(DRAFT) - Taxonomy
Species clam, softshell
Species Id M060160
Date 26 AUG 96
TAXONOMY
NAME - clam, softshell
OTHER COMMON NAMES - softshell clam and soft clam
ELEMENT CODE -
CATEGORY - Aquatic Molluscs
PHYLUM AND SUBPHYLUM - Mollusca,
CLASS AND SUBCLASS - Bivalvia,
ORDER AND SUBORDER - Myoida,
FAMILY AND SUBFAMILY - Myidae,
GENUS AND SUBGENUS - Mya,
SPECIES AND SSP - arenaria,
SCIENTIFIC NAME - Mya arenaria
AUTHORITY - Linnaeus, 1758
TAXONOMY REFERENCES - 168 and 136
COMMENTS ON TAXONOMY -
Other common names include Steamer (New England), long clam, gaper (Gosner
1978); long-neck clam (Light 1967); manninose (Chesapeake Bay) (Pfitzenmeyer
1972)
Taxonomy - 1 (DRAFT) - Status
Species clam, softshell
Species Id M060160
Date 26 AUG 96
STATUS
Coded Status
Commercial
Commercial/consumption
Commercial/industrial
Depleted
REFERENCES FOR STATUS - 52 and 136
COMMENTS ON STATUS -
Overfishing can drastically reduce the value of clam beds. Because of its
near-shore habitat, this valuable resource is easily endangered by
pollution. Mariculture efforts have been unsuccessful (Ritchie 1976).*52*
Soft clams are prominent members fo the benthic community in Chesapeake Bay
and contribute substantially to the economy of the region. Soft clams in
the Chesapeake Bay have decreased in abundance in recent years in the Bay.
Intense fishing pressure, loss of habitat, and water quality degradation
have been blamed for declines in the abundnace of this species *136*.
Status - 1 (DRAFT) - Distribution
Species clam, softshell
Species Id M060160
Date 26 AUG 96
DISTRIBUTION
Distribution - 1 HABITAT ASSOCIATIONS
HABITAT - AQUATIC
BENTHIC
REFERENCES FOR HABITAT - 52
NATIONAL WETLAND INVENTORY CODES
NWI NWICLS NWIMOD NWISPEC
Estuarine BB2 L 3
Marine, subtidal FL2 P 3
REFERENCES FOR NWI - 52
COMMENTS ON HABITAT ASSOCIATIONS -
Temperature
The most important factor in growth and reproduction of softshell clams is
temperature. 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 degrees C) depressed metabolism of
cold-acclimated clams (Kennedy and Mihursky 1972). 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 LC50 values for
summer-acclimated clams were 32.5-34.4 degrees C (Kennedy and Mihursky
1971). As temperature approached the upper lethal limit, a 1 degree C
increase often made the difference between total mortality and none (Kennedy
and Mihursky 1971).*52*
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 (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
havve a deleterious effect on softshell clams, which are isoconformers
(Stewart and Bamford 1976).
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 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 brought in over 12 cm of water in the watershed and
over 45 cm in isolated areas as well as high air temperatures, 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 degrees C.
Substrate
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
Habitat Associations - 1 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.*52*
ANIMAL/PLANT SPECIES ASSOCIATIONS -
finfish
blue crabs
waterfowl
REFERENCES FOR SPECIES ASSOCIATIONS - 136
COMMENTS ON SPECIES ASSOCIATIONS -
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 Urosalpiinx 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 canliculatum, 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. 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.*52*
Adult soft clams burrow deeply, feeding through a long extensible siphon.
Juveniles, burrowing less deeply, often fall prey to finfish, blue crabs and
waterfowl. Commercial harvesting of adults reduces adult populations and
exposes juveniles to predation before they can burrow back into the sediment
*136*.
Habitat Associations - 2 (DRAFT) - Food Habits
Species clam, softshell
Species Id M060160
Date 26 AUG 96
FOOD HABITS
TROPHIC LEVEL -
FILTER FEEDER/DETRITOVORE
REFERENCES FOR TROPHIC LEVEL - 136
LIFESTAGE FOOD FOOD PART
General Detritus Not Specified
General Phytoplankton Not Specified
General Bacteria Not Specified
General Zooplankton Not Specified
REFERENCES FOR GENERAL FOOD - 136
COMMENTS ON FOOD -
Feeding Habits
Adult softshell clams feed by filtering microscopic particles of organic
material, including detritus 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 micrometers in diameter.
