Reproductive Cycling in Female Fundulus heteroclitus.

The ovaries of female Fundulus heteroclitus living in the northeastern Florida saltmarsh recrudesce in January and the fish initially spawn heavily during the subsequent full moons (a lunar pattern); they later spawn with less intensity during both the new and full moons (a semilunar pattern), and then regress in late September. In the laboratory, fish spawning against a vertical screen showed only semilunar periodicities, as observed for seven spawning groups under constant conditions (temperature 26 +/- 1°C; photoperiod 14 h light to 10 h dark; excess food). Regardless of collection times (January, April, August, or September), these seven groups exhibited similar patterns of semilunar spawning for five to eight consecutive cycles whose periods (14.4 to 16.0 days) and phases (-1.7 to +8.4 days) were variable compared with concurrent full/new moon and spring tide cycles. These semilunar cycles, which occurred over the entire year in the laboratory, were thus free-running without entrainment and represent endogenous circasemilunar rhythms. In addition to annual and lunar/semilunar cycles, a tidal spawning cycle was also observed in the habitat. Fish apparently select the higher of the two semidiurnal tides for spawning, regardless of the daily light-dark cycle. This tidal cycle has not yet been tested in the laboratory.