Biological Report 82 (11.68) TR EL-82-4

b

August 1986

QL

155

.S63

no

11

. 82

.68

Species Profiles: Life Histories and

Environmental Requirements of Coastal Fishes

and Invertebrates (Mid-Atlantic)

SOFTSHELL CLAM

E

Coastal Ecology Group

L

Fish and Wildlife Service Waterways Experiment Station

U.S. Department of the Interior U.S. Army Corps of Engineers

Biological Report 82(11.68)

TR EL-82-4

August 1986

Species Profiles: Life Histories and Environmental Requirements

of Coastal Fishes and Invertebrates (Mid-Atlantic)

SOFTSHELL CLAM

Barbara J. Abraham

Department of Biological Sciences

P.O. Box 6565

Hampton University

Hampton, VA 23668

and

Perian L. Dillon

Department of Marine and Coastal Environmental Studies

Hampton University

Hampton, VA 23668

Project Manager

Carroll L. Cordes

Project Officer

David Moran

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Performed for

Coastal Ecology Group

Waterways Experiment Station

U.S. Army Corps of Engineers

Vicksburg, MS 39180

and

National Wetlands Research Center

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240

L
This series may be referenced as follows:

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

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

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

This profile should be cited as follows:

Abraham, B-J., and P.L. Dillon. 1986. Species profiles: life histories and

environmental requirements of coastal fishes and invertebrates

(mid-Atlantic)--softshell clam. U.S. Fish Wildl. Serv. Biol. Rep. 82(11.68).

U.S. Army Corps of Engineers, TR EL-82-4. 1B PP.
PREFACE

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

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

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

comprehensive sketch of the biological characteristics and environmental

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

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

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

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

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

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

and the U.S. Fish and Wildlife Service.

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

the following addresses.

Information Transfer Specialist

National Wetlands Research Center

U.S. Fish and Wildlife Service

NASA-Slide11 Computer Complex

1010 Gause Boulevard

Slidell, LA 70458

or

U.S. Army Engineer Waterways Experiment Station

Attention: WESER-C

Post Office Box 631

Vicksburg, MS 39180

iii
CONVERSION TABLE

Multiply !Y To Obtain

millimeters (mm)

centimeters (cm)

meters (m)

kilometers (km)

0.03937 inches

0.3937 inches

3.281 feet

0.6214 miles

square meters (m')

square kilometers (km2)

hectares (ha)

10.76 square feet

0.3861 square ,niles

2.471 acres

liters (1)

cubic meters (m3)

cubic meters

0.2642 gallons

35.31 cubic feet

0.0008110 acre-feet

milligrams (mg) 0.00003527

grams (9) 0.03527

kilograms (kg) 2.205

metric tons (t) 2205.0

metric tons 1.102

kilocalories (kcal) 3.968

Celsius degrees 1.8("C) + 32 Fahrenheit degrees

inches 25.40 millimeters

inches 2.54 centimeters

feet (ft) 0.3048 ineters

fathoms 1.829 meters

miles (mi) 1.609 kilometers

nautical miles (rmi) 1.852 kiloneters

square feet (ft')

acres

square miles (mi*)

0.0929 square meters

0.4047 hectares

2.590 square kilometers

gallons (gal) 3.785

cubic feet (ft3) 0.02831

acre-feet 1233.0

liters

cubic meters

cubic meters

ounces (02)

pounds (lb)

short tons (ton)

28.35

0.4536

0.9072

grams

kilograms

metric tons

British thermal units (Btu) 0.2520 kilocalories

Fahrenheit degrees 0.5556("F - 32) Celsius degrees

iv

Metric to U.S. Ctistomary

U.S. Customary to Metric

ounces

ounces

pounds

pounds

short tons

British thewal units
CONTENTS

Page

iii

i v

vi

1

1

3

3

3

3

4

4

5

5

5

6

7

7

8

9

9

10

10

10

11

11

12

15

PREFACE ....................................................................

CONVERSION TABLE ...........................................................

ACKNOWLEDGMENTS ............................................................

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

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

REASON FOR INCLUSION IN SERIES .............................................

LIFE HISTORY ...............................................................

Reproductive Physiology and Strategy .....................................

Spawning .................................................................

Larvae ...................................................................

Juveniles ................................................................

Adults ...................................................................

COMMERCIAL AND SPORT FISHERIES .............................................

Fisheries ................................................................

Population Dynamics ......................................................

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

Feeding Habits ...........................................................

Predators ................................................................

Competitors ..............................................................

Parasites ................................................................

ENVIRONMENTAL REQUIREMENTS AND TOLERANCES ..................................

Temperature ..............................................................

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

Substrate ................................................................

Oxygen and pH ............................................................

Pollutants ...............................................................

LITERATURE CITED ...........................................................

v
ACKNOWLEDGMENTS

We are grateful to Victor Kennedy, Horn Point Environmental Laboratory,

Cambridge, Maryland, for his helpful review of the rough draft of this

manuscript.

vi
to 10 days

@

.-*. .* Fertilization

. - and

Division

t\_

Veliger Larva

Late Veliger

Larva

Metamorphosis

1 to 2 years \

Figure 1. Life cycle of the softshell clam (after Anonymous 1983).

SOFTSHELL CLAM

NOMENCLATURE/TAXONOMY/RANGE

Scientific name........Mya arenaria L.

Preferred common name..... Softshell

clam (Figure 1)

Other common names.....Steamer (New

England), long clam, gaper (Gosner

1978); long-neck clam (Light 1967);

manninose (Chesapeake Bay)

(Pfitzenmeyer 1972)

Class.........................Bivalvia

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

Family........................Myacidae

Geographic range: along the Atlantic

coast from the Subarctic to Cape

Hatteras, less commonly to South

Carolina (Figure 2); introduced on

the west coast from Alaska to San

Francisco (Pfitzenmeyer 1972; Lucy

1976).

MORPHOLDGY/IDENTIFICATION AIDS

The softshell clam has a thin,

gray or chalky-white, egg-shaped shell

1
NEW YORK 1/ i-----q

f ~~~Y~

PE NSYLVANIA

eMARYLAND ,&

_7 BALTIMORE,

:--,

/

_-

i

IASHINGTON. D.C. _ 3:

ATlANTIC OCEAN

b

0N

. . RY KILOMETERS

Figure 2. Mid-Atlantic coast distribution of softshell clams. This species is

found the entire length of the mid-Atlantic region in bays and sounds, to a depth

of about 9 m, in a minimum salinity of 5 ppt (Gosner 1978).

