REFERENCECOPY 

Do Not Remove from the Libr~it?~ 

U. S. Fish and \JildlIfe Se;vi.ce 

TR EL-82.4 /00 Cajun Dome Eotile-~ord October 1983 Lafcryette, Louisiono 7Q506 

Species Profiles: Life Histories and 

Environmental Requirements of Coastal Fishes and 

Invertebrates (North Atlantic) 

HARD CLAM 

Coastal Ecology Group 

Fish and Wildlife Service Waterways ~xperiment Station 

U.S. Department of the Interior U.S. Army Corps of Engineers 
FWS/OBS-82/11.18 

TR EL-82-4 

October 1983 

Species Profiles: Life Histories and Environmental Requirements 

of Coastal Fishes and Invertebrates (North At1 antic) 

HARD CLAM 

Jon G. Stanley 

and 

Rachael DeWitt 

Maine Cooperative Fishery Research Unit 

313 Murray Hall 

University of Maine 

Orono, ME 04469 

Project Manager 

Larry Shanks 

Project Officer 

Norman Benson 

National Coastal Ecosystems Team 

U.S. Fish and Wildlife Service 

1010 Gause Boulevard 

Sl idel 1 , LA 70458 

Performed for 

Coas ta 1 Ecology Group 

Waterways Experiment Station 

U.S. Army Corps of Engineers 

Vicksburg, MS 39180 

and 

Na tiona 1 Coastal Ecosystems Team 

Division of Biological Services 

Research and Development 

Fish and Wildlife Service 

U.S. Department of the Interior 

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

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

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

Wi ldl . Serv. FWS/OBS-82/11. U.S. Army Corps of Engineers, TR EL-82-4. 

This profile should be cited as follows: 

Stanley, J. G., and R. DeWitt. 1983. Species profiles: life histories and 

environmental requirements of coastal fishes and invertebrates (North Atlantic) 

- - hard clam. U.S. Fish Wildl. Serv. FWS/OBS-82/11.18. U.S. Army Corps of 

Engineers, TR EL-82-4. 19 p?. 
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 requi rements of the species and to describe hob4 populations of the species may be 

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

profile has sections on taxonomy, 1 ife 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. 

A Habitat Suitability Index (HSI) model is being prepared by the U.S. Fish 

and Wildlife Service for the hard clam. HSI models are designed to provide 

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

habitat. 

Suggestions or questions regarding this report should be directed to: 

Information Transfer Specialist 

National Coastal Ecosystems Team 

U.S. Fish and Wildlife Service 

NASA-Sl idel 1 Computer Compl ex 

1010 Gause Boul evard 

Sl idel 1, LA 70458 

U.S. Army Engineer Waterways Experiment Station 

Attention: WESER 

Post Office Box 631 

Vicksburg, MS 39180 

iii 

CONTENTS 

Page 

PREFACE .................................. ii i 

CONVERSION TABLE ............................. vi 

ACKIVOWLEDGMENTS .............................. vii 

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

MORPHOLOGY/IDENTIFICATION AIDS ...................... 1 

REASON FOR 1NCLUSIONI:NSERIES ...................... 2 

LIFE HISTORY ............................... 2 

Spawning ................................ 2 

Fecundity and Eggs ........................... 3 

Larvae ................................. 3 

Juvenileseedclam ........................... 3 

Adult .................................. 4 

COMMERCIAL/SPORT FISHERIES ........................ 5 

Fisheries ................................ 5 

Population Dynamics ........................... 7 

GROWTHCHARACTERISTICS .......................... 8 

ECOLOGICAL ROLE .............................. 9 

Feeding Habits ............................. 9 

Predation ................................ 9 

ENVIRONMENTAL REQUIREMENTS ........................ 10 

Temperature ............................... 10 

Salinity ................................ 11 

Di ssolved Oxygen ............................ 12 

Substrate ................................ 12 

Currents ................................ 12 

Turbidity ................................ 13 

Habitat Alteration ........................... 13 

LITERATURE CITED ............................. 14 
CONVERSION FACTORS 

Metric to U.S. Customary 

-- To Obtain 

mi 11 imeters (mm) 

centimeters (cm) 

meters (m) 

ki 1 ometers (km) 

inches 

inches 

feet 

miles 

square meters (m') 

square ki lometers (km') 

hectares (ha) 

square feet 

square miles 

acres 

liters (1) 

cubic meters (m') 

cubic meters 

gal lons 

cubic feet 

acre-feet 

mi 11 i grams (mg) 

grams (g) ki 1 ograms (kg) 

metric tons (mt) 

metric tons 

kilocalories (kcal ) 

ounces 

ounces 

pounds 

pounds 

short tons 

BTU 

Celsius degrees Fahrenheit degrees 

U.S. Customary to Metric 

inches 

inches 

feet (ft) 

fa thoms 

miles (mi) 

nautical miles (nmi ) 

mi 11 imeters 

centimeters 

meters 

meters 

kilometers 

kilometers 

square feet (ftL) 

acres 

square miles (mi2) 

square meters 

hectares 

square ki lometers 

gallons (gal ) 

cubic feet (ft') 

acre-feet 

1 i ters 

cubic meters 

cubic meters 

ounces (oz) 

pounds (lb) 

short tons (ton) 

BTU 

grams 

kilograms 

metric tons 

kilocalories 

Fahrenheit degrees Cel sius degrees 
ACKNOWLEDGMENTS 

The cover is adapted from an illustration by Trudy Nicholson and is used 

with permission from Grass Medical Instruments and the artist. Figure 4 is 

reproduced with permission from the General Secretary of the Conseil International Pour L'Exploration de la Mer. We are grateful for the review by Robert 

L. Dow, Maine Department of Marine Resources, Augusta. 
Figure 1. Hard clam. 

HARD CLAM 

Scientific name . . . . Mercenaria 

mercenaria L. Widely known as Venus 

mercenari a before We1 1 s (1957 ) reassigned the species to the genus Linneaus original ly appl i ed 

Preferred common names . . Quahog in 

the Northern United States, hard 

clam in the Southern United States 

(Figure 1) 

Other common names . . . . Quahaug, 

hard-she1 led clam, round clam, cherrystone clam, little-necked clam 

Class . . . . . Bivalvia (Pelecypoda) 

Order . . . . . . . Eul amell i branchi a 

Suborder . . . . . . . . . Heterodonta 

Fami ly . . . . . . . . . . Veneri dae 

Geographical range: The hard clam occurs in intertidal and subtidal 

areas to depths of 15 m along the 

Atlantic and gulf coasts from the 

Gulf of St. Lawrence to Texas. 

The hard clam is most abundant 

from Massachusetts to Virginia. 

