Vertical migration and spawning inNodilittorina exigua (Gastropoda: Littorinidae)

The relation between the vertical migration in summer and spawning of a littoral-fringe periwinkle,Nodilittorina exigua, was studied by field observations and laboratory experiments. The snails moved down soon after the first appearance of their planktonic eggs in the sea and stayed at low shore-levels until early August. It was shown that the migration increased the chance of immersion of the snails, and therefore of releasing eggs by the females. Females immersed every day in the laboratory released more eggs than those which experienced some days of exposure to air, during the same period. The results obtained suggest the reproductive advantage of immersion at low levels. Nevertheless, the snails moved upward before the end of the spawning season. Typhoon swells increasing in frequency through the summer seemed to be responsible for the upward migration.