| dc.description | The vertical migration behavior of the the veliger larvae of the giant scallop (Placopecten magellanicus) was examined in detail. Replicated mesocosms (0.6 m diameter and 8.5 to 9.5 m deep) were held in a 10 m deep tank, allowing control of the light regime and thermal gradients. Vertical distribution was monitored by a remote controlled submersible video camera, designed for the purpose. Veligers exhibited strong diurnal migrations with intense surface aggregations at night. A 10$\sp\circ$C thermocline situated at mid-depth acted as a barrier to vertical migration, and few veligers were found below it until near settlement. A peak in veliger concentration just above the thermocline was frequently observed. Veligers fed only below the thermocline grew like those that were unfed, and veligers fed only above the thermocline grew like those fed throughout the water column. Response to thermoclines is probably behavior (a change in activity in response to perception of a stimulus) rather than the passive consequence of changes in viscosity or density of the water. Veliger responses to thermoclines as low as 1.2$\sp\circ$C varied with time of day and size of the veliger. Older veligers were clearly able to migrate in both directions through a 1.5$\sp\circ$C thermocline. Veligers from different populations (Georges Bank, Passamaquoddy Bay and Mahone Bay) exhibited different vertical distributions. Microsatellite DNA identification of progeny spawned from selected parents from two populations (Georges Bank and Passamaquoddy Bay) demonstrated that differences in vertical distribution between the populations were repeatable in different years with different parents, even when veligers were in the same mesocosm. I propose that vertical migration behavior of the veligers is in part determined by the horizontal transport consequences of the migration, and that population differences are the result of the different hydrographic properties in home regions. Veligers of P. magellanicus appear to migrate in response to both light and tidal stimuli. Moon rise early in veliger life may set an internal clock that controls migration at a tidal period, while the diel migration results from responses to changes in light at dawn and dusk. Migrating in response to both tide and light appears less powerful (in terms of distance gained) but more reliable (since the organism returns to the designated pattern even if it cannot perceive position or depth) than migrating only at a tidal period. Long term ($>$15 d) observations of the concentrations of veligers at the surface of mesocosms, re-analysis of the ZCM (mean depth) in previous mesocosm experiments, and re-examination of published field studies indicate that Georges Bank veligers have behavior that tends to move them in a northeasterly direction while Passamaquoddy Bay veligers have behavior that limits dispersal by tidal currents. | en_US |
