ONTOGENIC CHANGES IN THE VERTICAL-DISTRIBUTION OF GIANT SCALLOP LARVAE, PLACOPECTEN-MAGELLANICUS, IN 9-M DEEP MESOCOSMS AS A FUNCTION OF LIGHT, FOOD, AND TEMPERATURE STRATIFICATION

To understand how thermal stratification and food abundance affects th e vertical distribution of giant scallop larvae Placopecten magellanic us (Gmelin), a mesocosm study was conducted in January and February 19 92. The position of larvae was followed over 55 d in replicated 9-m de ep tanks in relation to a sharp thermocline and the presence or absenc e of phytoplankton. Growth and vertical position of larvae were monito red in separate treatments which included phytoplankton added above th e thermocline, below the thermocline, throughout the mesocosm, or abse nt from the mesocosm. Changes in the vertical position of larvae over time were quantified with a new, profiling, video-optical instrument c apable of semi-automatically identifying, counting and sizing larvae. The strong diurnal migration of scallop larvae resulted in aggregation s at two interfaces: the air/water interface during the night, and at the thermocline during the day. At times, the concentration of larvae within cm of the surface was > 100 times that in the remaining water c olumn. The formation of bioconvective cells of swimming larvae at the air/water interface allowed larval aggregations to persist throughout the period of darkness. Regardless of the distribution of food, larvae remained above the thermocline during most of the experiment. Therefo re, only in those treatments where food was also present above the the rmocline was larval growth relatively high. Larger larvae penetrated t he thermocline only after reaching a shell length of about 200 mu m; t hus larval size, rather than chronological age, was more important in describing their vertical distribution. The rapid increase in kinemati c viscosity with decreasing water temperature at the thermocline may r etard the movement of larvae and contribute to aggregation at this int erface. The influence of larval size on their vertical distribution, a nd the resulting potential for horizontal transport to settlement site s, points to the importance of persistent hydrographic features as cri tical factors contributing to settlement variance in scallops.