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Biological Report 82 (11.53) TR EL-82-4

June 1988

Library

Nat*onal Wetlands Research Center

us S. Ffsh and Wtidilfc Service

700 Cajundctrne -Bouhvard

Species Profiles: Life Histories and Lafayette, La. ?0506

Envhmental Requirements of Coastal Fishes

and Invertebrates (North Atlantic)

SOFTSHELL CLAM

WJ

us

.S63

PO. 82

11.53

Fish and Wildlife Service

U.S. Department of the Interior

Coastal Ecology Group

Waterways Experiment Station

U.S. Army Corps of Engineers

Biological Report 82(11.53)

TR EL-82-4

June 1986

Species Profiles: Life Histories and Environmental Requirements

of Coastal Fish and Invertebrates (North Atlantic)

SOFTSHELL CLAM

Carter R. Newell

Maine Shellfish Research and Development

Damariscotta, ME 04543

and

Herbert Hidu

Department of Animal and Veterinary Sciences

University of Maine

Orono, ME 04469

Project Officer

John Parsons

National Coastal Ecosystems Team

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_Coastal Ecosystems Team

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. 1983-19 . Species profiles: life histories

and environmental requirements of coaxal 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:

Newell, C.R., and H. Hidu. 1986. Species profiles: life histories and

environmental requirements of coastal fishes and invertebrates (North Atlantic)

-- softshell clam. U.S. Fish Wildl. Serv. Biol. Rep. 82(11.53). U.S. Army

Corps of Engineers, TR EL-82-4. 17 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 Coastal Ecosystems Team

U.S. Fish and Wildlife Service

NASA-Slide11 Computer Complex

1010 Gause Boulevard

Slidell, LA 70458

or

U.S. Army Engineer Waterways Experiment Station

Attention: WESER-C

Post Office Box 631

Vicksburg, MS 39180

iii
millimeters (mm)

centimeters (cm)

meters (m)

kilometers (km)

Multiply

square meters (m2)

square kilometers (km')

hectares (ha)

liters (1) 0.2642

cubic meters (m3)

gallons

35.31 cubic feet

cubic imeters 0.0008110 acre-feet

milligrams (mg) 0.00003527 ounces

grams (9) 0.03527 ounces

kilograms (kg) 2.205 pounds

metric tons (t) 2205.0 pounds

metric tons 1.102 short tons

kilocalories (kcal) 3.968 British thermal units

Celsius degrees 1.8("C) + 32 Fahrenheit degrees

inches 25.40 millimeters

inches 2.54 centimeters

feet (ft) 0.3048 meters

fathoms 1.829 meters

miles (mi) 1.609 kilometers

nautical miles (nmi) 1.852 kilometers

square feet (ft2)

acres

square miles (mi')

gallons (gal) 3.785 liters

cubic feet (ft3) 0.02831 cubic meters

acre-feet 1233.0 cubic meters

CONVERSION TABLE

Metric to U.S. Customary

EY

0.03937

0.3937

3.281

0.6214

To Obtain

inches

inches

feet

miles

10.76 square feet

0.3861 square :niles

2.471 acres

U.S. Customary to Metric

0.0929 square meters

0.4047 hectares

2.590 square kilometers

ounces (oz) 28.35

pounds (lb) 0.4536

short tons (ton) 0.9072

British thermal units (Btu) 0.2520

Fahrenheit degrees 0.5556("F - 32)

iv

grains

kilograms

metric tons

kilocalories

Celsius degrees
CONTENTS

Page

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

CONVERSION TABLE ................... iv

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

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

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

;

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

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

;

Spawning ...................... 3

Fecundity and Gametes ................ 4

Larvae ....................... 4

Juvenile Seed Clams .................

Adult Clams ..................... 5”

COMMERCIAL/SPORT FISHERIES .............. 6

POPULATION DYNAMICS ..................

GROWTH CHARACTERISTICS ................ !

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

Food and Feeding Habits ...............

;

'Predators .

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

t

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

Temperature ..................... ;

Oxygen ....................... 10

Substrate and Current ................ 10

Pollution ...................... 1 1

LITERATURE CITED ................... 13

V
.

