Study on the mechanism of lunar-related spawning in honeycomb grouper

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The honeycomb grouper ainephelus merra, which is distributed in the sub-tropical and tropical area in Pacific Ocean, is known as a lunar-related spawning. However, details of gonadal development and spawning in this species are not well understood. In this study, we investigated the gonadal development, hormonal changes, and expression of gonadotropin gene in female collected from the reef area around Sesoko Island and Iriomote Island in Okinawa during spawning season. In addition, we investigated the spawning migration to suitable environment for spawning using biotelemetry method.Ovaries of fish collected in both areas were occupied with oocytes of the tertiary yolk stage, when gonadosomatic index increased and reached the maximum level around full moon. However, mature size of fish in Ireornete Island was smaller than that in Sesoko Island. Moreover, fish of Treomote started female-to-male sex change immediately after the spawning season. On the other hand, sex change in fish collected from Sesoko Island stared in a few months after the spawning season. The different characteristics in both areas may depend on the difference in density, niche in the reefs, depth, water current, and water temperature. Estradiol levels indicated high just prior to spawning. Luteinizing hormone beta subunit gene expressed during reproductive period in female grouper, suggesting LH has important roles on gonadal development and steroid synthesis. In both areas, the fish migrated to suitable environment for spawning from habitat place just after full moon. It appears that the environment has water depth necessary to do the spawning behavior.