2
that gapes at both ends (Morris 1973;

Gosner 1978). The brittle shell

averages 75-100 mm in length, but

sometimes reaches 150 mm. The valve

surface is roughened and covered with

a dark brown periostracum. The hinge

of the left valve has an erect,

spoon-like tooth, the chondrophore,

which supports the resilium; the right

valve has a corresponding heart-shaped

pit (Gosner 1978). The siphons are

fused into a rigid siphonal process

that is too large to be completely

withdrawn into the shell and is

capable of great elongation (Purchon

1977).

REASON FOR INCLUSION IN SERIES

The softshell clam supplies the

third most important commercial clam

fishery in the United States. Meats

of this species averaged 12% by

volume, and 20% by value, of the

commercial harvest from 1965 to 1975

(Ritchie 1976). In 1984, commercial

landings totalled 7.9 million lb and

were valued at $19.8 million. Maine

landings were 5.2 million lb,

Massachusetts had 1.4 million lb, and

Maryland had 931,000 lb; the Maryland

figure represents a decrease from 1.9

million lb in 1983. The 5-year

average (1979-1983) for the east coast

fishery was 8.4 million lb (Thompson

1985).

Overfishing can drastically

reduce the value of clam beds (see

also "Fisheries"). Because of its

near-shore habitat, this valuable

resource is easily endangered by

pollution (see also "Environmental

Requirements and Tolerances").

Mariculture efforts have been

unsuccessful (Ritchie 1976).

During red tides caused by

Gonyaulax tamarensis, paralytic

shellfish poisoning (PSP) may result

from consumption of softshell clams.

The organisms do not harm the clams,

but result in temporary loss of the

fishery because they endanger humans.

Red tides causing PSP seldom occur

south of Cape Cod (Hanks 1963; Ritchie

1976).

LIFE HISTORY

Reproductive Physiology and Strategy

Softshell clams are dioecious

and nonprotandrous (Brousseau 1978a).

Shaw (1965) found no hermaphrodites in

a sample of more than 800 clams; Lucy

(1976) found 2 in a sample of 2,400.

The sex ratio of clams 25-95 mm long

was 1:l (Brousseau 1978a); Lucy (1976)

also reported a 1:l sex ratio in adult

clams.

Brousseau (1978a) found that

female body size and oocyte production

were correlated. Females less than 40

mm long were never gravid. Brousseau

reported that a 60 mm female produced

a mean of about 120,000 oocytes during

a single breeding season (two spawning

periods) at Gloucester, Massachusetts;

this would make a lifetime production

of about 1.5 million oocytes.

Reproductive processes for both

males and females have been described

as "inactive," "active," "ripe,"

"partially spawned," and "spent"

(Ropes and Stickney 1965; Shaw 1965).

Brousseau (1978a) preferred to divide

the developmental sequence into

"active" and "inactive" stages. Her

active stage included developing (=

active of Ropes and Stickney), ripe,

and partially spawned phases; the

inactive stage included spent and

indifferent (= inactive of Ropes and

Stickney). Criteria for determining

each phase corresponded to those of

earlier authors.

Spawning

There are two cycles of gonadal

development per year in both male and

female softshell clams in Chesapeake

Bay (Shaw 1965; Lucy 1976). These

gonadal cycles result in two spawning

periods. These are mid-March through

3
May and mid-October through November

in the Chesapeake Bay in Virginia

(Lucy 1976). Pfitzenmeyer (1965)

described two periods of spawning in

the Chesapeake Bay in Maryland, the

first in May-June and the second in

September-October.

Spawning depends upon water

temperature; therefore its timing

varies with latitude. Spring spawning

in Chesapeake Bay occurs when the

water temperature reaches 10 OC and

may continue at water temperatures up

to 20 OC; autumn spawning occurs when

water temperature has fallen from the

summer high of 25 OC to 20 OC (Lucy

1976).

According to Brousseau (1978a),

temperature is more important in

timing gonadal development than in

triggering release of gametes. She

found that at Gloucester,

Massachusetts, spawning occurred at a

surface (1 m) water temperature of 4-6

OC in March-April, but at 15-18 OC in

June-July.

Lucy (1976) noted that rapid

changes in water temperature in spring

may be detrimental to gamete

development. It takes about 60 days

for the water to fall from the maximum

summer temperature to the autumn

spawning temperature; the time from

minimum winter temperature to the

spring spawning temperature may be as

little as 40-42 days. Lucy recorded

that 18%-23% of softshell clams

spawned in the spring when water

temperature rose over a 62-day period;

only 2%-4% spawned when spawning

temperature was achieved in 40 days.

Shaw (1965) also reported a spring

spawning failure in Chesapeake Bay,

although he was not able to determine

the limiting factor.

Larvae

The fertilized egg takes about

12 h to develop into the planktonic

trochophore larva in cold New England

waters, and probably less in the

warmer waters of the mid-Atlantic

Region (Hanks 1963). This top-shaped,

ciliated larva feeds on suspended

particles. Within the next 24-36 h

the trochophore develops into the

veliger larva, which has two

calcareous valves. This stage remains

suspended in the water column by means

of a ciliated velum and drifts in

estuarine and ocean currents feeding

on phytoplankton. Veligers are

important food for the larvae of a

number of fish species. In samples

collected at water depths of l-17 m

off the coast of Maine, the density of

veligers was as high as 1,000

larvae/m3 (Anonymous 1983).

The veliger stage lasts for 2-6

weeks, depending on water temperature.

The mean period that larvae spend in

the water column before setting is

shorter during the spring spawning (4

weeks) than during the autumn (6

weeks) in Chesapeake Bay. The rate of

larval development is faster in spring

because the water temperature is at

the warmer end of the spawning

temperature range (Lucy 1976).

Juveniles

When the veliger reaches a length

of about 200 pm, its shell thickens, a

muscular foot replaces the velum, and

a byssal gland develops (Hanks 1973;

Perkins 1974; Lucy 1976). This late

veliger (the "setting stage") settles

to the substrate to become a juvenile

clam. A byssus (sticky thread) is

secreted to anchor the young clam to

the substrate. This may be retained

until the clam is 7 mm long (Perkins

1974). Adult habits are slowly

acquired, and bysally attached young

temporarily retain an active foot for

locomotion (Green 1975). Although

usually attached to the substrate by

the byssus, the juvenile clam is able

4
to move and attach itself in a more

favorable location (Hanks 1963).

Eventually the byssus is shed and

the adult lifestyle adopted: the

young clam burrows and becomes sedentary. The final settling location is

usually a sandy bottom with less than

50% silt. Very young softshell clams

apparently cannot tolerate highly

silted substrates (Pfitzenmeyer 1972).

It took 35 days for a clam to grow to

2 mm and an additional 95 days to

reach 12 mm at Gloucester,

Massachusetts (Brousseau 1978a).