ACKNOWLEDGMENTS

Dana Wallace and Walter Foster, Maine Department of Marine Resources,

Augusta, Maine, and John Moring, Maine Cooperative Fishery Research Unit,

University of Maine, kindly reviewed the manuscript.

vi


enclosed by the chon>rophore of the

left valve (Figure 1). The valves are

thin, and mean ratios of length (L),

width (W), and height (H) are: L/H1.65, H/W-l (Stanley 1970). Newell

and Hidu (1982) observed mean L/W

ratios of 2.6 in clams from gravel

beds, and 3.2 in clams from sand beds.

REASON FOR INCLUSION IN SERIES

The softshell clam is a dominant

member of many estuarine soft-bottom

communities and is commercially

important along much of the North

Atlantic coast. In New England, it is

the second most important commercial

clam after the hard clam (Mercenaria

mercenaria). In 1978, U.S. landings

yielded about 4.6 million kg (10.1

million lb) of meats (Pileggi and

Thompson 1979). Its inshore

distribution, however, makes it

vulnerable to contamination by municipal sewage, industrial effluent, and

coastal construction projects. Also,

the adults usually live in permanent

burrows, so excessive siltation can

suffocate them. The construction of

piers and jetties that alter clam

habitat is likely to have long-term

adverse effects on clam populations.

LIFE HISTORY

The softshell clam has seven life

history stages (Table 1).

Spawning

Sexual maturation of the softshell clam depends upon the size of

the clam rather than its age. Clams

longer than 20 mm in shell length are

usually capable of spawning (Coe and

Turner 1938). Clams from cold Maine

waters may spawn at a smaller size

than those from Massachusetts or Long

Island Sound. Sexes are separate, the

sex ratio is 50:50, and males are indistinguishable from females unless a

gonad smear is examined under a microscope. A low incidence of hermaprodites is common in most populations.

The seasonal development of the gonads

was summarized for the north Atlantic

coast by Ropes and Stickney (1965).

Gametogenesis begins in late winter

and early spring, and spawning peaks

from June to September, depending upon

location.

Clams usually spawn once a year

north of Cape Cod, and twice a year

south of Cape Cod. The differences in

Table 1. The life stages and characteristics of the softshell

clam.

Stage

Fertilized egg

Trochophore

Veliger

Prodissoconch I

Prodissoconch II

Spat (Dissoconch)

Juvenile

-

Adult

Age or

characteristic --I___ -

O-12 hours old.

13-24 hours old, top-shaped.

lo-20 days old, has a shell

and swims by using a velum.

Has straight-hinged shell.

Has umbo on shell.

Bottom-dwelling, after

settlement and velum cast

off.

Up to 20 mm shell length.

Sexually mature.

3

At the end of the larval staae.

the clam attaches to the bottom. Then

the velum is cast off and a foot

develops as the clam metamorphoses

a into a bottom-dweller called a spat.

Metamorphosis may be delayed later

than usual, until the clam locates a

) suitable substrate for attachment.

Juvenile Seed Clams

After metamorphosis, the young

spat (0.25 - 1 mm long) may undergo a

floating - crawling stage for 2 to 5

weeks (Belding 1930). During this

stage the clam spat sometimes holds on

to the substrate lusing a byssal

thread. The spat may first attach to

eelgrass, filamentous algae, and other

objects in the subtidal zone (Kellogg

1900), but as it continues to grow the

spat drops to the bottom and burrows

into the sediment. After the clam

becomes large enough to be noticed by

clam diggers (5 mm), it is referred to

&

as clam "seed" until it reaches market

size. Juvenile seed clams may migrate

up to several hundred yards towards

shore (Dow and Wallace 1961). The

byssus is used for attachment while

juvenile clams up to 13 mrr: long move

or burrow.

The movement of seed clams 2-15

mm long on a Massachusetts beach was

studied by Matthiessen (1960b). Clams

2-3 mm long that set in late summer

remained in the seaward portion of the

sampling area for 8-10 months (until

April, May, or June) and grew to 5 mm

or more, and then moved shoreward.

This movement was attributed to

hydrodynamic forces such as sediment

sorting by shoaling waves. During

spring storms, clams 5-15 mm long

showed a net horizontal displacement

shoreward along with coarse sediment

particles (Matthiessen 1960b). The

movement of first-year juveniles

peaked in September and October during

early growth, and again in May after

growth resumed in the spring (Dow and

Wallace 1961).