It has been introduced to Europe 

and California. A similar speci es, M. campechi ensi s, occurs 

from North Carolina southward to 

Mexico and is also called the 

hard clam. 

MORPHOLOGY/IDENTIFICATION AIDS 

The hard clam has a thick shell 

with a violet border and short 

siphons (Verri 11 1873; Stanley 1970; 

Morris 1973). The mean length of the 

thick solid shell is 60 to 70 mm, but 

some are 120 to 130 mm. The ratios of 

length (L), height (H) and width (W) 

are: L/H 1.25; H/W 1.52; L/W 1.90. The 

thickness Index (ratio of shell volume 

to internal volume) is 0.60. 
The external surface has numerous 

concentric 1 i nes, conspicuous and 

closely spaced near the ends, more 

widely spaced around the umbo, especially in younger shells. The center 

of each valve is smoother than the 

distal portion. The umbo is anterior 

and projects nearly to the front of 

the shell. The elliptical, somewhat 

pointed shell has a grayish-white 

exterior and a white interior with a 

dark violet border near the margins. 

The colored part of the shell was 

fashioned into wampum by the American 

Indians for use as money, hence the 

scientific name. The interior ventral margins are denticulate. 

The internal anatomy also has 

distinctive characteristics. Short 

siphons are united from their bases to 

near the ends; the incurrent siphon 

has a short fringe of tentacles. The 

siphon tubes are ye1 1 owi sh- or brownishorange toward the end and may be 

streaked with dark brown or opaque 

white. The foot is large, muscular, 

and plow shaped. The mantle lobes are 

separate a1 ong the front and ventral 

edges of the shell with thin edges 

folded into delicate frills, some of 

which are elongated near the siphons. 

Foot and mantle edges are white. 

REASON FOR INCLUSION IN SERIES 

Hard clams are the most extensi vely distributed commerci a1 clam in 

the United States and have the greatest total market value (Ritchie 1977). 

Thei r occurrence i n clean substrates 

accessible to the public makes the 

hard clam a popular recreational species. Thei r shore habitat is vulnerable to coastal construction projects 

and pollution from urban and industrial development. The absence of 

hard clam populations is an ecological 

i ndi cator of disturbances. Because 

adults do not move, repopulation of 

anni hi 1 ated hard clam beds depends on 

transport of larvae and several years 

growth. Hence, a temporary disturbance causes a long-term impact. 

LIFE HISTOKY 

Spawning 

The spawning season extends from 

May through August, dependent on latitude and temperature. In temperate 

1 atitudes the largest and densest 

spawns occur during July (Carriker 

1961). In the York River, Virginia, 

the peak is in May, and is progressively later in Raritan Bay, New 

Jersey, and Narragansett Bay, Khode 

Island (Jeffries 1964). Female hard 

clams require 2 to 2.5 months to spawn 

out completely, but the greatest release of ,eggs is during the initial 

spawning of the season (Ansell 1967a). 

Spawning is more intense during neap 

tides than spring tides, presumably 

because of hi gher temperatures during 

neap tides (Carriker 1961). 

Temperature is the decisive factor for final gamete maturation. In a 

2-year study in the Lower Little Egg 

Harbor, New Jersey, the meai an dai ly 

spawning temperature was 25.7"C with a 

range of 22" to 3U°C (Carriker 1961). 

Seventy -three percent of the spawni ngs 

occurred during 2 to 3 days of rising 

temperatures. Kenni sh and Ol sson 

(1975) cited 21" to 25°C as the required or preferred temperature range. 

Spawning in England takes places at 

18" to 2U°C (Mitchell 1974). When 

thresh01 d temperatures are reached, 

males release semen that contains 

pheromones. The pheromones are carried 

by water currents to the females, 

which are then stimulated to release 

eggs (Nelson and Haskin 1949). 

Sexual maturity usually is 

reached at 2 years of age (3 years in 

many areas in the North Atlantic region). The shell length at this age is 

between 32 and 38 mn. Size, not age, 

determines sexual maturity, so that 

slower growing individuals mature later than 2 years of age. The peak of 

reproducti ve potenti a1 is reached at 

60 mm; larger, older hard clams gradual ly 1 ose the reproductive capacity 

(Belding 1931). 
Fecundity and Eggs larvae are more evenly mixed in the 

water col umn. 

The average number of eggs released by a 60-mm female in the wild 

is about 2 million (Belding 1931). In 

laboratory tests, the average-sized 

fem.ale released 8 million eggs per 

season (Davis and Chanley 1956; Ansell 

1967a). The fecundity of one large female was 16.8 million eggs, whereas 

small clams (33 mm) had far fewer 

eggs (Brice1 j and Malouf 1980). About 

2,000 spermatozoa are shed for each 

ovum. 

The spherical eggs are 78 um in 

diameter with closely packed yolk 

granules (Belding 1931). A large 

gelatinous capsule distinguishes the 

hard clam egg from the eggs of otner 

moll usks. Eggs are re1 eased through 

the excurrent siphon, and the capsule 

swells after contact with water until 

it is 3.2 times the diameter of the 

egg. The gelatinous capsule imparts 

buoyancy, so that the eggs are pelagic 

and carried by tidal and coastal currents. Spermatozoa swimming in water 

come into contact with and penetrate 

the capsule, fertilizing the egg. 

At about 10 hr the embryo developing wit hi n the capsule becomes covered with cilia. The lashing of the 

cilia tears the membrane and gelatinous capsule; the ci 1 iated gastrula 

escapes into the water.. The egg may 

be carried 2 to 25 km from the spawning site. 

Larvae 

The larva develops into a trochophore larva 12 to 14 hr after hatching 

(Belding 1931). The shape, like a 

top, and the cilia on the blunt anterior end result in spiral swimming 

with rotation around the long axis in 

either direction. A functional mouth 

develops and the larva commences feedi ng on suspended particulates, especial ly d i n o f l a g e l l a t e s . The larvae 

concentrate near the surface during 

daylight at about 1 m below the surface (Carriker 1952). At night the 

A shell gland forms opposite the 

mouth by 24 hr after hatching, and a 

thin transparent she1 1 is secreted; 

the larva is now called a veliger 

(Belding 1931). The veliger drifts in 

ocean and estuarine currents with 

limited ability to swim horizontally. 

The veliger is able to move 7 to 8 

cm/mi n vertical ly by extending the 

ci 1 iated velum (Mi leikovsky 1973). 

Vertical swimming may enable the 

veliger to control horizontal displacement and thus travel to better 

areas (Mi leikovsky 1973). Vertical 

migration is stimulated by turbulence, 

which could bring vel igers into water 

currents for transport (Carriker 1961). 

