Rhythmic Larval Release in the Crab Dyspanopeus sayi: Entrainment by Light:Dark and Temperature Cycles

The release of larvae from the abdomen of the common subtidal crab Dyspanopeus sayi is not random, but instead is associated with several environmental rhythms, in particular light:dark rhythms. Previous work has given us a model which suggests the mother perceives the light:dark cycle and entrains this rhythm upon the embryos, which then stimulate release when the time is right. For this project the light:dark cycle and 24-hour temperature cycle were investigated. Crabs placed in constant conditions were able to display an endogenous circadian rhythm, with release occurring in the hours after sunset and with a period of 24.1 hours. Crabs with embryos of all stages without eye-slits were placed in a reversed light:dark cycle, and they were able to reset the entrainment and release in the hours following 06:00 hrs., the new sunset. The difference between the crabs in the altered cycle and the crabs in the field is significant. Finally, crabs placed in a reversed 24-hour temperature cycle were able to reset the entrainment after 4-5 days in the altered cycle, such that they also released in the hours after 06:00. The fact that the temperature is enough to cause entrainment suggests that maybe the female or embryos do not need to perceive the light in order to know when to induce larval release. In any event, this result opens up new areas of research involving decapod reproduction, in particular D. sayi.