Clams up to 12 mm move about

considerably over the substrate, and

only burrow down 1-2 cm. This exposes

them to wave action, and they are

moved shoreward and concentrated at

the break in the beach profile where

the slope increases suddenly. The

observed clumped distribution of

juvenile clams is therefore, according

to Lucy (1976), primarily due to

hydrodynamics, rather than predation

or other factors.

Young softshell clams may achieve

a length of 30 mm by the first winter

(Perkins 1974). Andrews (1970)

reported that it takes 18 months from

setting to steamer size in Chesapeake

Bay; according to Hanks (1963), the

acceptable commercial size of 5 cm is

achieved in 1.5-2 years in the same

area.

Adults

Maturity may be achieved in 5

years, and clams may reach 15 cm at an

age of 8 years. The lifespan has been

given as lo-12 years or, rarely, as

many as 19 years (Perkins 1974;

Brousseau 1978a). However, internal

shell growth lines indicate a lifespan

of as many as 28 years (MacDonald and

Thomas 1980).

Adult softshell clams inhabit

sandy, sand-mud, or sandy-clay bottoms

of bays and inlets. Density is

usually six to eight clams per square

foot; it is highest at depths of 3-4

m, temperatures less than 28 OC and

salinities not less than 4-5 ppt

(Pfitzenmeyer and Drobeck 1963; LUCY

1976). Adult clams burrow as far as

30 cm into the sediment, but the

siphonal process extends to the

sediment surface (Kennedy and Mihursky

1971).

COMMERCIAL AND SPORT FISHERIES

Fisheries

The softshell clam is a valuable

commercial species (see also "Reason

for Inclusion in Series") and is also

harvested recreationally in New

England. It is a popular delicacy

when fried, steamed in broth, or baked

in fire pits under seaweed (Hanks

1963; Lucy 1976).

Softshell clams have been

harvested commercially by hand on the

tidal flats of New England since the

mid-1800's. The first commercial

fishery began with a demand for salted

clams to be used as bait by cod

fishermen on the Grand Bank; more

lately, fresh, frozen, and canned

clams have been an important consumer

item (Hanks 1963). The hydraulic

escalator dredge, which efficiently

harvests subtidal clams, was introduced in 1951 (Ritchie 1976). This

allowed commercial harvesting in the

Chesapeake Bay, and

fishery began

the Maryland

to develop rapidly,

reaching a peak of 7.9 million lb in

1969 (Lucy 1976; Ritchie 1976). A

commercial fishery never developed in

Virginia because the best clam beds

are in oyster areas; oystermen claim

that the escalator dredge silts over

the oysters (Lucy 1976).

The New England beds were

overharvested in the late 1940's, and

declined from 15 million lb in 1940 to

5
2 million lb in 1958 (Lucy 1976;

Ritchie 1976). The decline is also

partly attributed to predation by

crabs (Hanks 1963). Between 1956 and

1970 the Maryland catch exceeded that

of New England, but in 1970 and 1971

the Maryland beds also showed signs of

overfishing (Ritchie 1976; Lucy 1976).

In June 1972 Tropical Storm Agnes

decimated the Maryland clam beds (see

also "Temperature" and "Salinity");

mortality was as high as 90% in some

upper bay areas (Lucy 1976). A

successful spawn in the autumn led to

rapid recovery; however, Maryland

closed its beds in 1973 because of

high levels of bacteria (Chesapeake

Research Consortium, Inc. 1976). In

1975 Maryland landed 1 million lb; New

England harvested 7.5 million lb.

In the Maryland Chesapeake Bay

acceptable commercial size for

softshell clams is 51 mm (Hanks 1963),

and the maximum allowable catch is 40

bu per day (Andrews 1970).

Population Dynamics

Larval softshell clams are among

the more abundant plankters in

estuaries. In sedentary bivalves,

settlement of recently metamorphosed

planktonic larvae is the only significant source of recruitment. Sizefrequency distributions show that

recruitment of young represents a

large proportion of the population,

but average settling rates of juveniles are low. Observed survivorship

at Jones River, Massachusetts, was

less than O.l% of the estimated egg

production (Brousseau 1978a); survivorship follows an exponential decline

(Figure 3).

Therefore, in the life history of

this species, high fecundity is offset

by high mortality during pelagic life,

metamorphosis, and early settlement

(Brousseau 1978b). Because spawning

cycles in which the greatest number of

oocytes were released did not

correlate with the periods of highest

recruitment, Brousseau considered that

Age (years)

II I I I

2040 60 60 85

Shell length (mm)

Figure 3. Survivorship curve for

softshell clams at Gloucester_

Massachusetts. (after Brousseau 1978b)

sources of mortality (predation,

disease) and the character of the

substrate were more important in

explaining fluctuations in recruitment

than was variability in fecundity.

The numbers of seed clams that

set at Gloucester Point and Fox Point,

Virginia, were reported to be

114-431/m2 and 133-578/m2, respectively (Lucy 1976). A good set

(3,000/m2) yields sufficient clam

densities (lOO-200/m2) to be

considered commercially productive.

Lucy has shown that predation and

stress from thermal and osmotic

fluctuations exact heavy tolls on

newly set bivalves. In the York River

in Virginia, he found 65%-100%

reduction in density of fall-spawned

juveniles by the next summer; the

lower figure occurred only once.

6
Adult population densities vary

according to natural mortality and

commercial harvesting. Estimates of

fishing mortality of adults in

Massachusetts have been reported by

Hruby (1981) to be 50%-60% of the

total population, or 3.2 million

animals (2,000 bu). In Massachusetts,

any clam over 51 mm (2 inches) may be

harvested; this effectively reduces

the lifespan to 2 years and may

endanger the long term stability of

<the resource (Brousseau 1978a).

Natural mortality rates decrease with

size and age. Mortality in fall and

winter may be considerably lower than

in summer, because predators become

inactive (Brousseau 1978b).

Factors influencing growth of

softshell clams include seasonal

availability of phytoplankton, water

currents, clam density, mudflat

topography, sediment type, tidal

level, water temperature, and spawning

activity (Anonymous 1983). Most rapid

growth occurs in sediments which are

easily penetrated (not coarse gravel

or hard clay), when clams are not

spawning, and when density is less

than 25 clams per square foot. Growth

slows as clams get larger.

In addition to the use of annular

ring measurements and mark-recapture

methods for determining growth rates,

Brousseau (1979) used the von

Bertalanffy equation, which relates

age and linear size:

L = a (1 - bemkt)

where L = linear size, t = age in any

convenient time unit, and a, b, and k

= constants. Plotted results (Figure

4) are decaying exponential curves.

