Mya arenaria
Softshell clam
Species Description
The softshell clam is a marine/estuarine bivalve species of the Family Myidae. This species attains a maximum shell length of about 15 cm (10-11 in the Chesapeake Bay), with an oval, elongated shell ranging from white to dark grey in color. Sediment type can dictate both coloration and size and shape with coarse sediment producing smaller and darker clams. The shell is thin and brittle and has a series of easily discernable concentric rings. It has two relatively long fused siphons used for filtering water for food and, important as an identification aid, the interior left valve has a chondrophore, supporting the hinge ligament.
(Abraham and Dillon 1986; Baker and Mann 1991).
Geographic Range
Mya arenaria inhabits intertidal zones and subtidal depths to almost 200m from Labrador, Canada through Florida along the east coast of the United States and in Western Europe from Norway to the Black Sea. This species has been introduced along the Pacific coast of the United States, from Alaska through California.
Although its Western Atlantic range appears extensive, softshell clam populations are most abundant from Maine to the Chesapeake Bay.
Current status, Chesapeake Bay.
Once widely distributed throughout the Bay, softshell clam populations of any consequence are now relegated to the Upper Bay, from the Patapsco River to the West River along the western shore and from Swan Point to Kent Point along the east shore, including the Chester River and Eastern Bay. Lesser populations are found in certain areas of the Choptank River, Tangier Sound and its tributaries and the Patuxent River. Since 1968, when the relatively modest fishery inexplicably collapsed, Virginia softshell clam populations may be characterized as remnant.
((Abraham and Dillon 1986; Baker and Mann 1991; Homer et al. 2011; Laursen 1966; Pfitzenmeyer 1972; Theroux and Wigley 1983).
Life History
Spawning/Larval Development. - Softshell clams typically have two spawns during a calendar year in the Chesapeake Bay, one in the spring and another during the fall. Both are triggered by temperatures in the 10-20oC range with optimal spawning temperatures 12-15oC.
Fertilization is external, sex ratio is close to 1:1, and egg production is highly correlated with shell length with females attaining sexual maturity at about 40mm. Large clams (>60mm) can produce 80,000-120,000 eggs per spawning period.
Within 12-24hrs, the fertilized egg develops into a trochophore (ciliated, free-swimming stage) and after another 2-3 days develops into a veliger (ciliated, rudimentary calcareous valves, free-swimming) larvae. After 2-6 weeks at about 0.2mm, veligers metamorphize into juvenile clams, secreting byssal threads used to attach to various types of substrate.
Juveniles/Adults. - Juveniles retain the ability to produce byssal threads until they reach a size of 12-15mm while they search for suitable habitat to dig burrows. These small clams, referred to as rice clams, are at great risk of predation as they move around the bottom and until they are able to dig relatively deep (>3 or 4cm) burrows, typically at shell lengths of >35mm. Juvenile softshell clams grow rapidly in the Chesapeake Bay region, often attaining the commercial minimum size of 50mm within two years.
Softshell clams reach sexual maturity after about 1.5 years and typically have a life span of 10-12 years although there are data indicating that this species can live nearly three decades. Adults inhabit permanent burrows up to 30cm deep, although strong storms may displace them.
(Abraham and Dillon 1986; Baker and Mann 1991; Brousseau 1978; Hanks 1963; Kennedy and Mihursky 1971; Loosanoff and Davis 1963; Pfitzenmeyer 1965; Shaw 1965; Smith 1955; Stickney 1964(a); Stickney 1964(b)).
Habitat Requirements
Although found in all types of habitat, softshell clams appear to thrive in areas with fine sediment, particularly mud, sand, and mud/sand mixtures. Growth rates are greatest in such areas as coarse sediments can restrict growth by impeding their burrowing ability. Survivorship, however, is greatly enhanced in areas thick with shells, gravel, and other large structures as this material provides refuge from most predators.
In the Chesapeake Bay, Mya arenaria can be found from intertidal zones to subtidal areas to at least 15m. Although it is assumed that the bulk of the clam population occurs in depths ranging from 1.5 to 5m, this range may be an artifact of where the commercial fishery operates.
