Spawning Period and Discovery of Juvenile Acanthaster Planci (L.) (Echinodermata: Asteroidea) at Northwestern Iriomote-Jima, Ryukyu Islands

In 1984 and in 1985, at northwestern Iriomote-jima, Ryukyu Islands, gonad indices for both sexes of Acanthaster planci showed a rapid decline in June when water temperatures were 27–28°C. This indicated a short breeding season in June, and there was no evidence of a secondary breeding season. Nine juvenile A. planci (0.55–12.5-mm total diameter) were collected in 1984 and 13 juveniles (5.5–14.9 mm) in 1985 at depths of 5.5–18.5 m on the floors of grooves and channels of the fringing reef and in small depressions on the reef slope. Feeding scars near juveniles indicated that they fed on crustose coralline algae.