ECOLOGICAL ROLE

Feeding Habits

Adult softshell clams feed by

filtering microscopic particles of

organic material, including detritus

1,8IIILI~f

1 3 5 7 9

Age (days) X IO- 3

Figure 4. Von Bertalanffy growth

curves for softshell clams from the

Bay of Fundy (l), Georgetown Island,

Maine (2), Gloucester, Massachusetts

(3), Monomoy Point, Massachusetts (4),

San Juan Island, Washington (5), and

Roskilde Fjord, Denmark (6) (after

Brousseau 1979).

and plankton, suspended in seawater.

Coe and Turner (1938) suggested that

softshell clams depend on abundant

plankton before and during spawning to

produce adequate gametes. Softshell

clams can also absorb and use

dissolved organic material, although

its importance has been difficult to

estimate (Stewart 1978).

Organic materials are drawn in

through the inhalent siphon where

branched cilia strain out suspended

particles as small as 2 pm in

diameter. Mucus, secreted by the

mantle, gills, and visceral mass,

collects the incoming particles, which

are carried to the mouth by cilia. At

7
the mouth, the labial palps sort and

reject large particles. Digestion

begins in the stomach and continues

intracellularly in the digestive

gland. Waste materials are expelled

through the exhalent siphon (Anonymous

1983).

Predators

Most predation on softshell clams

is on the larvae and juveniles. In

Chesapeake Bay, the jellyfish

Chrysaora quinquecirrha and the comb

jelly Mnemiopsis leidyi are efficient

feeders on the planktonic larvae of

infaunal bivalves (Holland et al.

1980). According to Andrews (1970)

cyprinodont fishes are voracious

feeders on bivalve larvae in ponds and

shallow areas.

Serious invertebrate predators on

juveniles in the lower Chesapeake Bay

include the oyster drill Urosalpinx

cinerea, the thick-lipped oyster drill

Eupleura caudata, several kinds of

crabs, and the flatworm Stylochus

ellipticus. Less important predators

in the mid-Atlantic Region include the

starfish Asterias, the horseshoe crab

Limulus polyphemus, the channeled

whelk Busycon canaliculatum, and the

lobed moon snail Polynices duplicatus

(Andrews 1970; Lucy 1976; Ritchie

1976).

The most important invertebrate

predator on softshell clams north of

Cape Cod is the green crab, Carcinus

maenas (Hanks 1963; Ritchie 1976;

Anonymous 1983); this species ranges

southward into New Jersey. The blue

crab, Callinectes sapidus, which

ranges southward from Cape Cod,

replaces the green crab as a major

predator on clams in Chesapeake Bay

(Andrews 1970; Lucy 1976; Holland

et al. 1980; Blundon and Kennedy

1982a).

Lucy (1976) strongly implicated

blue crabs as the major factor

contributing to mortality of juvenile

clams. In one experiment, he took

three 145-cm2 cores and found the

density of clams 4-18 mm long to be

4,360-6,000/m2; 1 month later, four

cores contained no clams, but broken

shells were scattered on the sediment.

Lucy considered the blue crab to be

the most important predator on

softshell clams for two reasons: abundance and ability to dig down 6-12 cm

into the substrate.

Although blue crabs bury

themselves in the mud during the

winter, which allows the fall spawn to

establish in their absence, the

juveniles cannot dig deeply enough to

avoid predation the following spring.

Lucy found juveniles of length 22-49

mm in July to be buried 6-16 cm.

Working in the Maryland Chesapeake Bay, Blundon and Kennedy (1982b)

found the fall set to be buried below

10 cm by the next May, and to achieve

maximum burial (25 cm) by June-July.

They showed that clams below 10 cm

were foraged by blue crabs less

efficiently than were those closer to

the surface.

Although invertebrate predation

has been cited as the major factor

determining post-settling survival of

softshell clams, predation by fish

may also be important. Kelso (1979)

showed that on intertidal mudflats in

Essex Bay, Massachusetts, predation by

mummichogs (Fundulus heteroclitus) may

equal or exceed that by invertebrates

for clams less than 12 mm long.

Bottom-dwelling fish also prey on

softshell clams. Spot (Leiostomus

xanthurus) are the most abundant

predacious fish which feed on infauna

of mesohaline regions in the

Chesapeake Bay (Holland et al. 1980).

As shown by stomach content analysis,

softshell clams are the preferred

food of the cownose ray (Rhinoptera

bonasus; Merriner and Smith 1979).

The American eel (Anquilla rostrata)

was found to subsist substantially

on young softshell clams (Wenner

and Musick 1975); winter flounder
(Pseudopleuronectes americanus) also

eat them (Gosner 1978).

Birds and mammals that prey on

clams in the mid-Atlantic region

include herring gulls (Larus

argentatus), diving ducks such as the

canvasback (Aythya valisineria) and

the oldsquaw (Clanqula hyemalis),

tundra swans

and raccoons

(Cyg;;soc;;Aumbianus),

r lotor;

MacGinitie and MacGinitie 1968; Gosner

1978; Holland et al. 1980).

Competitors

No data concerning intraspecific competition among softshell

clams are available. Since adults

are randomly distributed, there seems

be no

E74).

territoriality (Perkins

On a Maine tidal flat, Gemma

gemma has occurred with Mya for more

than 1,000 years (Bradley and Cooke

1959). m is numerically dominant

in the muddy areas, Gemma in the

sandy areas. Bradley and Cooke found

the two species to be incompatible

because of competition for food "or

for some other reason." Sanders

et al. (1962) thought that in

Massachusetts, large populations

of this small species excluded the

larger softshell clams by consuming

all available food. Kennedy and

Mihursky (1971) noted that the greater

resistance of Gemma to high temperature might allow this species to

occupy habitat vacated by the less

tolerant m, thus precluding resettlement. Thermal discharae

plants into Chesapeake Biy

might therefore influence

competitive interaction.

from power

in summer

Gemma-&a

Holland et al. (1980) suggested

that competition has a relatively

unimportant role in determining the

structure of Chesapeake Bay infaunal

communities, because he thought that

resources are probably rarely, if

ever, limiting.

Parasites

Observed incidence of parasites

is low (Hanks 1963). The pea crab,

usually considered an internal

commensal, has been reported to be an

endoparasite of adult softshell

clams (Ricketts and Calvin 1968).

MacGinitie and MacGinitie (1968)

reported Malacobdella grossa, a small,

white, fluke-like nemertean, in the

mantle cavity. Other parasites

include the sporocyst and cercaria

stages of various trematodes such as

Cercana !QE, Gymnophallus, and

Himasthla quissetensis (Sinderman

1970), and a ciliate, Trichodina

myicola (Hanks 1963).