Greatest numbers of veligers occur in 

the water column 3 hr after low tide 

(Moulton and Coffin 1954), which suggests differential tidal transport. 

By entering the water column on the 

incoming tide, the veligers would be 

transported up the estuary and thus be 

retained within the estuary. Vel i - gers, however, a1 so mi grate upwards 

during daylight regardless of tide 

(Carri ker 1961). Vel igers are important zooplankter; in estuaries during 

the summer (Carriker 1952; Moulton and 

Coffin 1954; Jeffries 1964). Densities may exceed 500/1. 

The veliger stage lasts 6 to 12 

days, depending on temperature. Metamorphosi s of the veliger occurs at 16 

to 30 days at 18"C, 11 to 22 days at 

24"C, and 7 to 16 days at 30°C (Loosanoff et a1 . 1951). 

Juvenile Seed Clam 

When the vel iger becomes 2 Lo 3 

mm long, the shell thickens, a foot 

replaces the velum, and a byssal gland 

develops, marking metamorphosis to the 

seed clam. Metamorphosis is inhibited 

at salinities below 17.5 to 20 parts 

per thousand (ppt) (Castagna and Chanley 1973), perhaps ensuring that seed 

clams avoid setting in an environment 

with salinities unsuitable for adults., 
Good sets occur in years with low 

freshwater inflow into the estuary 

(Hi bbert 1976). 

The byssal gland secretes a tough 

thread, the byssus, which anchors the 

animal to the substrate. Seed clams 

set more densely in sand than mud 

(MacKenzie 1979); b i t s o f shel 1 or detritus may also serve as anchors. Distribution of adults sugsests that the 

average size of substrate particles 

exceeds 2 mm diameter (Saila et al. 

1967) although in the laboratory size 

of sand grains was not associated with 

setting (Keck et al. 1974). The seed 

clams prefer setting on a firm surface 

with a thin layer of detritus (Carriker 1952) or on shells coated with mud 

(Carri ker 1961). 

The set may exceed 125 clams/m2 in 

good habitat (Carriker 1961) with extraordinary sets of 270,000/m2 (Dow 

and Wallace 1955), but set is not 

necessarily related to adult concentrations because of movements and 

mortality. Seed clams seek a preferred habitat: a bottom with a few 

small rocks and shells. They discern 

between silt and sand in the laboratory (Keck et al. 1974), explaining 

the selection for sand in nature. 

The seed clams begin a final migration to their ultimate habitat in 

their second summer (Burbanck et a1 . 1956). To move, the clam casts off 

the byssus and uses the foot for locomotion (Belding 1931). On finding 

desirable conditions, the young clam 

spins a new byssus and reattaches itself to a small object. Byssal fibers 

are used for anchorage for about a 

year, until the young clam is 10 mm 

1 ong ; the juveni les then metamorphose 

and assume the burrowing habits of the 

adults. A population in Mainewas 

displaced an average of 30 m by a 

storm (Dow and Wallace 1955). 

The habitat distribution of seed 

clams is altered by predation. Clams 

that set among oyster shel 1s or stones 

are protected (Maurer and Watling 

1973); without cover, seed clams 

largely disappear. Normally they do 

not occur in areas exposed to wave 

action or strong currents (Anderson et 

al. 1978), but in ,I saltwater pond 

they survived better on an unstable 

bottom because crab predation was absent (Carri ker 1959). 

Adul t 

The adult hard clam lives in the 

substrate and burrows with a muscular 

foot. It remains in essentially the 

same location for the remainder of its 

life. In 38 days adults moved laterally an average of 5 cm and a maximum 

of 15 cm from the place where seed 

clams first bedded (Chestnut 1951). 

Clams 20 to 30 mm long traveled up to 

30 crn in 2 months (Kerswill 1941). 

Thus, the adult habitat is determined 

by where the juvenile beds. 

Adults bury deeper in sand (mean 

depth 2 cm) than in mud (mean depth 1 

cm), and small adults burrow deeper 

than larger ones (Stanley 1970). If 

dug up, the hard clam reburrows, and 

if covered, can escape upward (Belding 

1931). A 6.8-cm long clam moved vertically at 44 cm/hr (Kranz 1974). A 

clam can escape 10 to 50 cm of overburden if the sediment dumped is the 

same as surroundings. Forei gn sedi - 

ment reduces escapabi 1 i ty . 

The adult is found in the intertidal and subtidal areas of bays and 

estuaries. Hard clams are most abundant in the lower estuary and are seldom found in the upper estuary (Turner 

1953). In some locations they are absent above the mean tide line (Hi bbert 

1976). Greenwich Cove, Maine, had 

about three times more clams at the 

seaward end of the cove than in the 

upper cove (Tiller 1950). In Rand's 

Harbor, Massachusetts, about 50% of 

the population was on the gravel 

slope, 25% in the muddy channel, and 

25% in the subtidal zone (Burbanck et 

al. 1956). In South Carolina, the 

hard clam is usually absent from open 

estuaries, but is present in small 
channels and protected areas (Anderson 

et a1 . 1978). In Georgia hard clams 

are largely in intertidal areas protected from wave action (Godwin 1968). 

Loosanoff (1946) a1 so mentioned into1 - 

erance to rough waves. There are, however, oceanic populations, e.g., in 

the shoals of Nantucket Sound (Turner 

1953). Several reviews (Belding 1931; 

Loosanoff 1946) state that hard clams 

occur to depths of 15 m; Burbanck et 

al. (1956) reported the maximum depth 

to be 8 m. 

COMMERCIAL/SPORT FISHERIES 

Fisheries 

The hard clam is harvested for 

commerce and recreation. It is more 

widely distributed than any other clam 

species in U.S. waters and is the most 

valuable commerci a1 species (Ritchie 

1977). The fishery is located chiefly 

along the mid-Atlantic Bight. North of 

Cape Cod (Figure 2) and in the Gulf of 

Mexico it is important only in isolated areas (McHugh 1979). In Maine, for 

example, the only hard clam fishery 

was in Casco Bay with good year classes in 1937, 1947, and 1952 (Dow 1955); 

t h e c a t c h is now insignificant (Table 1). 

Hard clams are harvested commercially by bull rakes, hand tongs, and 

power dredges. The power dredge disturbs the substrate no more than bullraking, and all evidence of harvesting 

disappears within 500 days (Glude and 

Landers 1953). A power dredge with an 

escalator caused only temporary di sturbance of the substrate and increased the catch of the more valuable 

small clams relative to larger clams 

(Godcharles 1971). Dredging, however, 

destroys seagrasses and benthic a1 gae 

that recolonize dredged areas slowly; 

thus dredging has a long-term impact. 