Because adult softshell clams are

sedentary, maintenance of each

population depends upon the settlement

of spat or the movement of juvenile

seed clams. Because of the influence

of oceanographic conditions on recruitment, the abundance of settling

clams (the set) may be irregular in

some locations: for example, enormous

quantities of seed clams may be

concentrated in

4

small area. A clam

set of 538,000/m was reported in an

eddy adjacent to a sand bar in Plum

Island Sound, Massachusetts (Belding

1930). The density of seed clams

usually is greatest in eddies, along

the sides of sand bars or islands, at

the mouths of rivers or Streams

emptying into shallow water, or in

slack water adjacent to a swift

current. Because of their small size

and shallow burrowing depth, juvenile

clams are subject to intense predation. The densit? of juvenile clams

approached 6,000/m in the subtidal

zone in Virginia, but dropped to zero

1 month later, presumably due to

predation (Lucy 1976); however, clams

in Chesapeake Bay were able to avoid

most predators by burrowing as deep as

IO cm (Blundon and Kennedy 1982).

Vegetation reduced predation of

softshell clams in the subtidal zone.

The movement of juveniles from

subtidal areas to the intertidal zone

in New England also reduces predation.

For example, experimental plantings of

seed clams at different intertidal

elevations along the Maine coast

revealed that growth is slowest but

survival is the greatest in the upper

intertidal zone (Newell 1982).

Adult Clams

In suitable habitat, the

softshell clam makes a substantial

contribution to the biomass of the

benthic community. For example, 1 ha

of mudflat containing clams abjut 62

mm long at a density of 269/m contains 1,442 bushels of clams in the

5
shell, and 21,635 lb of meats (D.

Wallace, Maine Dep. Marine Resources,

Augusta, pers. comm.).

COMMERCIAL/SPORT FISHERIES

The softshell clam is a valuable

sport and commercial species. The

sport fishery is locally important to

coastal resort towns, where clam

ordinances strictly regulate the

catch. In New England, where the

resource is primarily intertidal, the

commercial instruments are generally

constrained by law to hand implements.

Clam forks and hoes are common

instruments, although dredges are

sometimes used in subtidal areas in

Massachusetts and experimental lY in

Maine. In Chesapeake and Delaware

Bays, where the resource is subtidal,

clams are harvested with a hydraulic

escalator dredge. Damage to clams is

less from dredges than from forks and

hoes (Kyte and Chew 1975). Burial and

breakage of clams during hand harvesting sometimes reduces production

in New England (Glude 1954). Hydraulic clam rakes have been used in Canada (Bourne 1967) and Maine to collect

seed clams for transplanting. Annual

U.S. commercial landings of softshell

clams from 1977 to 1981 averaged 4.2

million kg (9.3 million lb), worth $15

million. At one time, sewage polluted

many of the clam beds along the New

England coast, so they were closed to

diggers and dredgers. In the 1970's,

the construction of municipal sewage

treatment plants sharply increased and

pollution decreased, so some of the

beds have been reopened and production

has proportionately increased.

POPULATION DYNAMICS

'Because the mortality of eggs,

larvae, and seed clams is extremely

high, clam populations are maintained

only because of a tremendous fecundity. Less than 0.1% of the eggs

produced in a spawning season result

in a successful settlement, and about

1% of the settled spat must mature and

reproduce in order to sustain the

populations. Mortality is heavy in

the planktonic stage and immediately

after settlement, and decreases as the

clam grows older. As the shell

becomes thicker and the clam digs

deeper, its survival rate increases.

Survival rate follows an exponential

decay, leveling off after 3 years of

age. Mortality rates are highest in

summer when predators are most

abundant (Brousseau 1978b). Fecundity

increases with clam size, so the

intrinsic rate of natural increase is

high. The high mortality of larvae is

offset by the high intrinsic rate of

natural increase (Brousseau 1978b).