The incidence of observed disease

is low and probably does not affect

clam production (Hanks 1963; Ritchie

1976). Several pathological

conditions have been observed: "water

belly," which causes watery, thin

meats unfit for market and may be due

to a nutritional deficiency, parasite,

or disease (Hanks 1963); neoplasia, an

uncontrolled new growth of tissue

which may be benign or malignant;

haemocytic proliferation, an increase

in blood cells; hypoplasia, defective

or incomplete development; hyperplasia, an increase in the number of

cells which ceases when the stimulus

is removed; and lipofucsin, fatty

pigments which may be related to

degenerating parasites (Walker et al.

1981).

Recent investigators have been

interested in a possible correlation

between pollution and increased

disease. Walker et al. (1981) compared the incidence of five pathological conditions (neoplasia,

haemocytic proliferation, hypoplasia,

hyperplasia, lipofucsin) in softshell

clams from 17 areas on the U.S. east

coast. They could not infer cause and

effect from their data, which seemed

to indicate a relationship between the

pollution histories of the areas and

the incidence of pathology in softshell clams. Although neoplasia was

9
found in both clean and polluted

environments, occurrence of both

neoplasia and hyperplasia affecting

more than 10% of the clam population

was found more often in areas impacted

with refined petrochemicals.

Brown (1978) reported on a survey

of diseases, which he divided into

three categories: disturbances of

growth (neoplasia, hyperplasia,

hypoplasia); reaction to injury

(haemocytosis, inflammation); and

parasites (bedsonia, protozoa,

metazoa, "accumulation of orange-brown

bodies"). Prevalence of these

conditions varied between sites, suggesting an environmental influence.

ENVIRONMENTAL REQUIREMENTS AND

TOLERANCES

Temperature

The most important factor in

growth and reproduction of softshell

clams is temperature (see also "Life

History"). Stewart and Bamford (1976)

found that uptake of the dissolved

amino acid L-histidine by clams 80-100

mm long increased with increasing

temperature. Respiratory rate also

varied directly with temperature;

however, high temperatures (30 "C)

depressed metabolism of coldacclimated clams (Kennedy and Mihursky

1972).

In a principal components

analysis, Appeldoorn (1983) found

"northness" to account for the most

variation in clam growth among 25

sites from Nova Scotia to Maryland.

He found a distinct latitudinal trend,

with growth decreasing toward the

north. Temperature, tidal height,

tidal position, and substrate all

systematically varied with latitude,

but temperature was the dominant

factor.

Softshell clams are eurythermal

(have a wide tolerance range for

temperature; Perkins 1974; Loi and

Wilson 1979). Overwintering clams can

survive temperatures below freezing

(Ricketts and Calvin 1968). The 24-h

,LY#

values for summer-acclimated

s were 32.5-34.4 OC (Kennedy and

Mihursky 1971). As temperature

approached the upper lethal limit, a

1 OC increase often made the

difference between total mortality and

none (Kennedy and Mihursky 1971).

Juveniles up to 15 mm long have

significantly higher heat tolerance

than do adult clams (Kennedy and

Mihursky 1971, 1972). This tolerance

is an adaptation for surviving at or

near the sediment surface, where

temperatures are higher. During times

of short-term natural heat stress,

adult clams survive by withdrawing the

siphons and living anaerobically in

the cooler mud (Kennedy and Mihursky

1971). In the Virginia Chesapeake Bay

at Gloucester Point, the summer water

temperature is potentially lethal near

the low watermark (33-35 "C), but the

temperature 15 cm into the sediment is

never over 30.6 OC (Lucy 1976). It

is partly due to high temperature

stress on the juveniles that softshell

clams in the southern part of their

range are not intertidal as in the

northern part (Pfitzenmeyer and

Drobeck 1963).

Anderson (1978) found that subtidal clams in Maine, if acclimated to

warm water, were metabolically temperature independent from lo-25 OC.

He also found that subtidal and high

intertidal clams were better able to

withstand high (25 "C) temperature

than low to mid-intertidal clams.

Kennedy and Mihursky (1972) also found

temperature compensation; at nonstress

temperatures, older clams compensated

as effectively as did younger.

Salinity

According to Holland et al.

(1980), salinity is the major environmental factor controlling presence of

Chesapeake Bay infaunal species.

Softshell clams are widely euryhaline

10
(Perkins 1974), being primarily

marine in the northern part of their

range and estuarine in the southern

(Pfitzenmeyer 1965).

The estuarine habitat in which

the softshell clam lives is constantly

exposed to changes in salinity from

about 10 to 25 ppt, mainly as a result

of freshwater runoff. Under normal

conditions, salinity fluctuations do

not have a deleterious effect on

softshell clams, which are isoconformers (Stewart and Bamford 1976).

No softshell clams died when they were

held at 27 ppt, then conditioned to

12.5 ppt, and finally subjected to a

reduction in salinity of 2.5 ppt per

week to a final salinity of 2.5 ppt

(Lucy 1976). However, rate of feeding

decreased as salinity fell from 31 ppt

and ceased at 4 ppt (Perkins 1974).

Stewart and Bamford (1976) reported a

significant reduction in uptake of two

amino acids by adult clams when

salinity was reduced to 75%, 50%, 25%,

and 10% of the 34 ppt in which they

B

had been collected.

Small clams are less tolerant of

low salinity than larger ones. When

placed in freshwater, clams 2-4 mm

succumb within 30-40 h, but clams over

20 mm survive more than 50 h. Within

their tolerance limit of 4 ppt, clams

can survive a change of 18 ppt in a

few minutes (Perkins 1974).

Low salinity coupled with high

temperature can cause mass mortality

of softshell clams. This was seen in

1972, after Tropical Storm Agnes

dropped large amounts of rain (over 12

cm throughout the watershed; 45 cm in

isolated areas) and brought high air

temperature, killing an estimated 90%

of the clam population in some areas

of the Chesapeake Bay (Chesapeake

Research Consortium 1976; Merrimer and

Smith 1979). Lucy (1976) measured

salinities of 2-6 ppt for 1 week at

various locations in the Virginia

portion of the bay; subsurface water

temperatures in the nearshore zone

were 24-25 OC.

Substrate

Within a salinity zone, substrate

is a primary determinant of the

distribution and abundance of infaunal

species (Holland et al. 1980). Softshell clams inhabit stiff sands and

muds which will not collapse against

the shell valves when they are closed

(Perkins 1974; Lucy 1976; Purchon

1977). Appeldoorn (1983) found that

sediment coarser and grainier than

silt or clay was beneficial to growth;

it allowed ample water percolation and

drainage and was associated with a

good current regime. Loi and Wilson

(1979) reported more clams on substrate with a high sand/clay ratio and

low organic content than on substrate

with high clay and organic content.