The annual landings of hard clam 

along the Atlantic seaboard average 

abaut 14 mi 11 ion pounds (McHugh 1979). 

A1 1 harvest reported is meat wei ght . The harvest in Massachusetts in 1970 

Table 1. Hard clam landings in Maine 

and Massachusetts (Current Fishery 

Statistics, National Oceanic and Atmospheric Admini stration; Hutchinson and 

Knutson 1978; and R. L. Uow, Maine Department of Marine Resources, Augusta). 

Meat Weight (100 kg) 

Year Maine Massachusetts 1 

1976 14 

'NA = Data not available. 
I 

I 

I 

A TL A N TIC OCEAN 

MILES 

0 50 1 W 

0 50 100 

KILOMETERS 

Area of concentration 

Figure 2. Major populations of hard clam in the North Atlantic region. 
was 532,000 lb, worth $1,461,132 (National Marine Fisheries Service 1979). 

In the same year, Maine landed only 

605 lb. Hard clam harvest peaked in 

New England in 1953, with 7.2 million 

pounds (Dow 1977). The U.S. landings 

declined between 1965 and 1975, concomitant with a 300% increase in value 

(Zakaria 1979). About 40% of the U.S. 

harvest is from Great South Bay on 

Long Is1 and (MacKenzie 1977). 

The price of the hard clam varies 

with their size and the season. The 

littlenecks (46 mm) command a higher 

price (860Ibu) than cherrystones (77 

mm, $22/bu), or chowder clams (97 mm, 

$l3/bu) (Ri tchie 1977). Hard clams are 

also processed and marketed as clam 

juice. The market for fresh hard 

clams is made possible because the 

clams remain a1 ive for 1 to 3 weeks 

out of water if kept cool. In contrast, Mercenari a campechiensis does 

not remain a1 i ve nearly as lonq out of 

water even though it "too has -a thick 

she1 1. 

In heavily fished areas clams are 

harvested as soon as they reach a marketable size (Ritchie 1977), i .e., at 

2 to 3 years old. Such harvest is the 

best use of the resource because the 

smaller clams are more valuable, and 

the larger clams grow more slowly. 

Older clams are found only in areas 

that are not actively fished (Greene 

1979); here the maximum life span of 

20 to 25 years may be reached (Belding 

1931 ) . 

Popul at i on Dynamics 

Larval hard clams may be one of 

the most abundant plankters in estuaries. Population densities of 2511 

( C a r r i k e r 1952) and 57211 (Carriker 

1961) have been measured. On the basis 

of these estimates and the bottom 

area, we calculated that there would 

be 50,000 to 1.1 mil 1 ion larvae/m2 in 

an estuary 2 m deep. About 6 mi 11 ion 

larvae are produced from spawning of 

the three pairs of adults found on a 

typical 1 m2 of bottom. 

The number of seed clams that set 

in Little Egg Harbor, New Jersey was 

estimated to be 125/m2 (Carriker 

1961). Populations of seed clams in 

restricted 1 o c a t i ons in Casco Bay, 

Maine, may reach 270,000/m2 (Dow and 

Wall ace 1955). 

Adult population density varies 

widely depending on numerous envi ronmental factors discussed below. In 

Mai ne, popul at ions i n Boothbay Harbor 

ranged from 4/m2 to 13/m2 (Tiller 

1950). In Rhode Island, populations 

in Greenwich Bay ranged from 2/m' to 

12/m2 (Stickney and Stringer 1957). 

Along the Georgia coast abundance 

ranged from 0.1/m2 to 21/m2 (Godwin 

1968). Introduced populations in Great 

Britain reached densities of 6 to 

8/rn2 (Ansell 1963). Biomass (meat 

weight) ranged from 1.6 g/m2 in poor 

habitat to 36 g/m2 in good habitat 

(0 'Conner 1972). In Maine, populations 

of 2,000 bushel s/acre (bu/A) , 1,500 

bu/A, and 1,250 bu/A were estimated 

in three areas (Dow21952). Densities 

of 110/m2 and 540/m were mentioned. 

Natural mortality is enormous in 

the larval and seed clam stages, but 

nil once the shell becomes thick 

enough to resist predators. Based on 

densities of different 1 ife stages, 

monthly mortality coefficients (Z) 

of 1.7 for eggs and 1.5 for larvae 

were calculated. The annual mortality coefficient from seed clam to 

adult was 3.0. Figures are available for calculating mortality coefficients of natural populations. Based 

on nine estimates of adult mortality 

in England, an average annual mortality coefficient was calculated to be 

0.80 (Hibbert 1976). The mortality 

coefficient of adult clams held in 

trays and protected from predators 

in South Carolina was 0.13 (Eldridge 

and Eversole 1982). These mortal i ties 

represent natural mortal ity , which 

equals the instantaneous total mortal - 

ity Z in the absence of harvest. Overwinter mortality of hard clams in two 

sites in Maine was 30% and 40% (Dow 

1965). 
Because of the method of fishing 

described above, it was not possible 

to arrive at a meaningful estimate of 

fishing mortality F. Hard clams tend 

to be completely harvested in any particular bed, resulting in instantaneous mortality of a different sort. 

Mortality of sublegal hard clams was 

estimated to be 30% each time a flat 

was disturbed by digging (Dow 1953). 

Survi vorship follows an exponential decay (Figure 3). Very few of 

the larvae successfully set, and few 

of the seed clams reach adulthood. The 

mortal i ty rate appears to decrease 

slightly in the adults. Obviously survi vorship depends on the microhabitat 

that individuals happen to occupy. 

Survival of hard clams planted in Casco Bay, Maine, was 91% over one summer 

(Gustafson 1954). There is little relationship between stock size and recruitment of young; a few adults produce sufficient offspring to sustain 

the populations. 

GROWTH CHARACTERISTICS 

The hard clam grows rapidly in 

favorable environments. The veli ger 

larvae grow from 10 um to 200 vm in 7 

days (Carriker 1952). At 18°C the larvae increased from 105 um to 183 um in 

20 days, whereas at 30°C they grew to 

this size in 12 days (Loosanoff et al. 

1951). The daily percent growth rate 

of veligers as a function of temperature and salinity is: 

where T is the temperature in "C and S 

is the salinity in ppt (Lough 1975). 

At 20°C and 30 ppt, for example, the 

daily growth would be 68%. 

Seed clams at the end of their 

first summer are 2 to 4 mm in Canadian 

waters, 5 to 7 mm in New York, and 16 

mm in Florida (Ansell 1967b). The 

size reached depends largely on the 

length of the growing season. 

The dependence of adult growth on 

the length of the growing season results in a pronounced latitudinal effect (Figure 4). The annual increment 

LC ddult 

Months 

Figure 3. Survivorship of hard clams 

from eggs to adult, based on a composite of the data cited in the text. 

