FORTNIGHTLY MOLTING AND REPRODUCTIVE CYCLES IN THE SAND-BEACH ISOPOD, EXCIROLANA CHILTONI

Abstract

1. The isopod, Excirolana chiltoni is found buried in the sand in a narrow zonal band on the high intertidal beach during low tide and emerges only during high tide to swim and forage in the wave wash. The position of the zonal band during low tides bears a consultant relationship to the level of the water line during the preceding high tide. Because of this relationship the isopod zone moves up and down the beach on a fortnightly schedule which corresponds to the semi-monthly variation in tide amplitudes (the spring-neap cycle). 2. Excirolana chiltoni stores considerable quantities of mineral prior to ecdysis in localized concretions of the integument (dermoliths). 3. Molting takes place in two steps. First the posterior portion of the isopod including the abdomen and last 3 thoracic segments is cast which is followed by the casting of the anterior portion of the animal. Isopods which are in the process of molting can be easily recognized since the newly exposed posterior exoskeleton is fully expanded before the anterior cast is made. 4. The development of dermoliths and the relative abundance of half-molted individuals were used to determine the frequence of molting in a field population which was sampled at 2 day intervals for a period of a month. These data indicate that molting is largely if not entirely restricted to the weeks preceding new or full moon when the amplitude of the tides is increasing. 5. Excirolana chiltoni also shows fortnightly periodicities in feeding and the degree to which it is infested with external parasites. 6. Indirect evidence suggests that the release of young also follows a fortnightly schedule and is most intense at the same phase of the spring-neap cycle that the population molts. Females retain their broods internally and might therefore be capable of controlling the timing of birth with considerable precision. 7. Synchronous molting was not as apparent in populations which were held under non-tidal conditions in the laboratory. 8. Synchronization of molting and birth to the ascending series of high tides and the storage of mineral reserves prior to ecdysis are interpreted as adaptations which serve to decrease the probability of an isopod being marooned above the waterline for extended periods of time where it would be subject to a high risk of death by desiccation.