Effects of Photoperiod and Temperature on Egg-Laying Behavior in a Marine Mollusk, Aplysia californica

Abstract

The primary purpose of these studies was to determine whether photoperiodic signals could influence seasonal egg-laying behavior in the marine mollusk, Aplysia californica. Egg-laying behavior was monitored from groups of animals that were collected at four times of year and maintained in different temperature and photoperiodic conditions in the laboratory. Animals that were obtained in autumn and kept in warm water laid eggs more frequently than those in cold water, regardless of photoperiod. Furthermore, animals maintained on short days and warm water laid eggs more frequently than those on long days and warm water. Animals in cold water showed little to no egg laying, and a photoperiodic response was not evident. Animals that were collected in either winter or spring and maintained in warm water showed little or no spontaneous egg laying throughout the study, regardless of photoperiod. As with the autumn animals, Aplysia individuals obtained in summer and kept on short days and warm water laid eggs more frequently than those kept on long days and warm water. These results provide the first evidence that the reproductive system of A. californica is responsive to photoperiod. Overall, the data suggest that warm water is permissive for egg laying, and that short days can further stimulate this behavior. However, there is a strong inhibition of spontaneous egg laying during the winter and spring, which neither warm water nor short photoperiod can overcome. The role of the eyes in mediating the photoperiodic response was also investigated. A control group of intact animals kept on short days laid eggs more frequently than those on long days, but this photoperiodic response was not evident in eyeless animals. These results suggest that the eyes play a role in mediating the effects of photoperiod on egg laying behavior.