Estimated Alie 

Figure 4. The increase in shell length 

with age of hard clams from Florida, 

North Carolina, New Jersey, Maine, and 

Prince Edward Island (Ansell 1967b). 
in shell length, estimated from Figure 

4 during the 2 to 5 years of linear 

increase, was 10 mn in Canada, 13 mm 

in Maine, 14 mm in New Jersey, and 23 

mm in North Carolina. The annual rate 

of shell formation was about the same 

between North Carol ina and Florida. 

In Casco Bay, Maine, 20- to 25-mm 

clams increased by 13 to 16 mm in one 

year, whereas 46- to 50-mm clams 

increased only 5 to 12 mm (Wallace 

1952). The daily shell increment is 

about the same during peak growth regardless of latitude (Ansell 1967b), 

again suggesting that it is the length 

of the growing season that is decisive 

in determining annual growth. 

Adult growth rate slows with 

increase in length. Clams of 35- to 

39-mm length grow about three times as 

fast as clams that are 65 to 69 mm 

(Pratt and Campbell 1956). 

Of interest to clam managers is 

the time required to reach the minimum 

legal size, which in most states is 

reached in about 3 years. In Massachusetts hard clams are about 3.5 

years old by the time they reach the 

50-mm legal size. In Rhode Island and 

Connecticut, where growth is faster, 

clams reach the 44-mm legal size in 

about 2.5 years. At the opposite extreme, Florida has a size limit of 56 

mm, and clams reach this size in about 

3 years. In Maine, however, the 51-mm 

size limit is not attained until about 

5 years. 

ECOLOGICAL ROLE 

Feeding Habits 

The adult hard clam feeds by filteri ng out plankton and microorganisms 

that are carried along the bottom by 

currents (Chestnut 1951). Ansell 

(1967a) suggested that hard clams 

depend on plankton abundance before 

and during spawning to furnish sufficient energy to ripen the gonads. If 

the food supply is inadequate, spawning will not occur. Food densities of 

300 mg/l of carbon are optimal for 

feeding (Tenore and Uunstan 1973). 

Food and other materials are 

taken in through the incurrent siphon. 

Tentacles on the siphon detect excessi ve concentrations or oversized particles in the water and cause the siphon to close. The mantle, visceral 

mass, and gills are ciliated and secrete mucus. Particles brought in 

through the incurrent siphon attach to 

the mucus. Deposits on the gills are 

collected by the cilia and carried towards the mouth (Kellogg 1903). The 

palps at the mouth entrance determine, 

by volume, whether the particle mass 

will be ingested or rejected. Only 

small masses are selected for digestion. Complex patterns of cilia movement remove the waste, called pseudofeces, .from palps and gills. Eventually al 1 waste materials are collected 

on the mantle and carried to the base 

of the incurrent siphon, avoiding the 

stream of incoming seawater. When 

sufficient waste has been collected, 

the adductor muscle suddenly contracts, forcibly ejecting a stream of 

water containing the waste mass from 

the incurrent siphon (Kellogg 1903). 

Predation 

Predation is the primary natural 

control of hard clam populations (Virstein 1977). It is preyed on by fish, 

birds, starfish, crabs, and other mollusks. Its defenses are burrowing and 

setting among shells or rocks. Without 

she1 1 or rock cover the juvenile hard 

clam is nearly extermi nated by predators. Survival in penned sites was 94% 

compared to 9% in an unpenned area 

(Kraeuter and Castagna 1980). 

Crabs are the most serious predators of hard clams. The crabs crush 

smaller clams with their claws, but 

chip the edges of the shells of larger 

clams. A rock crab (Cancer irruratus) 

may consume 30 small clams/hr; a mud 

sayi), 14 clams/hr 

. Mud crabs may be as 

One reason crabs are 

effective predators is that they 
2xtract the clam from the sediment. 

The rock crab, blue crab (Call inectes 

s a p i d u s ) , and green crab (Carci ni de? 

-m dig up the clams, whereas mud 

crabs bury themselves to crush the 

clam in place (MacKenzie 1977). Hard 

clams greater than 7 mm long are not 

vulnerable to mud crabs, and clams 

longer than 15 mm are not vulnerable 

to rock crabs (MacKenzie 1977). 

Mollusca are the next most important predators. Oyster dri 11s (Urosalpinx cinerea and Eu leura caudata) and 

the moon snails hces duplicata 

and Lunatia heros) drill holes in the 

shell and remove the clam's body tissues. The whelks (Busycon canal i cul atum and B. caria) chip off the outer 

edge oftheell to make a hole 

through which they insert their proboscises and ingest the clam's soft 

parts by a1 ternately rasping and swallowing (Carriker 1951). Hard clams 

are vulnerable to oyster drills until 

20 mm long and to moon snai 1 s unt.i 1 50 

mm (MacKenzie 1977). In addition, the 

adult hard clam may destroy its own 

larvae by taking them through in the 

incurrent siphon. 

The sea star (Asterias forbesi ) 

pulls the valves of adults apart with 

its tube feet and inverts its stomach 

into the body cavity (MacKenzie 1979; 

Doering 1982a). If a sea star is present, hard clams bury deeper (Pratt 

and Campbell 1956; Doering 1982b). 

Fish. such as flounder. and waterfowl 

feeds on larvae and young (Belding 

1931). 

ENVIRONMENTAL REQUIREMENTS 

Temperature 

Temperature is the most important 

factor in growth and reproduction. The 

harvest of the hard clam in Maine was 

highly correlated (r = 0.80) to the 

August sea temperature 2 years previously (Sutcliffe et al. 1977). Dow 

(1377) recorded a high significant 

correlation between mean annual sea 

temperature a~d populations of adult 

hard clams. 

Spawning occurred over the range 

22' to 3U°C median daily temperature 

in Little Egg Harbor, New Jersey (Carriker 1961) and 21' to 25'C in Barnegat Bay, New Jersey (Kennish and 01 sson 1975). Spawning generally occurred during periods of rising temperature. 

The optimum temperature for larvae was 22.5' to 25'C in brackish water and 17.5' to 30°C at a higher salinity (Davis and Calabrese 1964). 

Carriker (1961) stated that larvae 

tolerated 13' to 30'C. Lough (1975) 

found that eggs required temperatures 

above 7.Z°C, but that larva-! survival 

was highest between 19' and 29.b°C. 

