Serum hormone levels associated with spawning activity in the mummichog, Fundulus heteroclitus

Daily collections of the mummichog, Fundulus heteroclitus, from field populations during the first 45 days of the breeding season revealed a semilunar cycle in the sperm index. Peaks in the sperm index were preceded by 6 days with peaks in the serum testosterone concentration. Bihourly sampling of field populations during a 72-hr period at the new moon showed both diel cycling and an upward trend in serum progesterone levels in female mummichogs. Male mummichogs had 12-hr cycles in serum 17β-hydroxy-4-androstene-3,11-dione levels, the peaks of which preceded high tide by 4 hr. The physiological significance of these cycles are discussed.