Circadian rhythm in larval release by the crab Rhithropanopeus harrisii: entrainment model

The subtidal crab Rhithropanopeus harrisii has a circadian rhythm in larval release; under constant conditions eggs hatch in the 2-3-h interval after the time of sunset in nontidal estuaries. Eggs that are removed from the female hatch rhythmically, indicating that the circadian rhythm resides in the embryos. The model for entrainment is that mature embryos have functional sensory systems that detect and entrain to environmental cycles. This model was reexamined by confirming that the visual system of advanced embryos responds to light and thus could mediate entrainment to the light/dark cycle. We then determined whether the hatching rhythm of mature embryos that are removed from the female can be entrained to new light/dark cycles. Contrary to expectations, these embryos did not entrain to new cycles. Instead, they remained entrained to the light/dark cycle to which they were exposed when still attached to the female, suggesting that the female entrains the rhythm. Indeed, hatching by embryos collected from the field when they had not yet developed eye pigments, kept in constant conditions attached to their mother, exhibited the circadian hatching rhythm. They could also be entrained to a new photoperiod in the laboratory. The role of the female is further supported by experiments showing that the hatching rhythm in embryos carried by females lacking one but not both eyes can be entrained to a new cycle in the laboratory. Thus, the revised model is that the female perceives the light/dark cycle and entrains the circadian rhythm in the embryos.