Maximum growth occurred at 22.5' to 

36.5'C. Embryos and vel i ger larvae 

developed abnormally and died at 15'C 

and 33'C; hinged 1 arvae to1 erated 

these temperature extremes (Loosanoff 

et al. 1951). The minimum temperature 

for growth when clams were fed naked 

dinofl age1 lates was 12.5'C, but higher 

temperatures were needed t.o digest 

algae (Davis and Calabrese 1964). 

Thus, temperature, salinity and food 

are a1 1 interrelated. 

The adult hard clam tolerates 

temperatures from below freezing to 

about 35'C. The adult can survive to 

-6'C, but dies when 64% of the tissue 

water has changed to ice (Williams 

1970). Hard.clams located in bars 

elevated above the gradient of the mud 

flats had 100% winter mortality, probably because of freezing (Dow and 

Wall ace 1951). Summer temperatures of 

33" to 34'C are tolerated (Van Winkle 

et a1 . 1976; Mackenzie 1979). 

Sublethal effects of temperature 

include 1 ittle growth below lU°C 

(Pratt and Campbell 1956). Shell 

growth ceases below 8°C (Belding 

1931). The hard clam hibernates at 

temperatures of 5' to 6'C (Loosanoff 

1939). Pumping water required for 

feeding ceases below 6°C and above 
32°C (Hamwi 1968). The extension of 

the siphon also indicates pumping; the 

temperature range for siphon extension 

was 1" to 34°C (Van Winkle et al. 

1976). The limits for growth also may 

depend on the type of food. 

Estimates of the opt irnum temperature for hard clam growth vary from 

about 23°C (Pratt and Campbell 1956) 

to about 20°C (Ansell 1967b). An optimum mean annual temperature of 10°C 

was cited by Dow (1977). Other biological activities may indicate thermal 

optima. Hamwi (1968) found maximum 

pumping at 24" to 26°C. Siphon extension was greatest in the range of 11°C 

to 22°C (VanWinkle et a1 . 1976). There 

were two optima for shell calcium deposition: 13" to 16"C, and 24°C (Storr 

et al. 1982). The optimum range for 

burrowing is 21" to 31°C (Savage 

1976). 

Hard clams are adversely affected 

by rapid temperature changes. Rapid 

temperature fluctuations of + 5°C in 

the discharge from a nuclear power 

plant have caused breaks in she1 1 

growth (Kenni sh 1976). Summer growth 

was reduced 60% to 90% in hard clams 

transplanted to this discharge site. 

Salinity 

The hard clam occurs in environments with salinities ran5ing from 

about 10 ppt to about 35 ppt, with 

possible geographic difference. 

Belding (1931) cited 23 to 32 ppt as 

the general range of tolerance. In 

We1 1 f 1 eet Harbor, Massachusetts, salinity ranged from 20 to 34 ppt (Curley 

et al. 1972). The normal range of 

salinities given by MacKenzie (1979) 

was 15 to 35 ppt. In South Carolina 

hard clams do not usually occur below 

18 ppt (Anderson et al. 1978). Natural beds occur at salinities of 10 to 

28 ppt (Loosanoff 1946). 

Salinity appears to be most 

critical during the egg and larval 

,stages. The embryos in Long Island 

.Sound develop only in the range of 20 

to 32 ppt; at 35 ppt only 10% developed (Davis 1958). Veliger survival was 

low during high rainfall (Carriker 

1961). Veliger growth was best at 20 

to 27 ppt. Castagna and Chanley (1973) 

stated that 1 arvae requi red higher 

salinities than adults and noted that 

metamorphosis to seed clams did not 

occur below 17.5 to 20 ppt. Embryos 

developed normal ly between 20 and 35 

ppt, with an optimum at 27.5 ppt. The 

minimum salinity for larvae was 15 

ppt. In Southampton Water, England, 

young occurred only in years of low 

freshwater inflow from the Kiver Test 

(Mitchell 1974). 

Juveniles and adults close their 

shells during episodes of diluted seawater and hence tolerate low salinities. Juveniles remained alive in 

freshwater for 22 days in the laboratory (Chanley 1958). At 10 ppt they 

began dying at 28 days; at 1U and 15 

ppt there was little feeding or burrowing. Burrel l (1977) reported that 

adult hard clams exposed to salinities 

as low as 0.3 ppt in the Santee River 

system, South Carol i na, survived for 

14 days; less than 5% died because of 

heavy freshwater runoff. Pumping in 

the laboratory ceased below 15 ppt and 

above 40 ppt, with maximum pumping at 

23 to 27 ppt (Hami 1968). The siphons 

were rarely extended in the laboratory 

at salinities below 17 ppt and above 

38 ppt (VanWinkle et al. 1976). The 

optimum salinity range for siphon extension was 24 to 32 ppt. The slightly different findings noted above are 

probably a result of temperaturesalinity interactions. Davis and 

Calabrese (1964) reported an optimum 

salinity for larvae of 27 ppt. At reduced salinities, e.g., 22.5 ppt, the 

temperature to1 erance was reduced. 

Lough (1975) also measured a strong 

i n t e r a c t i o n between temperature and 

salinity; maximum survival of eggs was 

above 28 ppt and above 7.2"C and of 

larvae, between 21 and 29 ppt at 19" 

to 29.5"C. The larvae grew best between 21.5 and 30 ppt at 22.5" to 

36.5"C. 
Di ssol ved Oxygen 

Changes in dissolved oxygen do 

not affect hard clams as much as 

changes in temperature and sal ini ty . All life stages tolerate nearly anoxic 

conditions for long periods, but may 

cease growi ng. Embryos requi re only 

0.5 mg/l dissolved oxygen and die only 

at oxygen levels below 0.2 mg/l (Morrison 1971). Embryos at 0.34 mg/l fail 

to develop to the trochophore stage. 

Larval growth is nearly zero at such 

low oxygen levels. Growth occurs at 

2.4 mg/l but is best at 4.2 mg/l . Adults have tolerated low oxygen 

in the laboratory, but metabolism was 

depressed. The hard clam can tolerate 

less than 1 mg/l for 3 weeks and still 

be capable of reburrowing (Savage 

1976). Growth is surpressed at low 

oxygen. Below 5 mg/l , oxygen consumpti on progressively declines and an 

oxygen debt is incurred (Hamwi 1969). 

The oxygen debt is rapidly repaid in a 

few hours after return to aerobic conditions. Ultimately, hard clams succumb to hypoxi c environments. Hard 

cl ams nearly disappeared from a eutrophic environment near a duck rearing 

area on Long Island, New York (O1Conner 1972). 

Substrate 

Substrate is obviously important 

to a species that burrows, and numerous studies have shown that hard clams 

are associated with a sandy bottom 

rather than a mud bottom (Allen 1954; 

Maurer and Watling 1973; Mitchell 

1974). Water circulation may be the 

decisive element in the distribution 

of hard clams (Greene et al. 1978). 

