Gulf of Mexico, Publ. Inst. Mar. Sci. Univ. Tex., 5: 151-155.

1961. The marine and brackish water Mollusca of the State of Mississippi.

Gulf Res. Rep., 1 (1): 1-58.

PHILLIPS, RONALD C.

1960. Observations on the ecology and distribution of the Florida sea grasses. Professional Papers Series, Fla. Bd. Conserv., (2): 1-72.

PRICE, W. ARMSTRONG AND GORDON GUNTER

1942. Certain recent geological and biological changes in South Texas,

with a consideration of probable causes. Proc. Tex. Acad. Sci., 26:

138-156.

SIMMONS, ERNEST G.

1957. An ecological survey of the upper Laguna Madre of Texas. Publ.

Inst. Mar. Sci. Univ. Tex., 4 (2): 156-200.

STEPHENSON, T. A. AND ANNE STEPHENSON

1950. Life between tide marks in North America. 1. The Florida Keys.

J. Ecol., 38 (2): 354-402.

STRAWN, KIRK

1954. The pushnet, a one man net for collecting in attached vegetation.

Copeia, 1954 (3): 195-197.

1961. Factors influencing the zonation of submerged monocotyledons at

Cedar Key, Florida. J. Wildlife Mgmt., 25 (2): 178-189.

TABB, DURBIN C., D. DUBROW, R. MANNING AND L. THOMAS

1959. Annual report on ecology of Florida Bay estuaries. Univ. Miami Mar.

Lab. 89 pp. Typewritten.

TABB, DURBIN C. AND RAYMOND MANNING

1962. A checklist of the flora and fauna of northern Florida Bay and

adjacent brackish waters of the Florida mainland collected during the

period July, 1957 t.hrough September, 1960. Bull. Mar. Sci. Gulf &

Carib., 11 (4): 552-649.

TAYLOR, WILLIAM R.

1928. The marine algae of Florida, with special references to the Dry

Tortugas. Publ. Carneg. Instn., 379: 1-219.

THOMAS, LOWELL P., D. R. MOORE AND R. C. WORK

1961. Effects of hurricane Donna on the turtle grass beds of Biscayne Bay,

Florida. Bull. Mar. Sci. Gulf & Carib., 11 (2): 191-197.

Voss, GILBERT L. ANDNANCY Voss

1955. An ecological study of Soldier Key, Biscayne Bay, Florida. Bull. Mar.

Sci. Gulf & Carib., 5 (3): 203-229.

1960. An ecological survey of the marine invertebrates of Bimini, Bahamas,

with a consideration of their zoogeographical relationships. Bull. Mar.

Sci. Gulf & Carib., 10 (1): 96-116.

WOODBURN, K. D., B. ELDRED, E. CLARK, R. F. HUTTON AND R. M. INGLE

1957. The live bait shrimp fishery of the west coast of Florida (Cedar Key

to Naples). Fla. Bd. Conserv. Tech. Series, 21: 1-33.