Clamology 101

(from Washington Department of Fish and Wildlife)

6.53-pounder known as "Moby." Photo by Kristina Wilkening.

Clams belong to the group of animals called bivalves, which includes clams, mussels, oysters, and scallops. The soft body parts of these animals are enclosed between two shells; hence, the word bivalve. Bivalves are closely related to limpets, abalone, snails, slugs, squids, and octopuses. These animals, including bivalves, are collectively called mollusks.



Most bivalves reproduce by discharging sperm and eggs into the water where fertilization occurs. Some will spawn only when the water temperature reaches a critical level. The fertilized eggs develop into microscopic larvae, which are carried about by water currents for various lengths of time (usually 3-4 weeks) before setting, depending on the particular species and water temperatures. The larvae feed on microscopic organisms called plankton, and most species, after developing to the advanced stage, settle on and attach to gravel, shell, or sand grains, then burrow into the bottom. In contrast, oyster larvae cement themselves to hard, clean surfaces such as rocks and shells.



Since larval clams spend a considerable amount of time drifting about in the water before settling, the location where a particular larva ends up may be many miles from its parents. Many people assume that, if they transplant adult clams to their beach, they will soon populate the beach with clams. If the transplants survive and spawn, the resulting offspring will likely be scattered for several miles. A successful set of young clams on, any particular beach depends on a mass spawning of adult clams in a very large area, and on setting and survival on the beach in large numbers.

Hardshell clams include Manila and Native littlenecks, Butter clams, Cockles, Macomas and a few others of little harvest interest. These clams are found on beaches of mixed sand, gravel, and mud. They are commonly harvested using shovels or rakes. Except for the larger butter clams, rakes are usually most effective, and are less damaging to the clams and the beach.

Manila and Native clams are similar in appearance and grow to 3 or 4 inches in length. The shells of both species have concentric rings and radiating ridges, making a kind of lattice sculpture. Manilas seem to have more distinct radial sculpture at the posterior end. Both species can draw their siphons completely into their shell.



Manila clams (Venerupis philippinarum)

This important clam is not a native of North America, but was accidentally introduced to Washington State in oyster seed shipments from Japan. The animal quickly acclimated to our waters and is now found from British Columbia to northern California. They are similar in size and appearance to littlenecks; however, they are oblong in shape, being more long than high compared to littlenecks. The internal surface of the shells near the siphon end is normally stained a deep purple color or yellow. The shells completely close. Their siphons are short so they are buried to only about 4" fairly high in the intertidal zone. They inhabit a variety of substrates, from gravel to mud to sand, above the half-tide level, which is higher than the zone where butter and littleneck clams are found. The black siphon tips on Manilas are split. The inside edge of their shell is smooth to the touch. In Puget Sound, they are especially abundant in small outlets and lagoons. Growth is quite rapid with the clams reaching marketable size in two years.

This species has become a welcome addition to Washington's list of clams and is taken commercially by hand diggers and by sport diggers with shovels, forks and rakes. It is a shallow burrower with many found within the first two inches of substrate, and for this reason are easily harvested by hand-digging. Manilas account for 50 per cent of the annual commercial landings of hard-shell clams in Washington. This clam, like the native littleneck, is normally prepared by steaming. They are summer spawners.



Native littlenecks (Leukoma staminea)

The range of this species is from Alaska to Mexico. It is similar in size and appearance to the Manila littleneck. It is a medium-size clam, oval to round in shape. The external surface is marked with concentric rings and radiating ridges which produce a cross-hatched appearance. The color is variable, but is normally cream or grey, and sometimes mottled with brown markings. The shells close completely. They have an external hinge ligament and the black siphon tips of native littlenecks are fused together. The inside edge of their shell has a fine, toothed-edge, easily seen and felt. They are found in the top 4-6 inches of substrate of gravel-mud in protected bays. Normally this clam is found somewhat higher in the tidal zone than butter clams, being concentrated at about mid tide level, but are sometimes found mixed with butter clams in the lower intertidal and subtidal zones. They occur subtidally as deep as 60 feet. Sport diggers normally use rakes, shovels or forks to obtain them. On very rare occasions they can form pearls.They are delicious when steamed open and dipped in hot butter. This clams does not live as long out of the water as the Manila. Commercially they are dug by hand and mechanical harvesters. They are sold primarily in the fresh market with a large percentage of the production shipped to California. The littleneck clam is important to sport and commercial interests.

