Changes in plasma steroid and thyroid hormones and insulin during final maturation and spawning of the sea lamprey, Petromyzon marinus.

Circulating levels of plasma estradiol-17 beta, androgens, thyroxine, triiodothyronine, immunoreactive insulin, plasma fatty acids, and protein were measured at interval during the period of final gonadal maturation, prior to spawning of male and female sea lampreys. Plasma estradiol levels fluctuated significantly and generally covaried in males and females through time. In females, possibly in relation to environmental changes, mean plasma estradiol levels peaked four times during the final spawning period but decreased sharply at the time of ovulation. In males, mean plasma estradiol peaked seven times during the final prespawning period and, in contrast to females, peaked significantly at the time of final spermiation. Plasma androgens were extremely low and covaried in males and females through time. Like plasma steroid profiles, there were coordinated changes in plasma triiodothyronine and thyroxine levels in males and females through the prespawning season. There was a slight increase in plasma insulin during the terminal maturation of the lampreys. However, at ovulation, the insulin levels abruptly decreased in females, whereas in males they remained unchanged. Plasma protein and fatty acid levels gradually decreased until ovulation/spermiation. At ovulation plasma fatty acid levels increased.