Scallops swim for two reasons, as an escape response and to migrate.
Swimming by the bay scallop is often observed while snorkeling in shallow sea grass beds, an escape response triggered visually using their eyes or equally by water disturbance detected by the mantle tentacles.
Swimming, or a single jump, can be elicited repeatedly by touching the tentacles with a common predatory snail such as the banded tulip, another common sea grass inhabitant. In fact, if one touches the tentacles extending from the ears of the shell adjacent to the hinge the scallop will almost always swim.
Alternatively, if contact is made anywhere else around the shell the scallop will respond with a backward jump.
For fun, see if you can replicate these behaviors when scallop season opens in St. Joseph Bay.
How did these behaviors evolve, when the protective response to a disturbance by nearly all other bivalves is to “clam up”?
The adductor muscle of these bivalves is unique in being able to keep the shell tightly closed for extended periods without fatigue, as is familiar to anyone attempting to open an oyster. Paradoxically, the response in scallops is to open the shell widely, exposing the tasty innards. The effect, however, is to fill the interior of the shell with seawater, which the scallop then uses to jet away from a predator.
For swimming, the mantle tissues form a curtain around the front of the shell to enclose the water except for pores open at the shell ears where water is jetted backwards when the shell closes. Repeated rapid closures result in swimming, simulated as by a Pacman taking bites out of the water.
Alternately, the single jump involves contractions of the mantle curtain to form a pore at the point of contact through which water is jetted toward the irritant with the scallop retreating backward.
Two reasons contribute to the highly unusual swimming behavior in scallops.
Foremost, perhaps, is that the ability to migrate takes advantage of optimum conditions for survival. Cold-water species are capable of sustained swimming over long distances.
Also, in connection to swimming, the ears of the shell at the swimming pores do not close completely making the scallop susceptible to predation.
The ability to swim also involves muscle physiology, with the adductor classified as a fast twitch muscle that gives it its succulent tender gastronomic quality.
This is in contrast to other bivalves where only a small fraction of the muscle is fast contracting, nevertheless a necessary component used to expel sediments that collect inside the shell, as can be observed by oysters spitting at low tide on an oyster bar.