Surf clam (Spisula solidissima)

Description

Surf clams (Spisula solidissima) have thick, triangular, yellowish-white shells with rounded edges and concentric ridges. The shells do not close fully and therefore slightly gape (Fay et al. 1983). Surf clams are the largest bivalve in the mid-Atlantic Bight, with commercially harvested adults averaging 12 to 15 centimeters (Pearce et al. 1981; Caracciolo and Steimle 1983) and the largest individuals reaching approximately 20 centimeters (Weiss 1995). Surf clams utilize an unusual behavior in response to stress: they leap from the sediment surface in order to relocate. Surf clams have been observed using this avoidance behavior in response to crowding and the presence of predators (Caracciolo and Steimle 1983).

Background

Distribution. Surf clams are found from the Gulf of Maine to Cape Hatteras, North Carolina (Fay et al. 1983). Water currents play an important role in the distribution of juvenile clams and eventual settlement patterns. Adult beds are found from beach zones to an average of 50 meters in depth. During harsh winter storms, surf clams can be thrown onto beaches; one such storm placed an estimated 50 million clams along a 10-mile stretch of beach (Caracciolo and Steimle 1983). In New York/New Jersey Harbor, surf clams are found predominantly in the area where the harbor opens into the Atlantic Ocean.

Surf clams are mostly oceanic in distribution, preferring turbulent waters at the edge of the breaker zone. They can be found in some estuarine areas, but their distribution is limited by salinity (Fay et al. 1983). Juvenile clams prefer medium to fine, low organic sands averaging 9 to 25 meters in depth. Adults prefer medium- to coarse-grained sand and gravel, burying themselves just below the sediment surface. They are often found at evenly distributed positions relative to one another, with spacing interval negatively correlated to density. Additionally, adults often remain in their juvenile burrows unless they are displaced by storm events (Fay et al. 1983).

Feeding. Surf clams are planktivores during all life stages (Caracciolo and Steimle 1983, Fay et al. 1983). As larvae, surf clams consume algal cells, and adults primarily feed on diatoms, green algae, and naked flagellates. Adult surf clams have an incurrent siphon surrounded by a ring of papillae. Food particles are trapped on the siphon’s mucus lining, which is transported to the stomach for digestion, and excreta are removed via the excurrent siphon.

Fishery. Surf clams have only recently gained popularity as a commercially harvested species. Prior to World War II they were mostly used as a bait species, but increased demand in the early 1970s resulted in almost 75 percent of the U.S. clam market comprised of surf clams. According to the National Marine Fisheries Service (1999), until the 1970s, most of the fisheries for surf clams came from New Jersey. The U.S. Food and Drug Administration closed the New York fishery because of bacterial and chemical contamination (Caracciolo and Steimle 1983). Populations in most inshore areas are depleted; therefore the fisheries are currently offshore. In 1981 a minimum size catch was instituted, which further pushed claming offshore.

Life Cycle

Surf clams have two spawning periods, the first in mid-July through early August and the second in mid-October to early November, and these periods are believed to be synchronous across an entire bed. The timing depends on specific changes in temperature, with only one spawning cycle observed in cooler years (Pearce et al. 1981, Caracciolo and Steimle 1983, Fay et al. 1983). Eggs are spherical and approximately 1.5 millimeters in diameter. Planktonic larvae develop shell valves and "feet" prior to settling. Juvenile clams develop after 21 days and begin their sedentary life stage at less than 1 millimeter in size. Surf clams grow to approximately 4.5 centimeters by the end of their first year, and sexually mature by the end of their second year. After five or six years, surf clams reach commercial size (12.5 centimeters; Caracciolo and Steimle 1983). Adults are estimated to live 25 years with lengths reaching 20 centimeters; on average, open water adults live longer than inshore adults (Fay et al. 1983).

Environmental Influences

Salinity. Salinity is a critical factor limiting the distribution and survival of surf clams. Larval clams require a salinity of 16 parts per thousand (ppt) and adults can tolerate 12.5 to 52 ppt (Fay et al. 1983). According to Caracciolo and Steimle (1983), surf clams prefer salinities greater than 28 ppt, and this keeps them from most estuarine environments.

Predation. Though little is known about many of the species that prey on surf clams, two species of moon snail prey on clams approximately 80 millimeters in size (Fay et al. 1983). Suspected predators include boring snails, ducks, haddock, and cod, which typically consume smaller adults. Fish have mostly been observed feeding on surf clams only after a major storm event (Fay et al. 1983). Although salinity limits surf clam distribution, larval predation may play a role in habitat selection (Caracciolo and Steimle 1983). Predation by crabs, gastropods, and bottom-feeding fish have been observed to limit development of beds in nearshore areas colonized by larval surf clams.

Pollution. Surf clams are affected by sewage and metal pollution in the New York area. High fecal coliform counts were found in most of the clams within 11 kilometers of a sewage dumpsite, and these dense populations were unfit for human consumption. Metals such as silver, iron, copper, and even arsenic from oceanic dump sites and outfalls have also been the cause of much of the fisheries closures in the New York Bight area.

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