Current status, Chesapeake Bay.
During the last several decades, softshell clam populations have increasingly retreated from soft sediment type areas to bottom with significant structure. Where sand flats and muddy sediments once supported enormous populations of softshell clams, only occasionally are softshell clams now found in such places and these rapidly disappear, presumably to predation. The only areas that currently support significant populations of Mya arenaria are those with structure on the bottom, stones, rocks, shell, and man-made material. During softshell clam population surveys conducted between 2001 and 2008, over 80% of all Mya arenaria collected were taken from sites with significant substrate structural overburden.
It is unlikely that significant populations of softshell clams inhabit intertidal zones in the Chesapeake Bay. Surface summer water temperatures reach or exceed lethal levels on a regular basis in the Bay. The status of clams in depths greater than 5m is not known given the difficulty in sampling such areas. Given the high frequency of anoxic events in the Chesapeake, it is doubtful that this species inhabits depths greater than 8m.
(Abraham and Dillon 1986; Baker and Mann 1991;Chanley 1958; Chanley and Andrews 1971; Hidu 1981; Homer et al. 2011; Kennedy and Mihursky 1971; Matthiessen 1960; Newell and Hidu 1982; Pfitzenmeyer and Drobeck 1963; Pfitzenmeyer and Drobeck 1967; Shaw 1965)
Ecological Role
A filter feeder, Mya arenaria draws water through its inhalant siphon, collects suspended particles via secreted mucus, which is then carried to its mouth where labial palps sort the particles. Rejecta are compacted into pseudofeces and expelled.
Softshell clams are known to be prodigious filtering animals, but as rates vary from region to region, related to food availability, temperature, and size class structure, it's difficult to attribute an "average" filtration rate to this species. Some accounts have estimated that softshell clam populations can daily filter a volume equivalent to the entire water column encompassing the extent of the population. Suffice it to say, softshell clams, on an individual basis, either match or exceed filtration rates attributed to the Eastern oyster.
Burrow construction and maintenance can have a significant effect on microbial activity and geochemical cycles. Studies have shown greatly enhanced bacterial activity in and around softshell clam burrows resulting in significantly greater rates of sulfate reduction, polycyclic aromatic hydrocarbon (PAH) degradation, and aeration of sediments.
Just about every taxonomic group preys upon softshell clams, from Nemerteans to Cetaceans. The softshell clam has been an important, sometimes dominant prey item for species such as blue crabs, horseshoe crabs, summer and winter flounder, Atlantic croaker, and spot, just to name a few. Additionally, juvenile clams are prey for numerous other benthic invertebrate populations including several species of polychaetes, snails, mud crabs, and shrimp.
Current status, Chesapeake Bay.
Abundance data from 1962-1975 as compared to similar survey data from 2001-2008 indicate that softshell clam populations in Maryland have declined, in general, by over 90%. This differential is not insignificant to the Bay's ecosystem. It has been estimated that prior to the early 1990's collapse of clam populations, Mya arenaria numerically comprised over one-third of all the large bivalve species (oysters and softshell, razor, and hard clams) in Maryland. One must therefore conclude that this species no longer has a prominent functionality role in the Chesapeake Bay ecosystem.
(Baker and Mann 1991; Hansen et al. 1996; Haven 1970; Hidu and Newell 1989; Homer and Boynton 1978; Homer and Mihursky 1991; Homer et al. 2011; Lipcius and Hines 1986; MacKenzie 1997; Virnstein 1977).
Diseases/Parasites
Mya arenaria populations are beset by numerous diseases and parasites. Among these are several cancer-like diseases that attack gill, liver, and gonadal tissue and blood cells. Other diseases and conditions found in softshell clams include hypoplasia, gill epithelial cell nuclear hypertophy (GENH), apparently caused by a viral agent, procaryote or rickettsial infections, and concretions (lipofuscin) located in the kidney.
A variety of parasites have been found in softshell clams including Perkinus sp., ciliates, and trematodes.
Current Status, Chesapeake Bay.
