REPRODUCTIVE STEROIDS DURING MATURATION IN A PRIMITIVE TELEOST, THE PACIFIC HERRING (CLUPEA-HARENGUS PALLASI)

Concentrations of reproductive steroids were measured in the plasma of captive Pacific herring, Clupea harengus pallasi, (1) prior to ovulat ion and milt production, (2) during the periovulatory and newly milt-p roducing period, (3) during a ''ripe'' holding period after ovulation and during milt production, and (4) before and after spawning. 17 alph a,20 beta-Dihydroxyprogesterone (17,20 beta-P), despite being present only in low concentrations in the unconjugated (free) form (<10 ng/ml) , is likely to be the maturation inducing steroid in females and was a ssociated with the initiation of milt production in males since its le vels are elevated coincidentally with these events. Glucuronated 17,20 beta-P, free 17 alpha-hydroxyprogesterone (17-P), and free and glucur onated 3 alpha,17 alpha-dihydroxy-5 beta-pregnan-20-one (3 alpha,17-P- 5 beta) were present in high concentrations (140-250 ng/ml) in periovu latory females and newly milt-producing males. This steroid pattern su ggests that the low levels of 17,20 beta-P are due to glucuronation an d competitive conversion of its precursor, 17-P, to free and glucurona ted 3 alpha, 17-P-5 beta. Glucuronated testosterone was the principal steroid in preovulatory and premilt-producing fish (200-350 ng/ml), co incident with similar levels of glucuronated 11-ketotestosterone in ma les. After ovulation females did not spawn synchronously until 2 month s later, which may be partially due to reduced environmental cues in t he captive situation, while male fish released milt sporadically throu ghout the ripe holding period. Steroidal indicators of readiness to sp awn in females or males were not detected. Rather, levels of all stero ids gradually decreased in ripe holding fish (<30 ng/ml) to reach even lower levels (<1 ng/ml) after spawning. We suggest that ''runniness'' of gametes is a distinctive characteristic of females that are ready to spawn, but that this may result from relaxation of sphincter muscle s rather than being an additional maturational step. (C) 1996 Academic Press, Inc.