SYNCHRONIZED RELEASE OF MEDUSAE FROM 3 SPECIES OF HYDROZOAN FIRE CORALS

The release of medusae from three hydrozoan fire corals, Millepora dic hotoma, M. murrayi and M. platyphylla, was investigated at three sites in southern Taiwan from February 1994 to July 1995. All three species were gonochoristic, and developed and released several batches of med usae between April and May. The duration of open ampulla appearing on the surface of coralla was short, about three months, and could be use d to infer the reproductive season of the fire corals between April an d May. No obvious lunar cycles of medusa release were found in these s pecies. Medusa release started before dark at approximately 17:00 h an d continued for several hours. Males began releasing medusae earlier t han females. Synchronization of medusa release between colonies, i.e., the probability of occurring on the same nights, was significantly hi gher within a species than between different species. Hybridization in nature among the three species is, therefore, unlikely due to segrega tion in the spawning dates. Moreover, the synchronization within each species was often significantly higher within versus between sites. Th e free-swimming medusae released gametes within approximately one hour , and the spent medusae lived for a few more hours. Medusae may facili tate fertilization rates as a result of an apparently negatively geota ctic swimming response that results in medusa aggregation at the surfa ce. No differences in the sizes of medusae, eggs and sperm were detect ed among the three species; however, some characteristic differences o f medusa nematocysts were found.