In the Chesapeake Bay region, the most serious disease has been and is disseminated neoplasia (DN Disease), although the effects of GENH on softshell clam population levels is currently not known. The most damaging parasite afflicting Mya arenaria is Perkinsus chesapeaki, also known as dermo disease.
(Sindermann and Rosenfield 1967).
Chesapeake Bay Population/Fishery, History and Current Status
As previously mentioned, softshell clams were at one time a major component of the Chesapeake Bay's benthic community and, since the early 1950's, an important commercial species. The timeline given below lists major events that appear to have affected both the commercial fishery and clam stocks.
1951 Hydraulic escalator dredge first used to harvest softshell clams
1952 - 58 Various regulatory restrictions, including shoreline distances, exclusion from charted oyster bars, etc.
1954 Perkinsus sp. reported to infect Virginia softshell clams
1965 Major mortality event in Potomac River
1968 Collapse of the Virginia fishery
1971 Major mortality event in Maryland's Chesapeake Bay
1971 Unknown hyperplasia found in clam gill tissue
1971 Bruce Decision, overturning the regulation that restricted watermen from working in county waters other than that in which they reside
1971 Daily catch limit reduced from 40 to 25 bushels; cull size increased from 2 to 2.25 inches
1972 Tropical Storm Agnes floods Chesapeake Bay with freshwater, sewage, and sediments
1972 Fishery closed from June 1972 until June 1973, closed again in June 1973, and re-opened in September 1973
1973 Daily catch limit reduced to 15 bushels
1975 Cull size reduced to 2 inches
1980 Increase in American eel prices results in targeting of razor clams for blue crab bait
1984 Disseminated neoplasia (DN disease) found in Maryland softshell clams
1990 Perkinsus sp. found in Maryland softshell clams
2002 Major mortality event in Maryland
2002 - present Continued decline of softshell clam populations in Maryland, no
disease or parasite abatement, periodic recruitment eroded by disease, parasites, and predation
Literature Cited
Abraham, B.J. and P.L. Dillon. 1986. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (Mid-Atlantic): softshell clam. US Fish and Wildl. Serv. Biol. Rep. 82, TR-EL-82-4.
Baker,P.K. and R. Mann. 1991. Soft Shell Clam. In: S. Funderburk, J.A. Mihursky, S.J. Jordan, and D. Riley (eds). Habitat Requirements for Chesapeake Bay Living Resources. U.S. Fish.Wildlife Ser. Annapolis, Md.
Brousseau, D. J. 1978. Spawning cycle, fecundity, and recruitment in a population of soft-she11 clam, Mya arenaria, from Cape Ann, Massachusetts. U.S. Natl. Mar. Fish. Serv. Fish. Bull. 76(1): 155-166.
Chanley, P.E. 1958. Survival of some juvenile bivalves in water of low salinity. Proc. Natl. Shelf. Assoc. 48: 52-65.
Chanley, P.E. and J.D. Andrews 1971. Aids for identification of bivalve larvae of Virginia. Malacologia 11: 45-119.
Hanks, R.W. 1963. The soft-shell clam. U.S. Fish Wildl. Serv. Fish. Circ. 162.
Haven, D.S. 1970. A study of the hard and soft clam resources of Virginia. Virginia Inst. Mar. Sci. Grant Rep. 69p.
Hansen, K., G.M. King, and E. Kristensen. 1996. Impact of the soft-shell clam Mya arenaria on sulfate reduction in an intertidal sediment. Aquat. Microb. Ecol. 10:181-194.
Hidu, H. and C.R. Newell. 1989. Culture and ecology of the soft-shelled clam, Mya arenaria. In: J.J. Manzi and M. Castegna (eds.). Clam Mariculture in North America. Elsevier, Amsterdam, pp. 277-292.
Hidu, H. 1981. Mya arenaria – nonobligate infauna. J. Shellfish. Res. 1:116.
Hidu, H. and R.C. Newell. 1989. Culture and Ecology of the Soft-Shelled Clam, Mya arenaria. In: J.J. Manzi and M. Castagna (eds.). Clam Mariculture in North America. Elsevier, p.277-292.