Densities of larvae ranged from

0.1 to 1,000/m3 in New Hampshire

waters of the Gulf of Maine

(Normandeau Associates, Inc. 1978) and

the late veliger larval densities

ranged from 0 to 1,400/m3 in

Chesapeake Bay (Pfitzenmeyer 1962)

during the summer months. In one

study, settlement densities as high

as 107,600/m2 (10,000/ft2), decreased

;;te:1,500/m2 (2,000/ft2) 2 months

and then to 0 after 1 year

(Turnir 1953).

In Chesapeake Bay, spat (less

than 10 mm long) densities decreased

from 500/m2 in December to less than

lo/m2 by June (Blundon and Kennedy

1982). In New Hampshire, densities

of young-of-the-year spat ranged from

21/m2 to 8,200/m2 from 1971 to 1980.

High larval densities (530/m3 in the

summer of 1975) were followed by high

spat densities (8,200/m2) in 1976;

adults of the 1975 year class produced

a strong fishery from 1978 to 1980

(Savage 1981).

In Casco Bay, Maine, standing

crops in 1979 averaged 90 to 120

bushels of clams per acre. The total

inventory of the bay was 107,500

bushels (Card 1980).

6
Adults have an aggregated distribution, limited primarily to intertidal and shallow subtidal areas

(Saila and Gaucher 1966; Commit0

t

1982). The degree of aggregation may

depend upon the slope of the inter1

tidal area and current (Newcombe

1936). Juveniles may concentrate near

a steep shore profile (Matthiessen

1960b). Aggregation also may be

caused by predation, by hand

harvesting or dredging, and by the

concentration of spat by hydrograph

conditions.

GROWTH CHARACTERISTICS

The softshell clam grows rapid

in a favorable environment. Clams

usually reach market size (2 inches

long) in 1.5 years in Chesapeake Bay,

(Pfitzenmeyer 1972), in 2 to 3 years

in Rhode Island and Massachusetts

(Turner 1948; Brousseau 1979), in 3 to

6 years in Maine, and in 5 years in

New Brunswick, Canada (Turner 1948;

L

Spear and Glude 1957; Commit0 1982).

Growth may be modeled using the exponential von Bertalanffy growth

equation, expressed by the fol;,"yieg

formula: Shell length = a(1 -

where a, b and k are constants derived

from growth data, and t = time.

Growth rate constants of clams from

different geographic areas were

calculated by Brousseau (1979). Data

fit to the growth equation using best

fit computer-generated curves demonstrate widespread differences in

growth rates among locations.

Seasonal variations in growth rates

can be incorporated into the von

Bertalanffy equation by adding

temperature, .

(Munch-Peterson I9;3n).

day-degrees

Seasonal variations in growth

rates have been attributed in part to

food availability by Newcombe (1935),

Matthiessen (1960a), and Stickney

to temperature by Belding

Munch-Peterson

(1979). In New England, softshell

clams generally grow fastest in late

spring and early summer, and slowest

in the cold winter months (Belding

1930; Brousseau 1979). The months of

rapid clam growth are coincident with

rising abundance of phytoplankton and

seawater temperature in the Gulf of

Maine (Bigelow 1917; Petrie 1975).

Also, rapid clam growth in a salt pond

in Martha's Vineyard, Massachusetts,

was coincident with a high abundance

of flagellates (Matthiessen 1960a).

Seasonal variations in growth rates

are positively correlated with

seasonal changes in biochemical

( glycogen) levels and condition

indices (measurements of shellfish

"fatness"). Glycogen levels and meat

yields are high in the spring; the

glycogen is converted to gametes with

subsequent drop in meat yields

&ring the spawning season, and the

meat yield recovers after spawning

(Newell 1982). In Maine, glycogen in

shellfish of good market quality peaks

in late spring and is lowest in winter.

Growth rates are also closely

related to current, sediment type, and

intertidal height. Clams grow the

fastest in soft sediments on the lower

shore (where food is relatively

abundant) under good current

conditions in New England (Belding

1930; Newell 1982). In a laboratory

experiment, clams grew faster in soft

mud or sand than in gravel (Newell and

Hidu 1982). Similarly, shell form and

percent shell weight varied with

growth rate, sediment type, and

intertidal height. Slow-growing clams

from coarse sediments and from the

upper shore (where food is relatively

scarce) have higher percent shell

weights and larger shell length-depth

regression slopes (greater shell

globosity) than fast-growing clams

from the lower shore and from soft

sediments.

