Variation with semilunar periodicity of plasma steroid hormone production in the mudskipper Boleophthalmus pectinirostris.

Variation in the production of the plasma steroid hormones E(2), 17alpha-OHP and T in females and T and 11-KT in males, was investigated in the mudskipper Boleophthalmus pectinirostris during the spawning season. Females with oocytes at the vitellogenic stage (GSI 5.97-6.86%) and mature males with GSI of 0.255-0.288% were collected at intervals of 3-4 days within the two complete semilunar cycles from May 31 to June 30, 2006. The results showed that variations in the levels of plasma steroid hormones were synchronized obviously with semilunar periodicity in both females and males. Each steroid hormone level exhibited two cycles, each cycle with a peak. In females, the first peaks in plasma E(2), 17alpha-OHP and T levels were observed 3 days after the first lunar quarter, and the second ones, 4 days after the last lunar quarter. In males, the first peaks of plasma T and 11-KT levels occurred 3 days after the first lunar quarter, and the second ones, at the last lunar quarter. The fact that, in the present study, changes in the levels of plasma steroid hormones were synchronized with semilunar periodicity, although the fish were at the same stages of gonadal development, suggests that variation of plasma steroid hormones is basically regulated by biological rhythms (Zeitgebers), and that tidal movement (with its semilunar periodicity) is the major environmental factor stimulating steroid hormone production in B. pectinirostris.