Annual and lunar breeding rhythmicity in females of the terrestrial hermit crab Coenobita rugosus H. Milne Edwards, 1837 (Decapoda: Anomura: Coenobitidae) in Okinawa, Japan

Abstract

We investigated the reproductive characteristics of a terrestrial hermit crab (Coenobita rugosus H. Milne Edwards, 1837) inhabiting Okinawa-Jima. Monthly changes in the gonadosomatic index and ovarian histology revealed that this species had fully developed oocytes from May to October. Data showing that females with (ovigerous females) and without (non-ovigerous females) embryos on their pleopods appeared simultaneously on the shore from June to September 2014 demonstrates that this species undergoes larval release several times for four months. When non-ovigerous females were reared under a combination of two day lengths and two temperatures, oocyte development was induced in the high-temperature group, suggesting that temperature is a proximate factor in ovarian development. The weekly collection of females during the spawning season revealed that the proportion of fully developed oocytes in an ovary increased toward the time of the new moon and decreased around the first-quarter moon, suggesting that this species is a lunar-spawner with a new-moon preference. Some females releasing larvae around the new or full moon were recaptured at approximately 30 d intervals, suggesting at least two major groups with lunar-synchronized larval release. The ovarian development and larval release of C. rugosus are likely entrained to external factors, which change periodically in their habitats.