The Comparative Endocrinology of Sea Turtles

In recent years, an effort has been made to begin to understand the endocrine regulation systems of endangered sea turtles. The purpose of the present paper is to review this area. The glycoproteins follicle stimulating hormone (FSH), luteinizing hormone (LH), and thyroid stimulating hormone (TSH) have been purified from pituitaries of Chelonia mydas. A homologous radioimmunoassay for LH has been used to document a clear relationship between LH, ovulation and a pronounced progesterone peak which occurs as albumin is being secreted in the oviduct. A distinct role for FSH has yet to be proven, although, because estrogens seem to be out of phase with LH and progesterone, FSH may regulate estrogen. A complication with this suggestion, however, is that the reptiles which have been studied seem unique among tetrapods in not having LH and FSH receptor specificity. Circulating estrogens are at very low titers (20-50 pg/ml), with peaks in spring that may correlate with migration and ovarian maturation as well as during the internesting interval when subsequent follicular size classes are maturing. Testosterone is elevated slightly in female C. mydas during mating receptivity, but is much lower than early spring samples from mating males. The two species in the genus Lepidochelys hold mature eggs in their oviducts, apparently to ensure adequate recruitment of nesters for their unique mass nesting (arribada) system. Environmental temperature appears to directly affect some sea turtle endocrine systems. The unusually elaborate pineal complex in marine turtles produces the hormone melatonin which is reduced in the circulation and cerebrospinal fluid by the animal's exposure to light. Temperature, photoperiod and nutritional history appear capable of regulating reproductive cycling in these multiannual nesters. Much less is now known about males than about females. Growth hormone (GH) and to a lesser degree prolactin are somatotrophic in marine turtles. GH also appears to synergize with gonadotropin in inducing final testicular spermiation. TSH from sea turtles or mammals did not stimulate the sea turtle thyroid in two separate assays, an observation which is both unexpected and unexplained. The ACTH-interrenal-stress axis in sea turtles is similar to other vertebrates with the glucocorticoid corticosterone demonstrating peaks at hatching, entry into the ocean and during induced stress situations.