VELIGERS FROM DIFFERENT POPULATIONS OF SEA SCALLOP PLACOPECTEN-MAGELLANICUS HAVE DIFFERENT VERTICAL MIGRATION PATTERNS

Veligers spawned by adults collected from 3 different populations (Geo rges Bank, Passamaquoddy Bay and Mahone Bay, all on the northeastern c oast of North America) of giant scallops Placopecten magellanicus were maintained for 2 mo in replicated, 9.5 m deep, polyethylene mesocosms with a 1.5 degrees C thermal gradient at mid-depth and a 12/12 photop eriod. The populations came from varied hydrographic regimes that migh t require veligers to have different vertical migration patterns if th ey are to be in an appropriate area for settlement when they reach com petency. Veligers from all populations were found in shallower depths at night than during the day, developed strong bio-convective cells wh en numbers were high at the surface and were seldom found below the th ermocline until about 21 d after spawning. Under similar conditions, v eligers from the 3 populations exhibited significantly different verti cal migration patterns and depth distributions. These patterns of vert ical distribution are unlikely to be due to differences in buoyancy, g rowth rate, food density, settlement preference or mesocosm effect, an d they differ substantially from patterns of mussel veligers raised ac cording to the same protocol. Such differences may reflect selection f or different behaviors in different populations, arguing for an active and genetically controlled component to vertical movements of scallop veligers. The observed patterns of vertical distribution could produc e marked differences in horizontal transport in the various physical r egimes; increased self-recruitment seems the most likely explanation f or the intense selection required to fix such behavior in populations.