In laboratory flow-through experiments on mud, sand, gravel, and in

nets, Newell and Hidu (1982) found

significant differences in shell

length, dry meat weight, chondrophore

growth increment, and percent shell

weight. Growth was more rapid in

finer sediments than in coarser sediments or in nets; this might have been

partly due to food availability.

Oxygen and pH

The softshell clam is little

affected by oxygen fluctuations.

Juvenile and adult stages are able to

withstand long periods of anaerobiosis. At 14 OC, adult clams use

30-40 ul of oxygen per gram of body

weight per hour. Under experimental

conditions they can live for 8 days in

a medium lacking free oxygen; they

show no adverse effects after being

placed back into normal environmental

conditions, except for a decrease in

stored glycogen and an increase in

metabolic rate (Ricketts and Calvin

1968).

Clams normally use 3%-10% of

available dissolved oxygen, but after

21 h of anaerobiosis, as may occur

during low tide, the ventilation

current increases. Normal levels are

\

11
not restored for 3-4 h; meanwhile,

oxygen debt causes oxygen use to

increase to 25% (Van Dam 1935, cited

by Nichol 1967).

Digestion and absorption are not

adversely affected by decreasing pH

when the shell is closed for periods

of time (Stewart and Bamford 1976).

In laboratory experiments, uptake of

L-alanine was not significantly

different over a pH range of 6.2-8.8.

Pollutants

The shallow water habitat of the

clam is especially vulnerable to

pollution from urban and industrial

development. Because the adults are

sedentary, repopulation of destroyed

clam beds requires several years for

sufficient larval recruitment and

growth. In southern New England,

urbanization and pollution have

resulted in the closure of numerous

tidal flats to shellfishing (Whitlach

1982). For example, 30% of softshell

clam beds at Gloucester,

Massachusetts, were nonharvestable in

1980 because of discharge of untreated

sewage. This was estimated as a

$332,400 per year loss to the

community (Hruby 1981). Maryland's

Chesapeake Bay beds were also closed

in 1973 because of a high coliform

count; it is not known whether this

was due to Tropical Storm Agnes or

sewage pollution (Chesapeake Research

Consortium, Inc. 1976).

Chlorine is used as a disinfectant of sewage effluent and as a

biocide to combat fouling of condenser

tubes in steam electric power plants.

Modern facilities are frequently

located near estuaries and the ocean,

and meroplankton such as clam larvae

may be circulated through the cooling

systems, thus coming into contact with

this chemical. Larvae near the

outfall may also be exposed.

Roosenburg et al. (1980) reported

direct relationships between mortality

and both increasing concentration of

chlorine-produced oxidants (CPO) and

increasing exposure time. Setting

(pediveliger) larvae were more

tolerant than were young (straighthinge veliger) larvae.

Baltimore Harbor, a tributary of

the northern Chesapeake Bay, has

received and is receiving large

quantities of chemical pollutants.

Tsai et al. (1979) found lead,

chromium, zinc, copper, arsenic,

PCB's, and hexane extracts in the

harbor sediments. At five stations

the sediments were toxic enough to

produce 50% mortality in softshell

clams within 24 h. These investigators reported that clams can contract for a finite time to reduce the

surface area exposed to a toxin or reduce the rate of pumping water through

the gills.

Bioassays were conducted for

copper, zinc, nickel, manganese, and

lead in raw seawater under ambient

summer conditions (30 ppt salinity,

22 OC, pH 7.95, dissolved oxygen above

4.0 mg/l) by Eisler (1977). The 168-h

;i50's (mg/l) were as follows:

0.035; Cd, 0.150; Zn, 1.55; Pb,

8.80; Mn, 3.00; and Ni, 50.00.

Additional tests for zinc and cadmium

at 30 ppt at 17.5 OC in winter showed

that survival of clams increased with

decreasing temperature.

The hydrocarbons DDT, dieldrin,

endrin, a-endosulfan and B-endosulfan

all affect the rate of contraction of

isolated clam ventricles. Effects

range from reduction of amplitude to

complete arrest, depending on the

concentration and chemical (Roberts

1975).

Softshell clams are more seriously affected by oil pollution than

other co-occurring commercial shellfish. Effects may be outright death,

gonadal tumors, or stunting of growth.

The threat is most serious in the

Chesapeake Bay because of thermal

stress at the southern limit of their

distribution (Rose 1974).

12
Ecological, morphological, and

behavioral characteristics make

softshell clams particularly subject

to the deleterious effects of oil

spills. They tend to live in fine

sediments, in a low-energy environment

where oil persists for long periods

after a spill. An immediate

consequence of spilled oil is the

smothering of clams' burrows, which

reduces oxygen levels and promotes

bacterial action (Thomas 1973). The

siphons of the softshell clam prevent

it from completely closing its shell;

consequently the mantle and gills are

constantly exposed to sediments and

interstitial water. Since food is

obtained .from the boundary water,

which is in intimate contact with the

sediment surface, any oil leaching

from the sediment is taken in by the

clam (Gilfillan and Vandermeulen

1978).

In 1972 at Casco Bay, Maine,

oiled clams showed marked reduction in

metabolism and growth for 3 years

after a spill. In Seaport, Maine,

growth, survival, and recruitment were

reduced for up to 5 years after a

spill (Gilfillan and Vandermeulen

1978). Shortly after a Bunker C oil

spill, MacDonald and Thomas (1980)

compared clams in the polluted area

with those in an unpolluted 'area.

They repeated the comparison 9 years

later and found growth to be retarded

by 2-5 years in the oiled clams.

Petroleum hydrocarbons at rates

up to 200 ug/g tissue were found in

clams by Gilfillan and Vandermeulen

(1978). When compared to controls,

these clams exhibited decreased tissue

and shell growth and carbon flux.

Crude and refined hydrocarbons in

increasingly greater concentrations

increase mucus secretions and decrease

tactile response. Increased mucus

production drains the energy reserves

of clams, clogs the gills and mantle

cavity, and disrupts the normal feeding mechanism. Petroleum hydrocarbons

also affect respiration; very low concentrations cause a doubling of the

respiratory rate, but high concentrations cause depression of the rate.

Gill cilia can remove oil globules of

1,030 urn in diameter from sea water,

and handle the globules in the same

manner as food and detritus particles

(Fong 1976). An increase in respiratory rate increases filtration and

therefore mortality.

13

L

L

Anderson, G. 1978. Metabolic rate,

temperature acclimation and

resistance to high temperature of

soft-shell clam, -w arenaria, as

affected by shore level. Comp.

Biochem. Physiol. 61(3A):433-438.

Andrews, J.D. 1970. The mollusc

fisheries of Chesapeake Bay (USA).

Mar. Biol. Assoc. India, Proc. Symp.

Mollusca. Pt. 3:847-856.

