Vertical migration for horizontal transport while avoiding predators: II. Evidence for the tidal/diel model from two populations of scallop (Placopecten magellanicus) veligers

Abstract

We examined the vertical migration behavior of scallop (Placopecten magellanicus) veligers in mesocosms and in previously reported field studies. Evidence suggests that these bivalve veligers migrate in response to both tidal and diurnal stimuli in a manner similar to a proposed tidal/diel model. Both populations have a diurnal response to solar cues. The response to tidal cues differs between the Georges Bank and Passamaquoddy Bay populations. Georges Bank veligers appear to utilize the differences in tidal phase that occur with depth to transport them in a northeasterly direction, thus maintaining the population on the bank. Passamaquoddy Bay veligers respond by swimming up at slack water (high and low tides) and down when currents are strongest. Such behavior would minimize dispersal on the strong tidal currents in the Bay of Fundy and thus also tend to maintain a population within an area. Horizontal transport resulting from vertical migration is the most likely selective pressure to create and maintain these different behaviors against the homogenizing effects of migration between the two populations. The implications of inherited differences in behavior probably require consideration in the management of both wild and cultured populations. Common sampling practices that obscure the tidal part of tidal/diel migration, including averaging the results from several days of sampling, sampling too infrequently to perceive a tidal periodicity, and assuming that only behavior that changes at high and low tides will affect horizontal transport, need to be avoided in studies of vertical migration.