Ontogenetic 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 the vertical distribution of giant scallop larvae Placopecten magellanicus (Gmelin), a mesocosm study was conducted in January and February 1992. The position of larvae was followed over 55 d in replicated 9-m deep tanks in relation to a sharp thermocline and the presence or absence of phytoplankton. Growth and vertical position of larvae were monitored in separate treatments which included phytoplankton added above the thermocline, below the thermocline, throughout the mesocosm, or absent from the mesocosm. Changes in the vertical position of larvae over time were quantified with a new, profiling, video-optical instrument capable of semi-automatically identifying, counting and sizing larvae. The strong diurnal migration of scallop larvae resulted in aggregations 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 column. 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. Therefore, only in those treatments where food was also present above the thermocline was larval growth relatively high. Larger larvae penetrated the thermocline only after reaching a shell length of about 200 m; thus larval size, rather than chronological age, was more important in describing their vertical distribution. The rapid increase in kinematic viscosity with decreasing water temperature at the thermocline may retard the movement of larvae and contribute to aggregation at this interface. The influence of larval size on their vertical distribution, and the resulting potential for horizontal transport to settlement sites, points to the importance of persistent hydrographic features as critical factors contributing to settlement variance in scallops.