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 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-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 (370 cells/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 17alpha,20beta-DP was maintained at a low or undetectable level throughout the experimental period, but showed a surge (110 pg/ml) at 22:00. This short-lived 17alpha,20beta-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) 17alpha,20beta-DP production.