Freerunning circasemilunar spawning rhythm of Fundulus grandis and its temperature compensation.

Gulf killifish, Fundulus grandis, exhibit rhythmic reproductive activities that are synchronized with semilunar tidal fluctuations in their Gulf coast habitat. These cyclic activities, monitored through daily egg collections, persist in the laboratory for as long as four months with periods near the tidal period of 13.7 days. In nature, a specific semilunar spawning phase is maintained with respect to tidal cycles. However, in the laboratory, the phase may gradually advance or delay. These phase changes occur as a result of small differences between the periods of the spawning cycle and concurrent tidal cycle. Changes in the phase of the spawning cycle can be produced after several cycles by exposing fish to different temperatures (21 and 27°C). However, the period of the spawning cycle changes only slightly with temperature. Q10 calculations produce a value not significantly different from 1.0, indicating nearly complete temperature compensation. These results provide evidence for an endogenous circasemilunar cycle that is temperature-compensated and freeruns in the laboratory.