Semilunar Reproductive Cycles in Fundulus Heteroclitus (pisces: Cyprinodontidae) in an Area without Lunar Tidal Cycles

Although lunar spawning rhythms are relatively common in species of shallow-water fish, semilunar reproductive cycles have been reported in a small but growing number of species (Korringa 1947; Johannes 1978). Species with the best documented semilunar gonad and spawning cycles include the California grunion. LeurestMs tenuis, (Clark 1925); Atlantic silversides, Menidia menidia, (Middaugh 1981); the tropical coral reef saddleback wrasse, Tha.llassmna. dUpe1-rey, (Ross 1983); two tropical damselfishes. Pemuwentn.u; flavica1~da. and P wardi (Doherty 1983), and gulf kiilifish, Fundulus ~a:ndis: (Greeley and MacGregor 1983); and the salt marsh mummichog, Fun.dul:us h.eteroclit:us, (Taylor et aI. 1979; Taylor and DiMichele 1980). On the east coast of North America, F. Mt.eroclitus move up onto marsh surfaces during high spring tides to spawn either in empty mussel (Geukensia d.em1ssa) shells or in the outer leaves of salt marsh plants (Spa·rlina. alternijlora.) where oxygen levels are relatively high and sedimentation rates are relatively low (Able and Castagna 1975; Taylor et al. 1977; Kneib and Stiven 1978; Taylor and DiMichele 1983). Its eggs are extremely tolerant of desiccation and hatch on the next series of high spring tides. In addition, semilunar rhythms of larval hatching are well known for intertidal chironomid insects (Newmann 1978) and for several species of intertidal and estuarine crabs (see Christy 1982: Forward et al. 1982). In most cases, the proximal factors (sensu Giese and Pearse 1974) responsible for initiating and synchronizing these semilunar cycles are unknown. However, among the factors postulated as cues are tidal rhythms in water turbulence and hydrostatic pressures (Korringa 1947; Newmann 1978; Weld and Meier 1982; Ross 1983); tidal regimes in the habitat (Forward et al. 1982); photoperiod in combination with tidal fluctuations in water temperature (Miller et al. 1981), in combination with time of high tide (Middaugh 1981; Middaugh and Takita 1983), or in combination with interrupted current velocity (Middaugh and Hemmer 1984); and moonlight (Saigusa 1980). In addition to its wide distribution in coastal marshes with distinct lunar cycles of spring and neap tides, F. heteroclitus is abundant in extensive shoreline habitats and marshes of Chesapeake Bay, where tidal ranges are small and changes in water level caused by barometric pressure and wind frequently and unpredictably override lunar tidal