生物時計と潮汐によるサンゴ礁棲ベラ類の産卵時刻あわせ

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Many wrasses in coral reefs exhibit daily spawning that peaks around daytime high tide. In this study, we examined tidal-related ovarian development in the threespot wrasse Halichoeres trimaculatus, which is a common species in coral reefs of the Indo-Pacific Ocean. When the fish were collected in the morning at different tidal phases, the gonadosomatic index (GSI) and ovarian histology changed; concomitant with increases in GSI towards high tide, a clutch of the most advanced oocytes developed from vitellogenic to maturation stages. Ovulated eggs and post-ovulated follicles (POF) existed in most ovaries around high tide, but only POF remained around ebb tide, suggesting that spawning occurred during or after high tide. We noticed that tidal-related spawning was remarkable in the morning and that most ovaries collected at afternoon high tide exhibited post-spawning features. It was suggested that certain labrid species possess plasticity with regard to their spawning time and utilize potent environmental cues for their reproductive success. When the ovarian pieces were incubated with precursor steroids, high conversion of testosterone to 17β-estradiol occurred during high and ebb tides, while that of 17α-hydroxyprogesterone to 17α,20β-dihydroxy-4-pregnen-3-one and 17α,20β,21-trihydroxy-4-pregnen-3-one was observed during low and flood tides. Incubation of the ovarian pieces with human chorionic gonadotropin resulted in similar fluctuations of the steroid hormones with tide. Production of these steroid hormones correlated with oocyte development in the ovaries and was probably regulated by gonadotropin. These results demonstrate that the daily cycle is fundamental for oocyte development, and the tidal cycle is superimposed on this process.