Excessive density can also limit

growth rates because of conpetition

for food and space. Mature clam

7
densities of 161-269/m2 are generally

considered favorable for good growth

(Belding 1930).

ECOLOGICAL ROLE

Food and Feeding Habits

Clam larvae, juveniles, and

adults feed by filtering seawater.

Postmetamorphic clams draw in through

the incurrent siphon by beating the

gill cilia. The water passes through

the gills, where food particles are

removed, trapped in mucus, and swept

to the mouth. Particles too large for

ingestion, inorganic particles low in

nutrition, and particles of any type

in dense concentrations in Seawater

are usually rejected by ciliated

structures called the labial palps.

These particles are expelled through

the incurrent siphon as pseudofeces by

a rapid contraction of the adductor

muscles. Phytoplankton cell

concentrations greater than 30,00O/ml

of seawater cause a reduction of

filtration rates and the formation of

pseudofeces as undigested algae and

mucus (Stickney 1964).

Flagellates and diatoms are the

preferred diet, although clams can

obtain nutrition from bacteria and

organic detritus ’ resuspended

mudflat sediments (E8ion 1981) and

dissolved organic molecules (Stewart

and Bamford 1976). In a salt pond in

Massachusetts, clams grew faster on a

diet of flagellates than on diatoms

(Matthiessen 1960b).

Food filtration rates are influenced by water temperature,

seawater particle densities, and

particle type (Eaton 1981). In Maine,

Eaton found that clams filter and

assimilate higher quantities of algae

in the summer (14-20 “C) than in the

spring (3-8 "C). Clams are discriminatory feeders and increase filtration

rates when algae are added to diets of

suspended silt particles. Clams are

well-adapted to handling resuspended

silts in high concentrations, and may

sort algal cells from inorganic

particles prior to ingestion. Clams

of 0.3 g dry meat weights continue to

filter even if seawater silt particle

densities exceed 300 mg/l (Eaton

1981). High levels of oil pollution

in sediments (hydrocarbon levels over

1,500 ppm) caused a reduction in food

filtration rates and a lower carbon

flux in clams from Maine (Gilfillan et

1976).

$60b)

According to Matthiessen

filtration rates of clams

decline; when exposed to salinities

between 8 and 15 ppt and stopped when

salinities were below 4 ppt. Clams

continue to filter when seawater

temperatures are below 3 "C, but food

assimilation is low.

Predators

Predation is one of the most

important factors in the control of

natural populations of softshell

clams. Planktonic larvae are subject

to predation by other planktors, fish,

and filtering invertebrates; yOUn9

spat may be devoured by birds, fish,

shrimp, worms, crabs, snails and

flatworms. As the juvenile clam

grows, it burrows deeper into the

substrate, where it finds protection

from most predators. The rapid

juvenile growth and postponement of

gametogenesis are considered to be

adaptations for survival (Commit0

1982). In one instance, it was

estimated that in Massachusetts the

mummichog (Fundulus heteroclitus), a

small fish, consumed as many as

546,000 softshell clams (~12 mm long)

per km of shoreline per day (Kelso

1979). In a 1aborator.y experiment,

sandworms (Nereis ~~virens) 'consumed

softshell clams ~15 mm lone (D. Dean,

Department of Zoology, University of

Maine, Orono; pers. comm.). In

Massachusetts, Relding (1930) reported

that major predators are the blue crab

(Callinectes sapidus), lady crab

horseshoe crab

and especially

8
The green crab (Carcinus maenas)

is one of the most destructive clam

predators. For example, periodic declines in clam production in Maine

coincided with increases in the

abundance of green crabs when seawater

temperatures were unusually high

(Welch 1969). A green crab may consume

as many as 15 clams per day. When

abundant, green crab densities can

exceed 24,70O/ha (Spear 1953). Moon

snails (Lunatia spp.) preferentially

prey on clams 20 to 50 mm long and

prefer softshell clams over other

foods (Edwards and Heubner 1977). The

moon snail feeds by drilling a hole in

the clam shell with a rasping organ

and eating the contents. A single

snail may consume up to 100 clams a

year (Edwards and Heubner 1977).