Because water currents sort bottom 

substrates, the correlation between 

currents and bottom type is high. 

Without attempting to determine 

whether substrate or current is more 

important, we will review the relationships between each and hard clam 

distribution. Even if substrate per 

se is not critical, it does serve as 

an index to water currents. 

A series of studies indicate that 

larvae prefer to set on sand rather 

than mud. Larvae set more densely on 

sand than on mud (MacKenzie 1979), and 

Keck et al. (1974) found an association between grain size and setting; 

781 set on mud of 0.05-mm diameter, 

whereas 2,083 set on sand of 0.5U mn. 

There was not much difference in setting between sand grain sizes of 0.25, 

0.50, 0.71, and 1.00 mm. Larvae discriminated between sand (0.25 mm) and 

mud (0.05 mm). The highest concentration of seed clams was on shells coated with mud (Carriker 1961). The young 

can emerge from a depth of sediment at 

least five times their shell height. 

Abundance is related to substrate 

type. Twice as many hard clams were 

in gravely substrate as in mud (Burbanck et al. 1956). The biomass of 

clams depended on substrate: sand, 

25.52 g/m2; sand without vegetation, 324 g/m ; sand with vegetation, 11.3 g/m ; 

and sand with clayey silt, 1.6 g/m2 

(0' Conner 1972). A1 lee (1923) , however, reported a relative distribution 

of hard clams of 14 in sand, 19 in 

mud, 2 in gravel, 1 in eelgrass, and 4 

in rockweed. Dow (1955) found hard 

clams only in sand-clay-silt, and 

states that in the North Atlantic 

region sand substrate is not the usual 

habitat for the hard clam; they are 

more often found in mud. 

The growth of the hard clam is 

reflected by the substrate type. Clams 

grew 50% faster in sand than in mud 

(Greene 1975). Clams placed in sand 

grew 24% faster than those placed in 

mud (Pratt 1953). There was a high 

correlation (r = 0.88) between shell 

length and substrate particle size 

(Johnson 1977). The distribution of 

hard clams has been related to abundance of particle size greater than 2 

mm (Sai 1 a et a1 . 1967). 

Currents 

Water movement is important to 

all life stages of the hard clam. Currents transport eggs and larvae and 
bring food to the adult. 

Larvae occur in currents of 12 to 

130 cm/sec (Carri ker 1952). Carriker 

(1961) found lower densities near the 

inlet of an estuary where tidal exchange was greatest. The planktonic 

distribution of 1 arvae was not affected by individual tidal stages, but 

summing a1 1 observations suggested 

greatest numbers 3 hr after low tide 

(Moulton and Coffin 1954). 

The growth of adults is correlated with tidal currents (Kerswi 11 1949; 

Haskin 1952; Wells 1957). Hard clams 

grew better at a velocity of 7.5 

cm/sec than in a sluggish slough 

(Kerswi 11 1949). Very strong currents, 

however, may scour the bottom and 

reduce habitat qua1 i ty (We1 1 s 1957). 

Turbidity 

Because hard clams filter water 

to obtain food they also collect other 

suspended materi a1 . Processing this 

material requires energy and clogs the 

fi 1 teri ng apparatus (Pratt and Campbell 1956). Turbidity can thus reduce 

the growth of hard clams. The eggs 

and larvae are also sensitive to 

turbidity . Embryos developed normally in the 

presence of silt or sediment except at 

high concentrations of these suspensions (Davis 1960). Some embryos developed normally with 4 g/l of clay, 

chalk, or Fuller's earth, but the number developing decreased as the concentration increased above 0.75 g/l . Silt above 3 g/l impeded development. 

Sand had 1 ittle effect on eggs except 

for the smallest particles at the 

highest concentrations (Davis and Hidu 

1969). Development was normal at 2 

g/l of particle sizes between 5 and 50 

um diameter. 

Larvae are more sensitive to 

turbidity than are embryos. Ninety 

percent of the larvae died at concentrations of chalk above 0.25 g/l and 

of Fuller's earth above 0.5 g/l (Davis 

1960). The larvae, however, tolerated 

silt of 4 g/l, and in fact, grew 

faster in low concentrations of silt 

than did controls in silt-free water. 

Growth was depressed by 0.5-911 clay 

(Davis and Hidu 1969). 

Little is known about the effects 

of turbidity in adults, despite the 

postulation of adverse effects on 

theoretical grounds. Menzel (1963) 

mentioned that high turbidity in 

summer may have inhibited growth in 

Florida. Pratt and Campbell (1956) 

hypothesized that processing of particles accounted for the reduced 

growth they observed in muddy habitats. Adults in mud expelled pseudofeces 107 timeslhr; in fine sand, 19 

timeslhr; and in coarse sand, 7 times/ 

hr. Rhoads et a1 . (1975) , however, 

believed that a turbid layer near the 

bottom in Buzzards Bay, Massachusetts, 

enhanced the growth of hard clam. The 

layer probably contained detri tal food 

utilized by the clams. 

Habitat Alteration 

Dredging may reduce populations 

of hard clams. Hard clams in the path 

of a dredged channel though a lagoon 

on Long Island, New York were destroyed (Kaplan et al. 1974). Hard clams 

that were not directly disturbed and 

were further than 400 m from the 

dredge site were unaffected. Commercial clammers in this area reported no 

noticeable reduction in harvest the 

following year, whereas scientists 

found a significant reduction in 

standing crop. In Boca Ciega Bay, 

Florida, the hard clam population 

failed to return to previous population level after dredging (Taylor 

and Soloman 1968). 
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the United States. U.S. Fish 

Wildl. Serv. Bur. Commer. Fish. 

Fish. Leaf. 13, 12 pp. 

Lossanoff, V.L., W.S. Miller, and P.B. 

Smith. 1951. Growth and setting 

of larvae of Venus mercenaria in 

relation to temperature. J. Mar. 

Res. 10(1):59-81. 

Lough, R.G. 1975. A reevaluation of 

the combined effects of temperature and salinity on the survival 

and growth of bivalve larvae 

using response surface techniques. U.S. Natl. Mar. Fish. 

Serv. Fish. Bull. 73(1):86-94. 

MacKenzie, C.L.,Jr. 1977. Predation 

on hard clam (Mercenaria mercenari a) populations. Trans. Am. 

Fish. Soc. 106(6):530-537. 

MacKenzie, C.L.,Jr. 1979. Relation of 

biological and environmental factors to soft-shell and hard-shell 

clam management. Pages 67-78 in 

Proceedings of the Northeast clx 

industries: management for the 

future. Ext. Sea Grant Advisory 

Program, U. Mass. and M.I.T. Sea 

Grant Program SP-112. 