Native littlenecks spawn in the spring and early summer. Water temperatures of 60° F have been used successfully to initiate spawning in the laboratory.



Butter clams (Saxidomus gigantea)

This clam is found from Alaska to California. The shells are large (up to five inches in length), heavy, oval to square-shaped and externally marked with concentric rings, but without radiating ridges. They are yellow in color when young, changing to gray-white with age. When disturbed, the clam withdraws completely within the shell, leaving a slight opening between the valves in the area of the siphon or neck. Butter clams are normally buried in the substrate between 8-14 inches; sport diggers use shovels or forks. They prefer sand-gravel beaches and are concentrated in the lower intertidal and shallow subtidal zone. Some beds have been located in water as deep as 60 feet. They are an important commercial and sport clam. Many people consider them the best for chowder. Butter clams have a tendency to accumulate and retain PSP toxins, so be sure to check the Department of Health's PSP hotline (1-800-562-5632) before harvesting. Commercially, they are dug intertidally by hand during low tides and by mechanical harvesters in deeper water. A large portion of the commercial catch is canned. They spawn in the summer. Experiments indicate that water temperatures of about 70' F are required for spawning in the laboratory. Three to four weeks are needed to complete the larval period in laboratory cultures.



Cockles (Clinocardium nuttallii)

Cockles are found from the Bering Sea to California. Their shells are medium size, slightly higher than long, somewhat triangular in shape. They are easily recognized by the prominent, radiating, ribs which originate at the hinge line and fan out to the outer shell margin and are evenly spaced on the exterior of the shell. The shells, which are light brown, completely close. Cockles have short siphons and, therefore, are rarely buried more than an inch or two in the substrate. Shallowly buried, they are easily harvested by sport diggers at low tide who pick them from the surface by hand or with a garden rake. They inhabit sand and mud beaches intertidally and subtidally to 50-60 feet. They have never been observed in large concentrations in Puget Sound, but are common and widespread. The cockle has a powerful muscular foot, which gives it a high degree of mobility. They have been observed moving along the bottom by springing with the foot. Each hop can cover two to three feet. They frequently enter the commercial harvest with butter and littleneck clams but are not important commercially. They spawn in the summer.



Macoma clams (Macoma nasuta and Macoma brota)

Are found in sand or mud and have wafer-thin, chalky-white shells and reach a maximum length of about 4 inches. They appear to have two necks, but these are actually in-current and ex-current siphons. Macomas are generally found 4 to 6 inches below the surface in the middle of the intertidal zone.



Horse clams (Tresus nuttallii and Tresus capax)

