Most scallops are able to swim using a simple form of jet-propulsion. Scallops, like all bivalves, have two valves that are connected by a hinge. By clapping their valves together, scallops force water through small openings on either side of their hinge. Swimming scallops appear to take bites out of the water as they move forward. If you are old enough to remember arcade games, watching a scallop swim will probably remind you of Pac-Man. Although scallops do not swim gracefully, many species are able to travel several meters at a time and are able to reach speeds of several body lengths per second (~ 50 cm/s).
Scallops swim in response to chemical or tactile cues from predators such as crabs and sea stars. Scallops may also visually detect and swim towards preferred habitats. For some species, this means a dark crevice in which to hide. For other species, such as the bay scallop Argopecten irradians, this means swimming towards patches of eelgrass (Zostera sp).
Along with swimming away from predators and towards preferred habitats, it is possible that scallops migrate long distances. Observations of scallops from across the globe support this possibility, but definitive proof is lacking. For example, I have heard fishermen on the North Carolina coast say that bay scallops migrate from deeper to shallower water during the spring. Tidal currents may also influence the distribution of scallops. Perhaps scallops ride currents to aid their migrations?
My goal is build up a database of observations supporting or refuting the possibility that scallops (or at least certain species of scallop) migrate. I would also like to hear your ideas on how to study migration in scallop. Mark-and-recapture studies have potential, but these would be difficult to pull off without broad community support. I have also considered tagging scallops with radio beacons, but the costs of this may be prohibitive. I suspect that we underestimate the mobility of scallops and that studying the migratory behaviours of these animals will help us make better decisions about fisheries and conservation.