Diurnal Periodicity in Testicular Activity in the Japanese Flounder Paralichthys olivaceus

Testicular activity, as represented by the amount of each type of testicular germ cell in the seminal lobules, and the serum levels of three major male teleost steroids—testosterone (T), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP)—were investigated during the daily spawning cycle of the male Japanese flounder Paralichthys olivaceus. B-type spermatogonia and spermatocytes increased in number from the lowest level at 22:00 (186 cells/lobule), and peaked at 10:00 (292 cells/lobule). The number of spermatozoa increased from 14:00 (370cells/lobule) and peaked at 22:00 (782 cells/lobule). In contrast, spermatid numbers decreased from 14:00 (579 cells/lobule) and showed the lowest level at 22:00 (348 cells/lobule). These results indicate that spermatogenesis (spermatogonial proliferation, the formation of spermatocytes and spermatids) occurs between 22:00 and 10:00, and spermiation (release of spermatozoa into the lobular lumen from the cysts) occurs between 14:00 and 22:00. Thus, spermatogenesis and spermiation occur on a daily basis in the male Japanese flounder. Serum T and 11-KT levels showed no significant fluctuation throughout the day. Serum 17α,20β-DP was maintained at a low or undetectable level throughout the experimental period, but showed a surge (110pg/m/) at 22:00. This short-lived 17α,20β-DP surge corresponded to the peak of spermiation. This study demonstrates, therefore, the existence of a diurnal periodicity in (1) spermatogenesis and spermiation, and (2) 17α,20β-DP production.