Reproduction in the Apodid Sea Cucumber Patinapta ooplax: Semilunar Spawning Cycle and Sex Change. Many fertile individuals of the apodid holothurian Patinata ooplax, living in the intertidal area of the Ichiki Fishing Port in southern Kyushu, Japan, spawned during the two days after every full and new moon, probably the first and second days, in the period from the middle of July to the end of August during 1990, 1992 and 1993. Matured individuals were divided into three sexual types: males, hermaphroditic males with an early stage of oocytes, and females, using a dissecting microscope. The distribution frequency and gonadal histology of these sexual types indicate that some individuals changed from male to female or in the reverse direction at two-week intervals between spawnings, and suggest that some change first from male to female, and then back to male during the main breeding season. In addition, it was found that during the main breeding season, synchronous gametogenesis occurred in association with the sex changes, and that the period from the initiation of spermatogonia proliferation to sperm release during the same season was two weeks, and the period from the initiation of oocyte growth to egg shedding was probably slightly longer than two weeks.