Comparison of semilunar cycles of spawning activity in Fundulus grandis and F. heteroclitus held under constant laboratory conditions

Abstract

AbstractTwo closely related killifish, Fundulus grandis of the Gulf coast and F. heteroclitus of the Atlantic coast, were held under standard laboratory conditions and monitored for spawning activity. Daily egg collections for 4–5 months during summer revealed semilunar cycles of spawning activity in both F. grandis (three aquaria) and F. heteroclitus (five of six aquaria). The mean 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, which is an expression of the declinational cycle of the moon (tropical month: 27.3 days). The longer duration in F. heteroclitus closely approximates the tidal cycle along the Atlantic coast, which, along with the moonlight cycle, is an expression of the synodic month (29.5 days). Unless the fish respond to subtle exogenous cues that can influence spawning under isolated laboratory conditions, these results indicate that both killifish have endogenous cycles with very precise periods. These cycles free run for several months in specific phase relationships with tidal (F. grandis) and tidal/moonlight cycles (F. heteroclitus) present in their respective natural habitats.