Reduced reproductive success in scleractinian corals that survived the 1998 bleaching in Okinawa

Okinawan coral reefs experienced extensive coral reef bleaching in the summer of 1998 (Fujioka 1999, Taniguchi et al. 1999, Sugihara et al. 1999, Yamazato 1999). Around Sesoko Island (Okinawa, Japan), about 90% of hard corals and soft corals disappeared from the reef flat after the bleaching event (Yamazato 1999). We have been studying early development of zooxanthella-containing eggs of scleractinian corals belonging to the genus Pocillopora since 1997 (Hirose et al. 2000), and studying the incorporation of zooxanthellae into oocytes of Montipora species since 1995 (Hirose et al, in preparation). During the 1999 reproductive season, we tried to observe the early development of Pocillopora verrucosa and Montipora digitata. However, the number of colonies which spawned gametes was low in P. verrucosa, and even when we got a sufficient amount of gametes for fertilization experiments, fertilization rates were extremely low. Szmant and Gassman (1990) reported that corals that remained bleached over an extended period were not able to complete gametogenesis during the reproductive season following the bleaching, while those corals that recovered and regained their zooxanthellae by the next spring were able to follow a normal gametogenic cycle. However, the effects of thermal stress on the reproductive biology of corals are only partly understood (HoeghGuldberg 1999). Here we report preliminary observations on gamete production, the percentage of colonies which spawned gametes, and the fertilization rate of two scleractinian corals, Montipora digitata and Pocillopora verrucosa during the reproductive season following the 1998 bleaching. Both species are hermaphroditic spawners and release zooxanthella-containing eggs (Hirose et al. 2000). While most colonies of P, verrucosa at our collection site died after the 1998-bleaching event, colonies of M, digitata survived, even in shallow waters.