Softshell clam (Mya arenaria)

Description

The softshell clam (Mya arenaria) has white to gray shell valves that are very brittle and thin (Abraham and Dillon 1986; Weiss 1995). The shell has an egg-shaped form that gaps at both ends and is generally 75 to 100 millimeters in length. Softshell clams have long incurrent siphons that draw in food but which cannot be fully withdrawn into the shell (Abraham and Dillon 1986).

Background

Distribution. Softshell clams are a nearshore species that can be found along the Atlantic coast from the subarctic to Cape Hatteras, and sometimes extending to South Carolina. Softshell clams were introduced to the West Coast from Alaska to San Francisco (Abraham and Dillon 1986). In New York/New Jersey Harbor, softshell clams have been observed in the intertidal flats and shallow water areas throughout the harbor except the northeastern portion (MacKenzie 1992). MacKenzie (1992) documented the highest abundance of softshell clams in the broad flats of Great Kills Harbor and from Keyport to Atlantic Highlands; they have also been observed in the Navesink and Shrewsbury rivers.

Soft clams are commonly found in polyhaline and mesohaline zones (Ristich et al. 1977), from the littoral zone to 9 meters (Gosner 1971). Preferred substrate is a sandy bottom with less than 50 percent silt, but they have been observed in sandy, sand-mud, or sandy-clay substrates (Abraham and Dillon 1986). Densities of softshell clams have been observed at 18 to 24 clams per square meter, though they can reach higher quantities in polyhaline to upper mesohaline areas such as the mouth of an estuary (Ristich et al. 1977). The highest densities are found at 3 to 4 meters, temperatures less than 28 degrees Celsius, and salinities greater than 4 to 5 practical salinity units (psu; Abraham and Dillon 1986). According to Steimle and Caracciolo-Ward (1989) and Dean (1975), softshell clam populations show cyclic occurrences with periods of high and low densities.

Feeding. Softshell clams feed on small detrital particles, phytoplankton, small zooplankton, and bacteria (Chesapeake Bay Program 1987). The larger incurrent siphon draws food into the clam, and wastes are released through the excurrent siphon.

Fishery. Known commonly as a "steamer clam," the softshell clam is the third most important commercially harvested clam in the U.S., with 7.9 million pounds harvested in 1984 (MacKenzie 1992). Historically, the soft clam was a staple food for the Indians of the New York/New Jersey Harbor area. Since settlers arrived in the area, the softshell clam has been harvested from Keyport to Atlantic Highlands, and currently clam populations can support moderate harvesting, mostly in rivers adjacent to New York/New Jersey Harbor (MacKenzie 1990). According to National Marine Fisheries Service data (1999), harvesting reached a peak in 1969 and has been slowly declining since that time. Commercial harvest of adult clams results in disturbance of juveniles, exposing them to predation before they can rebury (Chesapeake Bay Program 1987). Loss of eelgrass habitat in the 1930s has been suggested as a possible indirect cause of softshell clam population declines in the 1940s (MacKenzie 1990). Protection from wave energy provided by the eelgrass beds was lost, and as a result, mudflats used by harvesters were almost completely washed away by the 1980s. Abraham and Dillon (1986) estimated that to be considered commercially productive, the clam must have a density of 100 to 200 per square meter.

Life Cycle

Softshell clams spawn twice each year, generally in May and October, when water temperatures range 10 to 20 degrees Celsius. Softshell clams have high fecundity and very low survivorship (estimated at 0.1 percent of total egg production; Abraham and Dillon 1986). After fertilization, eggs develop into planktonic larvae that pass through various developmental stages, marked by growth and the formation of calcareous shell valves. After 2 to 6 weeks, the larvae settle onto hard substrates and attach via secreted byssus. The attached juveniles continue to grow and foot development occurs. At 7 millimeters, juveniles burrow into the sediments and take up active, adult-like lifestyles. In approximately two years, the juveniles reach the commercial size of 5 centimeters, and sexual maturity is reached at approximately 5 years. Life span is estimated between 10 and 12 years, but some shells have been estimated at greater than 28 years (Abraham and Dillon 1986).

Environmental Influences

Salinity. The most significant factor affecting the distribution of soft clams is salinity (Abraham and Dillon 1986). The clams tend to be euryhaline, with some living in primarily marine environments (35 psu) and others in estuarine habitats (10 to 25 psu). In laboratory experiments conducted in 1974 to 1976, inhibition of feeding was observed in response to rapid decreases in salinity, but no significant mortality was observed (Abraham and Dillon 1986).

Predation. Most of the predation on soft clams occurs during the larval and juvenile stages. Larvae are an important food source for larger planktonic organisms, including larval fish, jellyfish, and comb jellies; juveniles are preyed on by oyster drills, crabs, starfish, horseshoe crabs, whelks, and moon snails (Abraham and Dillon 1986). Additionally, softshell clams are an important food source for many adult and juvenile bottom-dwelling fish, such as spot (Leiostomus xanthurus) and winter flounder (Pleuronectes americanus).

Pollution. Soft clams are relatively tolerant to changes in sediment organic content (Diaz and Boesch 1982), and they have been observed with the pollution-tolerant polychaete, Streblospio benedicti (Reish 1979). Numerous beds have been closed as a result of sewage contamination and accompanying high coliform bacteria counts. Softshell clams are seriously affected by oil pollution, more so than other shellfish, and may suffer from inhibited growth, development of gonadal tumors, or increased mortality (Abraham and Dillon 1986). Additionally, softshell clams are known to take up silver and other metals from industrial wastes in very high concentrations. Once beds are destroyed, repopulation can take many years for sufficient larval recruitment and growth (Abraham and Dillon 1986).

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