Larval Release Rhythms of Decapod Crustaceans: An Overview

Decapod crustaceans release larvae rhythmically in relation to lunar, light-dark and tidal cycles. Rhythms occur in a wide range of species. Those rhythms related to lunar phase are usually semilunar with larval release mainly occurring at the time of largest amplitude nocturnal ebb tides, which usually correspond to spring tides at the new and full moon. Semilunar rhythms are most common among littoral and supralittoral species and depend upon when in the lunar month the female lays her eggs and the length of embryo development. The specific time of larvae release depends upon the interaction of LD and tidal cycles. Release relative to the LD cycle occurs most often in the first half of the night. If timing is related to tides, releases always occur around the time of high tide. Rhythms are under endogenous control. The known zeitgeber for entrainment of lunar, diel and tidal rhythms are cycles in moonlight, LD, and salinity, respectively. Other untested environmental cycles may also be used for entrainment. The site of endogenous control among brachyurans probably varies with adult habitat. For species living in high littoral and supralittoral areas, the "clock" may reside in the female while in the sublittoral it resides in either the female or the developing embryos depending upon the species. The functional significance of the timing of larval release for each rhythm is usually related to survival of the adult female and her larvae.