Mucus, secreted by the mantle, gills, and visceral mass, collects the
incoming particles, which are carried to the mouth by cilia. At 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).*52*
Soft clams are important benthic species in the Bay. These clams are
infaunal suspensions feeders, ingesting small detrital particles and
phytoplankton, as well as bacteria and microzooplankton in the case of Mya
spp. Adult soft clams burrow deeply, feeding through a long extensible
siphon. Juveniles, burrowing less deeply, often fall prey to finfish, blue
crabs and waterfowl. Commercial harvesting of adults reduces adult
populations and exposes juveniles to predation before they can burrow back
into the sediment *136*.
Food Habits - 1 (DRAFT) - Environment Associations
Species clam, softshell
Species Id M060160
Date 26 AUG 96
ENVIRONMENTAL ASSOCIATIONS
G = General A = Adult
LIM = Limiting RA = Resting Adult
J = Juvenile FA = Feeding Adult
RJ = Resting Juvenile BA = Breeding Adult
FJ = Feeding Juvenile P = Pupae
L = Larvae E = Egg
RL = Resting Larvae
FL = Feeding Larvae
LIFESTAGE ENVIRONMENTAL ASSOCIATIONS
G Water Temperature: Between 15-21 degrees C
G Dissolved Oxygen: Low [less than 5 mg/l] oxygen concentrations
G Substrate: Sand
G Water Depth Preference: Specified in Comments
G Estuarine habitat zone: specified in comments
G
BA Water Temperature: Specified in Comments
BA Water Temperature: Below 15 degrees C
BA Water Temperature: Between 15-21 degrees C
L Water Temperature: Specified in Comments
G Flow: Specified in Comments
G Turbidity: Specified in Comments
G Water Depth Preference: Less than 1 ft.
G Water Depth Preference: 1-5 ft.
G Water Depth Preference: 5-10 ft.
G Water Depth Preference: 10-25 ft.
G Water Depth Preference: 25-50 ft.
G Water Depth Preference: Specified in Comments
LIM Water Temperature: Greater than 27 degrees C
LIM Water Temperature: Specified in Comments
A Water Temperature: Specified in Comments
REFERENCES FOR ENVIRONMENTAL ASSOC_ - 52 and 136
REFERENCES FOR LIMITING ENVIRONMENTAL ASSOC_ - 136
REFERENCES FOR ADULT ENVIRONMENTAL ASSOC_ - 136
REFERENCES FOR BREEDING ADULT ENVIRONMENTAL ASSOC_ - 136
REFERENCES FOR LARVAE ENVIRONMENTAL ASSOC_ - 136
COMMENTS ON ENVIRONMENTAL ASSOCIATIONS -
The most important factor in growth and reproduction of softshell clams is
temperature and salinity is the major environmental factor controlling
presence of Chesapeake Bay infaunal species. 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). The softshell clam is
little affected by oxygen fluctuations. Juveniles and adult stages are
Environment Associations - 1 (DRAFT) - Environment Associations
Species clam, softshell
Species Id M060160
Date 26 AUG 96
able to withstand long periods of anaerobiosis.*52*
Soft clams in the Chesapeake inhabit shallow subtidal (10 m) estuarine
waters to intertidal areas in the oligohaline through the polyhaline zones
*136*.
Soft clams are vulnerable to sediment disturbances since they are slow
reburrowers. As such, they are impacted by harvesting practices, waves,
currents and bioturbation. Regowth of SAV would benefit these bivalves by
reducing the amount of sediment resuspension and the resulting turbidity.
Soft clams are also impacted by anoxia which restricts their distribution
to shallow waters less than 10 m in depth *136*.
COMMENTS ON LIMITING ENVIRONMENTAL ASSOC_ -
Temperatures of 32.5 oC or greater are lethal to adult soft clam limiting
intertidal distribution in the species' southern range *136*.
COMMENTS ON ADULT ENVIRONMENTAL ASSOC_ -
Temperatures of 32.5 oC or greater are lethal to adult soft clam limiting
intertidal distribution in the species' southern range *136*.
COMMENTS ON FEEDING ADULT ENVIRONTAL ASSOC_ -
Food availability is a significant factor dictating their survival. Foods
of critical sizes are needed for the different life stages; with the cell
sizes generally ranging from 3-35 um *136*.
COMMENTS ON BREEDING ADULT ENVIRONMENTAL ASSOC_ -
Soft clams spawn when a critical temperature occurs. In the Chesapeake,
soft clams spawn in the spring when water temperature reaches 10 oC and
spawning may be repeated in the fall when water temperature falls to 20 oC
*136*.