Maurer, D., and L. Watling. 1973.. 

Studies on the oyster community 

in Delaware: the effects of the 

estuarine environment on the 

associated fauna. Int. Rev. 

Gesamten Hydrobiol . 58(2): 161- 

201. 

McHugh, J.L. 1979. United States clam 

industry: where is it going? 

Pages 7-24 in Proceedings of the 

Northeast clam industries: management for the future. Ext. Sea 

Grant Advisory Program, Uni v. 

Mass. and M.1 .T. Sea Grant 

Program SP-112: 

McHugh, J.L., M.W. Summer, P.J. Flagg, 

D.W. Lipton, and W.J. Behrens. 

1982. Annotated bibliography of 

the hard clam (Mercenaria mercenaria). NOAA Tech. Rep. U.S~ 

Mar. Fish. Serv. SSRF-756. 845 

PP. 

Menzel, R.W. 1963. Seasonal growth of 

the northern quahog, ~ercenaria 

mercenaria and the southern quahog, M. campechiensis, in alligator H;rbor, Florida. Proc. Natl. 

She1 1 fish. ~ssoc. 52: 37-46. 

Mileikovsky, S.A. 1973. Speed of active movement of pelagic larvae 

of marine bottom invertebrates 

and their ability to regulate 

their vertical position. Mar. 

Biol. (Berlin) 23(1):11-17. 

Mitchell, R. 1974. Aspects of the 

ecolom of the 1 amel 1 i branch 

~erceGri a m e r c e n a r i a in British 

water. Hydrobiol. Bull. 8: 124- 

Morris, P.A. 1973. A field guide to 

the shells of the Atlantic and 

gulf coasts and the West Indies. 

3rd ed. W. J. Clench, ed. Hough- 
ton Mifflin Co., Boston. 330 pp. 

Morrison, G. 1971. Dissolved oxygen 

requirements for embryonic and 

larval development of the hardshe1 1 clam Mercenaria mercenari a. 

J. Fish. Res. Board Can. 28(3): 

379-381. 

Moulton, J.M., and G.W. Coffin. 1954. 

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tidal cycle and during the summer 

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17: 1-51. 

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105 (3): 484-485. 

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142 (1 ): 84-102. 

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Pratt, D.M., and D.A. Campbell. 1956. 

Envi ronmental factors affecting 

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Invest. Conf. Clam Res. 1:18-19. 

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biology of some commerci a1 m o 1 - 

1 uscs of the east coast of North 

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Shellfish Mass., Mass. Dep. Nat. 

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upon the invertebrate animals of 

Vineyard Sound and the adjacent 

waters, with an account of the 

physical characteristics of the 

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on the condition of thesea fisheries of the south coast of New 

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Virstein, R.W. 1977. The importance of 

predation by crabs and fishes on 

benthic infauna in Chesapeake 

Bay. Ecology 58(6): 1199-1217. 

Wall ace, D.E. 1952. Age determination 

and growth rate of soft clams and 

quahogs in Maine. U.S. Fish 

Wildl. Serv. Clam Invest. Conf. 

Clam Res. 3:l-3. 

Wells, H.W. 1957. Status of the name 

Venus. Ecology 38(1): 160-161. 

Williams. R.J. 1970. Freezins tolerM tilus edulii. Comp. 

:!::he:: 3345-161. 

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relates to the hard shell clam of 

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the Northeast Tam industries: 

management for the future. Ext. 

Sea Grant Advisory Program, 

Univ. Mass. and M.I.T. Sea Grant 

Program, SP-112. 
50171 -101 

j 1 

Rt"" NO 

I? 

I 3 Recipaent s Accerr~on No 

FWS/OBS-82/11.18* I - -- -- 

1 

Species Profiles: Life Histories and Environmental Requirements i ~~~~~~; 1983 I 

of Coastal Fishes and Invertebrates (North Atlantic) -- Hard Clam 6 i -. - j 7. Au~hor(~) 18 Pcrlormlng Organizaliln Rent. No. Jon G. Stanley and Rachael DeWitt - _- _ I 9. Performing Organ,ratlon Name and Address .- 

I lo. Pro,ect/Tark/Work Unit No. 

U.S. Fish and Wildlife Service 1 

Maine 313 Murray Cooperative Hal 1 Fishery Research Unit 

i 

! 

University of Maine 

Orono, ME 04469 ~ / (G) 

12. Swnrorlng Organizatlor, Name and Addrerr i 13. Type of Repom L Pcrnod Covered 

National Coastal Ecosystems Team U.S. Army Corps of Engineers 

Fish and Wild1 ife Service Waterways Experiment Station 

u.S. Dept. of the Interior P.O. Box 631 

Washington , DC 20240 Vicksburg, MS 39180 15. Supbkmcnlary Noler 

*U.S. Army Corps of Engineers report No. TR EL-82-4. 1 

! - I - - -- I6 Atrlract (Cnrn~t 200 word~) 

Species profiles are literature summaries on the taxonomy, morphology, range, life history, 1 

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

envi ronmental impact assessment. The hard clam, Mercenaria mercenaria, is the most exten- I sively distributed commercial clam in the United States, but at the northern end of its 

range in the North Atlantic region it has large fluctuations in population. Spawning occurs 

in summer at 18" to 30°C. Eggs and larvae are carried by currents in estuaries for 6 to 12 

days, and then seed clams set on sand or pebbles. Seed clams that lack cover of shells or 

stone 1 argely perish because of predation. Adults filter feed on phytoplankton. Adults r 

survive temperatures of 17" to 30°C and salinities of 10 to 35 ppt, but can withstand 

freshwater for several days by closing the shel 1. LJhen the shel 1 is closed they must 

tolerate anoxic conditions, and they survive less than 1 mg/l oxygen in the water for I 

several days. Even the larvae tolerate 0.5 mg/l of oxygen. 

I 

- - ~ ~ ~ 

17. Cocument Analysis a Dzscriptors 

Estuaries 

Cl ams 

Growth 

Feeding 

0 laentlflen/Ooen Ended Terms I Hard clam Spawning 

Mercenari a mercenari a 

Habitat 

Salini ty requirements 

Teppgr;ia~uy-ett~~,"i remen ts Fi sheri es 

Unl imi ted 

- - . ~~ .. ~ - --- .-. . 

15 SCCUC,:~ tl-s; uh.5 fieoonl 

Unclassified . - - - . - . - 

20 Errurlt> Cjo~l nh15 F>~el 

21 ho 01 Pager 

19 I 

! Unclassified I 

--- (see ~rrs1-Z33.181 OPTIONAL FORM 772 (L-'? 

, 1.1, ' 
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