Clams over 60 mm long are not usually

attacked by crabs or moon snails. The

blue mussel (Mytilus edulis) and

another clam (Gemma gemmb)ompete

with the softshell clam for space and

food in the lower intertidal zone in

k

some coves in Maine (Dow and Wallace

1961).

ENVIRONMENTAL REQUIREMENTS

Salinity

The salinities of softshell clam

habitat decrease from north to south.

Preferred salinities were 20-32 ppt in

Maine (Dow and Wallace 1961; Gilfillan

et al. 1976), lo-33 ppt in Massachusetts (Belding 1930; Brousseau 1978b),

and 4-15 ppt in upper Chesapeake Bay

(Pfitzenmeyer 1972). When clams were

planted in salinities near their

lowest tolerated level, they showed

signs of distress and reduced their

pumping rates. The rate of pumping was

sharply reduced when the salinities

were reduced to the stress point. The

stress point is about 15 ppt for

Massachusetts clams (Matthiessen

1960a), 22-24 ppt for Medomac River,

Maine, clams (Welch and Lewis 1965),

and 5 ppt for Chesapeake Bay clams

(Schubel 1973). Larvae are more

sensitive to low salinities than

adults (Belding 1930). The lowest

tolerable salinity for adult softshell

clams is about 5 ppt.

Growth rates of clams in

Massachusetts are positively

correlated with salinities from 3 to

14 ppt in Massachusetts (Matthiessen

1960a). Mortalities were as high as

90% in Chesapeake Bay when salinities

dropped to 2 ppt because of the

freshwater runoff after Hurricane

Agnes (Shaw and Hammons 1974).

Softshell clam survival at extremely

low salinities is inversely related to

water temperature (Allen and Garrett

1971). For example, softshell clams

in the Bay of St. Lawrence survived

salinities below 1 ppt for as long as

1.5 days at low temperatures.

However, at higher water temperatures,

clams acclimate faster to moderately

decreasing salinities (Creaser and

Clifford 1977). For example, in an

experiment in Maine, acclimation time

for a change from 30 ppt to 22 ppt was

reduced from 60 hours at 4 "C to 10

hours at 10 'C. The tolerance of

clams 2-25 mm long to low salinities

is somewhat positively correlated with

their size (Matthiessen 1960a). Amino

acids regulate the osmotic characteristics of the clam hemolymph

(blood) in different salinities.

Temperature

Water temperature regulates the

time of spawning and influences the

distribution of clams along the north

Atlantic coast. The range of softshell clams is limited in the north by

temperatures too low (12 "-15 "C) for

spawning; the range in the south is

limited by excessively high temperatures (Lawson 1966). Clams do not

usually survive water temperatures

above 28 "C in Chesapeake Bay; high

temperatures restrict their

distribution and abundance there

(Pfitzenmeyer 1972). In l-day

bioassays, the LT50

temperature for 50%) was

(lethal

32.5 "C for

adult clams from Chesapeake Bay and

9
34.4 "C for juveniles (Kennedy and

Mihursky 1972). Clams from the subtidal zone and the high intertidal

zone better withstood high temperatures (> 25 "C) than clams from low

and middle intertidal levels. Water

temperatures regulate the length of

larval life (low temperature-long

life); optimal temperature for

maximum growth is about 20 "C (S.

Chapman, Ira Darling Center, Univ. of

Maine, Walpole; pers. comm.). In a

study of softshell clams from Maryland

to Nova Scotia, water temperature was

the dominant environmental factor

affecting 68% of the observed

9

rowth

rate fluctuations (Appeldoorn 982).

Food availability, water current

velocity (Belding 1930), and sediment

type (Newell and Hidu 1982) contribute

to local fluctuations in growth rates,

which sometimes mask the general

latitudinal trend (Dow and Wallace

1961).

The pumping rates of softshell

clams increase with increasing

temperature up to 16 'Y (Harrigan

1956). At lower temperatures (l-2

"C), clams continue to pump but rates

of food assimilation are low

(Gilfillan et al 1976). Water temperatures of -1.7 "C are tolerated in New

England. During icing, clams survive

by utilizing glycogen energy reserves

(Newell 1982).

Rafting of the surface sediment

and the destruction of shallow

burrowing juveniles by ice have been

observed in mudflats in Maine.

