The Puget Sound cockle called Nuttall's Cockle, or the Heart Cockle for its heart-like shape, is one of our more interesting clams.
Unlike other clams that live deep in the sediment to avoid predators, the cockle lives just under the surface using very short siphons to suck in plankton and oxygen. This exposes it to gulls, crabs and large sea stars. However, it has several features to help it avoid being eaten by gulls, which drop it on hard surfaces, or by red rock crabs using strong claws. The first is a thick shell that is heavily ribbed, giving the shell strength to resist breakage. Also, the margin of the ribs are wavy so the shell halves interlock, keeping the shell margin from slipping when dropped or squeezed.
For protection from sea stars, the outer lip of flesh around the margin of the shell sports tiny tentacles and eyes that are used to detect nearby movement and chemicals. This allows the clam to sense, then avoid, the sunflower sea star or the pink short-spined sea star by using its well-developed foot to flip away.
The cockle can be found in muddy to pure sand in sheltered conditions. Areas with more muddy sediment that supports eelgrass also can have high populations. Their vertical range is from the mid-intertidal to a depth of 100 feet. They have been found from Japan and the Bering Sea south to Southern California. Similar species are found worldwide, where they are often used for human food.
They can live for a long time, between 15 and 19 years, based on counts of the prominent growth rings of the northern variety. The rings aren't formed during the winter, when the cockle greatly reduces feeding. They mature by their second year in Puget Sound and spawn in July and August. This 3.5-inch specimen, which has at least several growth rings, could grow to a maximum of 5.5 inches.
Another interesting feature is that they may contain a small commensal pea crab inside the mantle cavity that feeds on material strained out of the incurrent water stream by the clam's gills.
David W. Jamison is a marine biologist and Boston Harbor resident.