Hormonal changes in relation to lunar periodicity in the testis of the forktail rabbitfish, Siganus argenteus.

Correlation of hormonal changes in the testis and the lunar periodicity was studied using the forktail rabbitfish, Siganus argenteus, which spawns synchronously around the last quarter moon. Weekly change in sperm motility peaked around the last quarter moon. The pH and osmolality in the seminal fluid increased and decreased around the same lunar phase, respectively. These results suggest that the testis of this species develops toward the specific lunar phase. Human chorionic gonadotropin (hCG) stimulated in vitro production of 11-ketotestosterone (11-KT) in the testicular fragments around the full moon. When the testicular fragments and the sperm preparations were incubated, respectively, with testosterone (T) and 17alpha-hydroxyprogesterone (17alpha-OHP), conversion of T to 11-KT in the testicular fragments decreased and, alternatively, that of 17alpha-OHP to 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) in the sperm preparations increased from the full moon to the last quarter moon. Administration of the fish with hCG or DHP prior to the predicted spawning lunar day resulted in increases in the sperm motility and the seminal fluid pH. Plasma level of DHP, but not T and 11-KT, increased after hCG injection. These results indicate that gonadotropin (GtH) and DHP are related to the final stage of testicular maturation, and that GtH acts through production of DHP in the testis. Moreover, the present study shows that use of the lunar cue(s) in the rabbitfish occurs in the higher part of the hypothalamus-pituitary-gonadal axis.