Changes in temperature regulate

the rate of burrowing. In Chesapeake

Bay, burrowing was greatest at 18 "C,

tnz;n;;;e from 9 to 21 'C, and greatly

at higher or lower

temperatures (Pfitzenmeyer and Drobeck

1967).

Oxygen

Oxygen utilization by softshell

clams is governed largely by body

size, water temperature, rate of

metabolism, and oxygen concentrations

in the water. A positive linear

relation between oxygen consumption

and wet weight was reported by Fong

(1976). Oxygen intake in softshell

clams is independent of oxygen

concentration down to about 2.8

mg/liter (van Dam 1935). Oxygen

intake is greatest at about 20 "C

(Kennedy and Mihursky 1972). Intertidal softshell clams can function as

facultative anaerobes (Collip 1920),

switching to anaerobic metabolic

pathways at low tide. Calcium carbonate from the shell is used to buffer

the acidic products of anaerobic respiration. Upon reimmersion, clams

undergo rapid ventilation to oxidize

the end products of anaerobic respiration. Excessively low oxygen

concentrations cause increased stress

and heartbeat rates (Lowe and Trueman

1972). Softshell clams survive

periods of anaerobiosis longer at lOW

temperatures, for example, during

icing. High respiration rates in the

spring have been attributed to warm

water acclimation (Gilfillan et al.

1976) or metabolic costs of active

gametogenesis.

Substrate and Current

Clams live in soft muds, sands,

compact clays, and coarse gravel, and

between stones. Sediment type is

important in controlling growth rate

and shell allometry. Under identical

current conditions, clams grow faster

in fine sediments than in coarse

sediments (Newell and Hidu 1982), and

grow fastest in sand or sandy mud

(Belding 1930; Dow and Wallace 1961),

though few live in shifting sand.

Slow-growing clams from gravel have

more globose shells than those from

mud or sand, and clams in sand are

longer and narrower than those in mud.

Differences in growth rates and shell

form are attributed to the physical

properties of the sediment and its

resistance to burrowing. Sediment

particle size also affects the

10
reburrowing of clams. Clams had

difficulty burrowing in sediments

larger than 0.5 mm (Pfitzenmeyer and

Drobeck 1967).

The characteristics of the

sediments reflect the rates of the

bottom currents and establish, from a

physical standpoint, the suitability

of the bottom for clams. Coarser

sediments usually reflect faster

currents, which support greater

population densities and cause faster

growth (Appeldoorn 1982). If the currents are too slow, the sediments

usually have a high silt-clay content,

which, in excess, can clog the gills

of the clams, reduce growth rates, and

in extreme cases cause smothering (Dow

and Wallace 1961). Most clams in

Chesapeake Bay thrive in a substrate

that is less than 50% silt (Pfitzenmeyer 1972). Clams may continue

pumping when total suspended solids

exceed 300 mg/l, but the production of

mucus and the loss of energy during

the ejection of pseudofeces strain the

\r

energy budget of the clam (Eaton

1983).

Coarse sediments and mats of

vegetation help protect clams from

most predators (Blundon and Kennedy

1982). During winter months when

phytoplankton populations are lowest,

resuspended bottom sediments are an

important food source for softshell

clams.

Pollution

Softshell clams in polluted water

accumulate pesticides (Dow 1972), oil

(Mayo et al. 1975), heavy metals

(Eisler 1977), and sometimes the

bacteria and viruses in municipal

sewage. Toxic materials are most

damaging to fertilized eggs and

larvae, and less damaging as the clams

grow larger. Oil pollutants can cause

a reduction in the carbon flux of

clams (Gilfillan et al. 1976). More

refined petrochemicals may have a

greater effect on the incidence of

pathological tumors in clams than

unrefined oils and heavy metals

(Walker et al. 1981). Clam larvae are

sensitive to chlorine-produced

oxidants but in general adult clams

are relatively tolerant of pollution

(Brown et al. 1977). Despite the

tolerance of softshell clams to

pollution, bacteria and viruses from

municipal effluent accumulate in the

clam's body tissues, and if eaten, are

a threat to public health. Until

municipal pollution is adequately

abated, clamming in some waters will

continue to be prohibited.

11

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13
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__--

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1 7