Homer, M., C.F. Dungan, and M. Tarnowski. 2011. Assessment of Chesapeake Bay Commercial Softshell Clams, Mya arenaria and Tagelus plebeius, with emphasis on abundance and disease status. Completion Rep. to NOAA Ches. Bay Fish. Sci. Prog., NA07NMF4570326.
Homer, M. and W.R. Boynton. 1978. Stomach analysis of fish collected in the Calvert Cliffs Region, Chesapeake Bay-1977. Final Rep. To Md Dept. Nat. Res., Power Plant Siting Prog., Univ. Md, Ches. Biol. Lab. Refernce No. UMCEES 78-154-CBL.
Homer, M. and J.A. Mihursky. 1991. Spot. In S.L. Funderburk, S. Jordan, J.A. Mihursky, and D. Riley (eds). Habitat Requirements For Chesapeake Bay Living Resources. Second Edition, Ches. Research Consortium, Solomons, Maryland, p 11-1 to 11-19.
Kennedy, V.S., and J. A. Mihursky. 1971. Upper temperature tolerances of some estuarine bivalves. Ches. Sci . 12(4): 193-204.
Laursen, D. 1966. The genus Mya in the Arctic region. Malacologia 3: 399-418.
Lipcius, R.N. and A.H. Hines. 1986. Variable functional responses of a marine predator in dissimilar homogeneous microhabitats. Ecology 67: 1361-1371.
Loosanoff, V.L. and H.C. Davis. 1963. Rearing of bivalve mollusks. Adv. Mar. Biol. 1: 1-136.
MacKenzie CL, Jr. 1997. The molluscan fisheries of Chesapeake Bay. In: CL MacKenzie Jr., editor. The history, present condition, and future of the molluscan fisheries of North and Central America and Europe. NOAA Tech. Rep. 127:141-169.
Matthiessen, G.C. 1960. Observations on the ecology of the soft clam, Mya arenaria, in a salt pond. Limnol. Oceanogr. 5: 381-388.
Newell, C.R. and H. Hidu. 1982. The effects of sediment type on growth rate and shell allometry in the soft shell clam Mya arenaria. J. Exp. Mar. Biol. Ecol. 65: 285-295.
Pfitzenmeyer, H. T. 1972. Tentative out1ine for inventory of molluscs: Mya arenaria (soft-shel1 clam). Ches. Sci. 13(Suppl.): 182-184.
Pfitzenmeyer, H. T. 1965. Annual cycle of gametogenesis of the softshelled clam, Mya arenaria, at Solomons, Maryland. Ches. Sci . 6: 52-59.
Pfitzenmeyer, H. T. and K.G. Drobeck. 1963. Benthic survey for populations of soft-shelled clams, Mya arenaria. Ches. Sci. 4: 67-74.
Pfitzenmeyer, H. T. and K.G. Drobeck. 1967. Some factors influencing reburrowing activity of soft-shell clam, Mya arenaria. Ches Sci. 8: 193-199.
Shaw, W.N. 1965. Seasonal gonadal cycle of the male soft-shell clam, Mya arenaria, in Maryland. U.S. Fish Wild. Serv. Spec. Sci. Rep. Fish. 508.
Sindermann, C.J. and A. Rosenfield. 1967. Principal diseases of commercially important marine bivalve Mollusca and Crustacea. Fish. Bull. 66: 335-385.
Smith, O.R. 1955. Movements of small soft-shell clams (Mya arenaria). U.S. Fish Wildl. Serv. Spec. Sci. Rep. Fish. 159.
Stickney, A.P. 1964(a). Feeding and growth of juvenile soft-shell clams, Mya arenaria. Fish. Bull 63(3): 635-642.
Stickney, A.P. 1964(b). Salinity, temperature, and food requirements of soft-shell clam larvae in laboratory culture. Ecology 45: 283-291.
Theroux, R.B. and R.L. Wigley. 1983. Distribution and abundance of east coast bivalve mollusks based on specimens in the National Marine Fisheries Service Woods Hole collection. NMFS Spec. Sci. Rep. 768.
Virnstein, R.W. 1977. The importance of predation by crabs and fishes on benthic infauna in Chesapeake Bay. Ecology 58: 1199-1217.