Anonymous. 1983. Increasing clam

harvests in Maine: a practical

guide. Maine/New Hampshire Sea

Grant College Program, University of

Maine, Orono, and Maine Department

of Marine Resources. 61 PP.

Appeldoorn, R.S. 1983. Variation in

the growth rate of Mya arenaria and

its relationship to the environment

as analyzed through principal

components analysis and the w

parameter of the von Bertalanffy

equation. U.S. Natl. Mar. Fish.

Serv. Fish. Bull. 81(1):75-84.

Blundon, J.A., and V.S. Kennedy.

1982a. Mechanical and behavioral

aspects of blue crab. Callinectes

sapidus (Rathbun), predation on

Chesapeake Bay bivalves. J. Exp.

Mar. Biol. Ecol. 65:47-65.

Blundon, J.A., and V.S. Kennedy.

1982b. Refuaes for infaunal

bivalves from biue crab, Callinectes

sapidus (Rathbun), predation in

Chesapeake Bay. J. Exp. Mar. Biol.

Ecol. 65:76-81.

LITERATURE CITED

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Serv. Fish. Bull. 137:304-334.

Brousseau, D.J. 1978a. Spawning

cycle, fecundity, and recruitment in

a population of soft-shell clam, Mya

arenaria, from Cape Ann,

Massachusetts. U.S. Natl. Mar.

Fish. Serv. Fish. Bull.

76(1):155-166.

Brousseau, D.J. 1978b. Population

dynamics of the soft-shell clam Mya

arenaria. Mar. Biol. (Berl.)

50:63-71.

Brousseau, D.J. 1979. Analvsis of

growth -rate in m arenaria using

the von Bertalanffy equation. Mar.

Biol. (Berl.) 51:221-227.

Brown, R.S. 1978. A disease survey

of New England soft-shell clams, w

arenaria. Proc. Natl. Shellfish.

Assoc. 68:75. (Abstr.).

Chesapeake Research Consortium, Inc.

1976. The effects of Tropical Storm

Agnes on the Chesapeake Bay

estuarine system. Chesapeake Res.

Consort. Publ. 54. 639 pp.

Coe, W.R., and H.J. Turner, Jr. 1938.

Development of the gonads and

gametes in the soft-shell clam (m

arenaria). J. Morphol. 62:91-111.

Eisler, R. 1977. Acute toxicities of

selected heavy metals to the

soft-shell clam Mya arenaria. Bull.

Environ. Contam. Taxicol.

17(2):137-145.

Bradley, W.H. and P. Cooke. 1959.

Living and ancient populations of

the clam Gemma gemma in a Maine

Fong, W.C. 1976. Uptake and retention of Kuwait crude oil and its ’

effects on oxygen uptake by the

15
soft-shell clam Mya arenaria. J.

Fish. Res. Board Can. 33:2774-2780.

Gilfillan, E.S., and J.H. Vandermeulen. 1978. Alterations in

growth and physiology of softshelled clams, @ arenaria, chronically oiled with Bunker C from

Chedabucto Bay, Nova Scotia 1970-76.

J. Fish. Res. Board Can. 35~630-636.

Gosner, K.L. 1978. A field guide to

the Atlantic seashore. Houghton

Mifflin Co., Boston. 309 pp.

Green, J. 1975. The biology of

estuarine animals. University of

Washington Press, Seattle. 401 pp.

Hanks, R.W. 1963. The soft-shell

clam. U.S. Fish Wildl. Serv. Fish.

Circ. 162. 16~~.

Holland, A.F., N.K. Mountford, M.H.

Hiegel, K.R. Kaumeyer and J.A.

Mihursky. 1980. Influence of

predation on infaunal abundance in

upper Chesapeake Bay, USA. Mar.

Biol. (Berl.) 57:221-235.

Hruby, T. 1981. The shellfish resource in a polluted tidal inlet.

Environ. Conserv. 8(2):127-130.

Kelso, W.E. 1979. Predation on softshell clams, w arenaria, by the

common mummichog, Fundulus

heteroclitus.

2(4):249-254.

Estuaries

Kennedy, V.S., and J. A. Mihursky.

1971. Upper temperature tolerances

of some estuarine bivalves. Chesapeake Sci. 12(4):193-204.

Kennedy, V.S., and J.A. Mihursky.

1972. Effects of temperature on the

respiratory metabolism of three Chesapeake Bay bivalves. Chesapeake

Sci. 13(1):1-22.

Light, S.F. 1967. Intertidal invertebrates of the central California

coast. S.F. Light's laboratory and

field text in invertebrate zoology.

Rev. by R.I. Smith, F.A. Pitelka,

D.P. Abbott, and F.M. Weesner.

University of California Press,

Berkeley. 446 pp.

Loi, T.N., and B.J. Wilson. 1979.

Macroinfaunal structure and effects

of thermal discharges in a mesohaline habitat of Chesapeake Bay,

near a nuclear power plant. Mar.

Biol. (Berl.) 55(1):3-16.

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.

Lucy, J.A. 1976. The reproductive

cycle of t&a arenaria L. and

distribution of juvenile clams in

the upper portion of the nearshore

zone of the York River, Virginia.

Master's Thesis. The College of

William and Mary, Virginia Institute

of Marine Science, Gloucester Point.

131 pp.

MacDonald, B.A., and M.L.H. Thomas.

1980. Aae determination of the

soft-shell- clam Mya arenaria using

shell internal growth lines. Mar.

Biol. (Berl.) 58:105-109.

MacGinitie, G.E., and M. MacGinitie.

1968. Natural history of marine

animals, 2nd ed. McGraw-Hill Book

co., New York. 523 pp.

Merriner, J.V., and J.W. Smith. 1979.

A report to the oyster industry of

Virginia on the biology and management of the cownose ray (Rhinoptera

bonasus Mitchill) * lower

Chesapeake Bay. Virginia'tnst. Mar.

Sci. Sch. Mar. Sci., College of

William and Mary Spec. Rep. Appl.

Mar. Sci. Ocean Engineer. 216.

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

the shells of the Atlantic and Gulf

coasts and the West Indies, 3rd ed.

Houghton Mifflin Co., Boston.

16
L

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Newell, C.R., and H. Hidu. 1982. The

effects of sediment type on growth

rate and shell allometry in the

soft-shelled clam Mya arenaria L. J.

Exp. Mar. Biol. Ecol. 65(3):285-295.

Nicol, J.A.C. 1967. The biology of

marine animals, 2nd ed. Sir Isaac

Pitman & Sons, Ltd., London. 699

PP.

Perkins, E.J. 1974. The biology of

estuaries and coastal waters.

Academic Press, New York. 678 pp.

Pfitzenmeyer, H.T. 1965. Annual

cycle of gametogenesis of the softshelled clam, - Mya arenaria, at

Solomons, Maryland. Chesapeake Sci.

