Semilunar Spawning in Killifish, Fundulus Grandis.

Gulf killifish, Fundulus grandis, spawned daily in aquaria exposed to 12-hr daylengths and 21 to 28$\sp\circ$C temperatures. The number of eggs spawned varied daily producing a peak about every 13.7 days. Ovarian studies indicate that increased maturation of peripheral germinal-vesicle oocytes in preparation for spawning begins five days before the semilunar spawning peak. There is an increase in numbers of hydrated oocytes and ovulated eggs reaching a maximum on or a day before peak spawning. Semilunar spawning cycles of F. grandis are expressions of an endogenous rhythm. Although a specific phase of the semilunar cycle is maintained with respect to a tidal cycle in the natural environment, the spawning cycle freeruns in the laboratory with periods ranging from 12.7 to 14.1 days so that different phases occur respecting the concurrent tidal cycle. The period of the spawning cycle changes only slightly with temperatures (21 and 27$\sp\circ$C) so that there is nearly complete temperature compensation (Q$\sb{10}$ is not significantly different from 1.0). The period of the freerunning semilunar spawning cycle is apparently a long-term adaptation to the local environment. F. grandis, collected from the Louisiana coast of the Gulf of Mexico, and its close relative, F. heteroclitus, collected from the Delaware coast of the Atlantic ocean, were monitored side by side. The mean cycle periods were near 13.7 days for F. grandis and near 14.8 days for F. heteroclitus. The shorter duration in F. grandis closely approximates the tidal cycle in the Gulf of Mexico. The longer duration in F. heteroclitus closely approximates the tidal cycle along the Atlantic coast. Whereas endogenous freerunning semilunar cycles have been demonstrated previously in aquatic invertebrates, the present research is the first unequivocal evidence of such a cycle in fish. Contrary to mammalian reproductive cycles (e.g., estrual and menstrual) the semilunar cycle in F. grandis is not merely a reproductive rhythm, but rather an expression of a more basic periodicity of the fish neuroendocrine system. Otolith growth patterns indicate that a semilunar cycle of growth occurs in reproductively immature fish and in adult fish in or outside the breeding season