Most fertile individuals of the apodid sea cucumber Polycheira rufescens, living in the intertidal zone near Iso Marine Station in southern Kyushu, Japan, spawned on day one and two before every full and new moon in the period from the second half of July until August or October from 1983 to 1989, the month differing in different years. Synchronous spawning associated with the same phases of the moon was also observed at Sakurajima Marine Station near Iso in 1991. Individuals ready to spawn were divided into four sexual types: two sorts of hermaphroditic males with small or larger growing oocytes and males and females. Further investigation revealed that some individuals changed from male to female or from female back to male in the two week interval between spawnings. Moreover synchronous gametogenesis occurred in association with sex changes throughout the breeding season, the period from initiation of spermatogonia proliferation to spermatozoa release was three weeks, and the period from initiation of oocyte growth to egg shedding was about five weeks.
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