These two species are distributed from Alaska to California. T. capax is the more northern form and T. nuttallii is more abundant in the south. These species are similar in appearance and most people are unaware that two species of horse clams exist. They are large clams with shells up to eight inches long. The shells of both species are oval and chalky-white or yellow with patches of brown periostracum (leather-like skin) on the shell. The shells are flared around the siphon and do not completely close (hence the name gaper). The clams are unable to completely retract the siphon within the shell. Horse clams prefer sand, mud, and gravel substrates. They are normally buried 12-16 inches in the substrate and are, therefore, much easier to dig than geoducks. They are found in the lower intertidal zones on out into water as deep as 50-60 feet. They are often found with butter and littleneck clams and, for this reason, are taken incidentally with these clams in the commercial harvest. These clams often harbor small commensal pea crabs which are easily observed when opening the clams for cleaning and which in no way affect the clam as a human food source. The meat is of good flavor and makes excellent chowder. They are largely ignored by sport diggers in Washington but are an important sport clam in Oregon. They are abundant and widespread in Puget Sound. Present sport regulations encourage more use of these clams. Perhaps the reason they tend to be avoided by sport diggers is that the edible portion compared to the total weight is low and the tough skin which covers the siphon is difficult to remove. The two species differ in several aspects, but probably the easiest method to differentiate the two is that T. nuttallii normally has larger siphonal plates (horny plates found at the tip of the siphon often with algae or barnacles attached.) than T. capax, and T. nuttallii shells are longer compared to their height than T. capax. T. nuttallii is a summer spawner and T. capax spawns during the winter.



Eastern soft shell (Mya arenaria)

This clam is distributed on the west coast of North America from Alaska to California. Eastern softshells were thought to have been introduced to the west coast from the east coast of the United States in shipments of oyster seed. Recent studies, however, indicate that they may be native to this coast. They are medium-sized clams, with shells which are easily broken. Softshell clams are often mistaken for small horse clams. The shell is rounded at the foot end and rather pointed at the siphon end. The external surface of the shell is marked with uneven concentric rings. They have brittle, thin elongated shells, colored chalky-white to gray with brown or yellow periostracum at the edges. Their siphons are dark. This clam, as well as other clams in this genus, has a large spoon-shaped structure in the inside of the left valve near the hinge. The shells gape, but the clam is able to completely retract its body within the shells. Soft shells are normally found in sand and mud, and are most abundant in the upper half-tide level near river mouths or heads of bays where low salinity water occurs. Soft shells normally bury to a depth of 8-14 inches, and are taken with shovels or standard garden forks by sport diggers. These clams are not as popular with sport diggers in Puget Sound as are butter or littlenecks. A small commercial fishery has recently developed on these clams on private intertidal ground in Skagit Bay and Port Susan, where extensive beds exist. This species is highly prized for food on the Atlantic Coast and contributes to an important commercial fishery there.



Geoduck (Panopea generosa)

The most impressive clam in the Pacific Northwest is the geoduck (Panopea generosa). The world's largest burrowing clam, the geoduck reaches an average size of 2.07 pounds (including the shell) in subtidal waters of Puget Sound (based on surveys of commercial beds before fishing). The average size of recreationally caught geoducks on intertidal public beaches in Puget Sound is 2.47 pounds. The largest geoduck ever weighed and verified by WDFW biologists was a 8.16-pound specimen dug near Adelma Beach in Discovery Bay in year 2000. Much larger specimens have been reported by commercial harvesters. Geoducks grow rapidly, generally reaching 1.5 pounds in three to five years. They attain their maximum size by about 15 years, and can live at least as long as 168 years. They are extremely abundant in the inland waters of Puget Sound, British Columbia and Alaska, where the subtidal populations support important commercial fisheries. Their range extends from Alaska to Baja California, but they are rarely found along the Pacific coast, and populations are likewise scarce west of Clallam Bay in the Strait of Juan de Fuca. Geoduck clams are found buried two to three feet deep in mud, sand, or gravel. The gaping, oblong shell is white with concentric rings, and generally has thin patches of flaky brown covering (periostracum) at the edges. The siphon and mantle are so large that they cannot be withdrawn into the shell.



Geoducks have been observed with underwater video cameras living as deep as 360 feet in Puget Sound, and the vast majority of the population is subtidal. They are not nearly as abundant intertidally, and sport diggers generally find them on beaches only at extreme low tides (lower than -2.0 feet). For this reason, most of the sport digging is restricted to less than 20 tides a year.

The clam's name, pronounced "gooey-duck" is of Native American origin and means "dig deep." It is variously spelled goeduck, goiduck, or gweduck.

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