Soft Shell Clams Spawn Twice A Year

A juvenile Soft-shell clam

By Joe Reynolds
NY Harbor Nature Blog

Bees do it and birds do, but both do it mainly in the spring. For soft-shell clams (Mya arenaria), once is just not enough. They like to do it twice a year.
 
The soft shell clam is a common bivalve mollusk within estuarine waters of New York Harbor with a thin, oval, elongated shell. They usually spawn once in late spring and once in mid-to-late autumn. Which means the clams are getting ready to once again create new life.
 
There is a reason for the twin spawning times. Clams are low on the food chain, which means just about anything loves to eat a clam. These little bivalves need to reproduce often and with many young if they wish for the species to survive.
 
Predators are quite abundant and include several species of crabs and horseshoe crabs, snails, sea stars, killifish, rays, skates, and even diving ducks and raccoons. Humans also harvest soft shell clams commercially. With so many predators, it’s no wonder why soft shell clams are capable of reproduction after their first year of life. They need to hurry up and just do it.
 
Similar to hard clams, soft-shell clams reproduce by releasing eggs and sperm into the water column. Once the eggs have been fertilized, free-swimming larva develops within a few hours or within one day. Though the larva are able to swim freely, currents in the bay, winds and wave action often disperse the little critters a great distance away from the site of fertilization. No worries, though, this action helps to increase genetic diversity among the species. Disbursement takes place for about 1 to 3 weeks,
 
During this time, many clams will become food for plankton eating fish and shellfish. But those that survive will go on to change into juvenile clams, resembling small adults with a small foot and shell.
 
Next they will find an appropriate place to settle by crawling along the bottom of the bay looking for suitable sandy substrate and low wave action. Ultimately each living clam will find a permanent place to burrow and call home. They will anchor to the bottom of an estuary using thin threads secreted from a gland on the foot and dig into the sediments using their muscular foot.
 
When soft shell clams become adults, the remainder of their life is spent beneath the sandy sediments, up to a foot in some places, in relatively the same location with minimum movement, often buried in a mixtures of sand, mud and gravel. They can tolerate a variety of salinities and withstand below freezing temperatures too, which makes them well adapted to an estuarine life.
 
Like all other bivalves, soft shell clams are filter feeders extracting their food, which is mostly plankton, from the water column by use of a slimy siphon. One clam can filter up to a gallon of water per day.
 
When a soft-shell clam feels threatened, it will often spew out a long spurt of water via their siphon and dig down deeper into the sand. This squirting behavior has earned the soft-shell clam a nasty nickname of "piss clam."