Soft-shelled Clam (Mya arenaria)
Alternate common names: Steamer, Longneck.
Color: Shell is chalky white to dark grey.
Size: Up to 4 inches long. Siphons can extend several inches out of the shell.
Habitat: Burrowed in sandy or muddy bottoms of bays and estuaries.
Seasonal appearance: All year.
Soft-shelled clams are thin, oval-shaped bivalves that can grow up to four inches long. They have two long siphons covered in a thick, black skin. Within the shell is a soft body composed of two gills, a heart, a stomach, a kidney, and a large muscular foot. The gills are used for respiration and for feeding.
Life History and Behavior
The soft-shelled clam is a suspension or filter feeder. Water is brought into the clam through an intake siphon, which extends from its shell to the surface of the mud. As the water passes over the gills, oxygen is removed for respiration and small hair-like structures known as cilia trap plankton for food. The particles of food are transported to the mouth, and the water is passed out through the exit siphon. When the tide is high, the siphons are extended out of the burrow; they are retracted during low tide. The siphons are encased in a fleshy tube and cannot be fully retracted into the shell. Because soft-shelled clams burrow, the siphons may be the only part of the clam a beachcomber will see.
Soft-shelled clams spawn in early summer. The eggs develop into free-swimming larvae, or plankton, and eventually settle onto a hard substrate, attaching themselves with a sticky thread secreted from a large, mobile foot. This byssal thread keeps them from being swept away by waves. Once mature, the foot reduces in size, and each animal releases from the substrate to burrow into the sediment where it will remain for life.
The moon snail is the primary predator of the soft-shelled clam. Moon snails secrete an acidic chemical that softens the shell, and the snails easily drill through and eat the soft-shelled clams. Large groups of moon snails can destroy soft-shelled clam beds. Digging fish, such as sea robins, also prey upon soft-shelled clams; their digging behavior disrupts the burrows. Once dislodged from the burrow, a soft-shelled clam cannot burrow quickly and is easy prey.
Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.