COMMENTS ON LARVAE ENVIRONMENTAL ASSOC_ -
Soft clam eggs develop into planktonic trochophore larvae in about 12
hours. Larvae remain in the water column for about 6 weeks during the fall.
The faster spring rate of larval development is caused by temperatures at
the warmer end of the soft clam's spawning temperature range. Setting of
soft clams may occur twice in the same year. Frequently, heavy predation on
the spring set by blue crabs and bottom-feeding fish results in
unsuccessful recruitment *136*.
COMMENTS ON EGG ENVIRONMENTAL ASSOC_ -
Soft clam eggs develop into planktonic trochophore larvae in about 12 hours
*136*..
Environment Associations - 2 (DRAFT) - Life History
Species clam, softshell
Species Id M060160
Date 26 AUG 96
LIFE HISTORY
Morphology/Identification Aids
The softshell clam has a thin gray or chalky-white, egg-shaped shell 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 ridgid siphonal process that is too
large to be completely withdrawn into the shell and is capable of great
elongation (Purchon 1977).*52*
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 1:1 (Brousseau 1978a); Lucy (1976) also reported a 1:1 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.
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, ripe, and partially spawned phases; the inactive
stage included spent and indifferent. Criteria for determining each phase
corresponded to those of earlier authors. *52*
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
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 degrees C and may continue at water temperatures up
to 20 degrees C; autumn spawning occurs when water temperature has fallen
from the summer high of 25 degrees C to 20 degrees C (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, MA, spawning occurred at a surface (1 m) water temperature of
4-6 degrees C in March-April, but at 15-18 degrees C in June-July.*52*
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
Life History - 1 (DRAFT) - Life History
Species clam, softshell
Species Id M060160
Date 26 AUG 96
fall from the maximum summer temperture 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; 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.*52*
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 1-17 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).*52*
Juveniles
When the veliger reaches a length of about 200 micrometers, 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 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).
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
Life History - 2 (DRAFT) - Life History
Species clam, softshell
Species Id M060160
Date 26 AUG 96
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 10-12 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, temperature less than 28 degrees C and
salinities not less than 4-5 ppt (Pfitzenmeyer and Drobeck 1963; Lucy 1976).
Adult clams burrow was far as 30 cm into the sediment, but the siphonal
process extends to the sediment surface (Kennedy and Mihursky 1971).*52*
Soft clams in the Chesapeake inhabit shallow subtidal (10 m) estuarine
waters to intertidal areas in the oligohaline through the polyhaline zones.
Soft clams spawn when a critical temperature occurs. In the Chesapeake,
soft clams spawn in the spring when water temperature reaches 10 oC and
spawning may be repeated in the fall when water temperature falls to 20 oC.
Soft clam eggs develop into planktonic trochophore larvae in about 12 hours.
Larvae remain in the water column for about 6 weeks during the fall. The
faster spring rate of larval development is caused by temperatures at the
warmer end of the soft clam's spawning temperature range. Setting of soft
clams may occur twice in the same year. Frequently, heavy predation on the
spring set by blue crabs and bottom-feeding fish results in unsuccessful
recruitment *136*.
LIFE HISTORY CODES -
Reintroduced Native
Stocked
REFERENCES FOR LIFE HISTORY- 52 and 136
Life History - 3 (DRAFT) - Management Practices
Species clam, softshell
Species Id M060160
Date 26 AUG 96
MANAGEMENT PRACTICES
RESULT MANAGEMENT PRACTICE
Beneficial Regulating harvest - setting size limits
Beneficial Regulating harvest - setting bag/creel limits
REFERENCES FOR BENEFICIAL MANAGEMENT PRACTICES - 52
COMMENTS ON MANAGEMENT PRACTICES -
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).*52*
Management Practices - 1 (DRAFT) - References
Species clam, softshell
Species Id M060160
Date 26 AUG 96
References
52* Abraham, B., P. Dillon. 1986. Species Profiles: Life Histories
and Environmental Requirements of Coastal Fishes and
Invertebrates (Mid-Atlantic) -- Softshell Clam. U.S. Fish and
Wildlife Service Biol. Rep. 1986(11.68) pp 18.
136 * Chesapeake Bay Program. 1988. Habitat Requirements for
Chesapeake Bay Living Resources. Chesapeake Executive
Council pp 86.
References - 1