Dr Nishikawa is a Post Doctoral Fellow for Research Abroad of JSPS [Japan Society for Promotion of Science] and is currently working in Prof. David Millers lab at JCU.
The 1998 coral bleaching event was the most extensive and severe recorded to date. Mortality rates during 1998 bleaching varied among localities. Coral populations, particularly those of branching species, declined severely in the Okinawa Islands (Middle Ryukyus). In contrast, the Kerama (Middle Ryukyus) and Yaeyama Islands (South Ryukyus), located respectively 30 and 400 km southwest of the Okinawa Islands, maintained healthy coral communities on many reefs. The study of ocean currents supports the hypothesis that the Kerama populations are a source of larvae for the Okinawa populations. To test this hypothesis, we assessed genetic differentiation (FST) among populations in Yaeyama, Kerama, and Okinawa using allozyme electrophoresis for four species (Acropora tenuis, A. digitifera, Goniastrea aspera, and Stylophora pistillata). The FST values between the Kerama and Okinawa populations were lower than those between the Yaeyama and Okinawa populations. Geographic distance, ocean currents, and the present population genetic analysis largely support the larvae source hypothesis. Based on reproductive modes, relatively high genetic differentiation was found among regions in the brooding coral Stylophora pistillata compared to spawning corals. Planula behavior due to reproductive mode may influence genetic differentiation. These results indicate that a longer period is needed for the recovery of brooding species than for spawning species in the Okinawa Islands.