6:52-59.

Pfitzenmeyer, H.T. 1972. Tentative

outline for inventory of molluscs:

Mya arenaria (soft-shell clam).

Chesapeake Sci. 13(Suppl.):182-184.

Pfitzenmeyer, H.T., and K.G. Drobeck.

1963. Benthic survey for populations of soft-shelled clams. Mva

arenaria. Chesapeake ~' .Sz

4(2):67-74.

Purchon, R.D. 1977. The biology of

the mullusca. Pergamon Press,

Oxford. 560 pp.

Ricketts, E.F., and J. Calvin. 1968.

Rev. by J.W. Hedgepeth. Between

Pacific tides, 4th ed. Stanford

University Press, Stanford, Calif.

614 pp.

Ritchie, T.P. 1976. The U.S. clam

industry. Univ. Del. (Newark) Sea

Grant Publ. DEL-SG-26-76. 119 pp.

Roberts, D. 1975. Sublethal effects

of chlorinated hydrocarbons on

bivalves. Mar. Pollut. Bull. (Woods

Hole) 6(2):20-23.

Roosenburg, W.H., J.C. Rhoderick,

R.M. Block, V.S. Kennedy, and S.M.

Vreenegor. 1980. Survival of Mya

arenaria larvae (Mullusca: Bivalvia)

exposed to chlorine-produced

oxidants. Proc. Natl. Shellfish

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Spec. Rep. 5. 15 pp.

Sanders, H.L., E.M. Goudsmit, E.L.

Mills, and G.E. Hampson. 1962. A

study of the intertidal fauna of

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Shaw, W.N. 1965.

cycle of the male

Mya arenaria, in

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Fish. 508. 3 pp.

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Spec. Sci. Rep.

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diseases of marine fish

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Principal

and shellNew York.

Stewart, M.G., and D.R. Bamford.

1976. The effect of environmental

factors on the absorption of amino

acids by isolated gill tissue of the

bivalve Mya arenaria (L). J. Exp.

Mar. Biol. Ecol. 24:205-212.

Stewart, M.G. 1978. The uptake and

utilization of dissolved amino acids

by the bivalve w arenaria (L.).

Pages 165-176 in D.S. McLusky and J.

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of marine organisms. Pergamon

Press, New York.

Thomas, M.H. 1973. Effects of Bunker

C oil on intertidal and lagoonal

biota in Chedabucto Bay, Nova

Scotia. J. Fish. Res. Board Can.

30:83-90.

Thompson, B.G. 1985. Fisheries of

the United States. 1984. U.S.

Natl. Mar. Fish. Serv. Curr. Fish.

Stat. No. 8360. 121 pp.

17
Tsai, C.-F., J. Welch, K.-Y. Cheng,

J. Shaeffer, and L.E. Cronin. 1979.

Bioassay of Baltimore Harbor sediments. Estuaries 2(3):141-153.

Walker, H.A., E. Lorda, and S.B.

Saila. 1981. A comparison of the

incidence of five pathological

conditions in soft-shell clams, Mya

arenaria, from environments with

various pollution histories. Mar.

Environ. Res. 5(2):109-123.

Wenner, C.A., and J.A. Musick. 1975.

Food habits and seasonal abundance

of the American eel. Anguilla

rostrata, from the lower Chesapeake Bay. Chesapeake Sci. 16(l):

62-66.

Whitlach, R.B. 1982. The ecology of

New England tidal flats: a community profile. U.S. Fish Wildl.

Serv. Biol. Serv. Program

FWS/OBS-81/01. 125 pp.

18
b

B

L

272 -101

‘GpoRT DGCUMENlAflGN I* nLmllT NO- 2

PAGE Biological Report 82c11.68)"

ntk and lubtltl.

Species Profiles: Life Histories and Environmental

?equirements of Coastal Fishes and Invertebrates

[Mid-Atlantic)--Softshell Clam

Aumor(S)

Barbara J. Abraham and Perian L. Dillon

hrformlne O~w~lz~tton Ilane and Addm8

1(0)

t. Sgonsoviq Or9anlutian N*m. and Addews

National Wetlands Research Center U.S. Army Corps of Engineers 13. Ty~ofR*potiL PoriadCmmnc

Fish and Wildlife Service Waterways Experiment Station

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

Washington, DC 20240 Vicksburg, MS 39180 14.

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

5. Ab8tnct (Umi(: 200 wards)

The softshell clam supports the third most valuable commercial clam fishery in the

United States. Density is highest at depths of 3-4 m, temperatures less than 28 OC,

and salinities greater than 3 ppt.

by pollution.,'

Its near-shore habitat makes it easily threatened

bacteria.

Clam beds in some places have been closed because of contamination by

Softshell clams are more sensitive to oil pollution than are the other

clams that share its habitat.

The softshell clam spawns in spring (sometimes in early summer) and again in fall.

In 36-48 h after fertilization,

2-6 weeks.

a pelagic veliger larva develops and persists for

Then it settles out of the plankton. It attaches to the substrate and

can move and reattach itself. Eventually, it adopts the adult lifestyle and occupies

a permanent burrow, usually in sandy bottom with less than 50% silt.

Adult clams feed by filtering small particles from the water column. Predators of

adult clams include crabs, fish, birds, and raccoons.

i!te?;,hsLaCn58 adults

values for summer-acclimated clams have been reported as 32.5-34.4 OC.

ca n withstand long periods of anaerobiosis.

7. Docunwnt An8lysiS l . D*scrlPtws

Estuaries, Fisheries, Temperature, Salinity, Sediments, Feeding habits,

Clams, Life cycles, Oxygen, Contaminants, Water pollution

b. Idntlthn/Oven-Ended Terms

Softshell clam, f& arenaria, enVirOnment.al requirements

c. COSATI fl~ld/Gmup

2 Av~llabllity Stat,m.nt

Unlimited availability

19. Scurity Class CThis Rwwt)

Unclassified

20. kcurib Class (This Paw)

Unclassified

". No' Of '*" 18

72 MC9

n ANSl-239.lUl -AL fOau 272 w-7

(Fomurty NTIs-39~

~dCO#tWW-
TAKE PRIDE

in America

DEPARTMENT OF THE

U.S. flSH AND WILDLIFE

INTERIOR

SERVICE

As the Nation’s principal conservation agency, the Department of the Interior has responsibility for most of our.nationally owned public lands and natural resources. This includes

fostering the wisest use of our land and water resources, protecting our fish and wildlife,

preserving theenvironmental and cultural values of our national parks and historical places,

and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in

the best interests of all our people. The Department also has a major responsibility for

American Indian reservation communities and for people who live in island territories under

U.S. administration.