Environmental and Endocrine Influences on Reproduction of Fundulus heteroclitus

Reproduction in Fundulus heteroclitus is adapted to the temperateclimate and the tidal environment of the coastal marshes whichare its primary habitat. Egg deposition and spawning are closelylinked to the tidal cycle. Concentration of spawning on springtides gives the fish access to sites in the high intertidalzone. Eggs are deposited near the high water mark, usually insand in New England populations and in Spartina alternifloraor empty Geukensia demissa shells in Middle Atlantic and southernpopulations. Aerial incubation of eggs appears to be essentialfor their survival in the salt marsh. Loss of eggs due to lackof fertilization, mortality during development and predationwas always less than 30 percent in our observations. Hatchingoccurs only when the eggs are inundated, usually on spring tides.An endogenous semilunar gonadal cycle appears to be involvedin control of spawning, but an effective environmental synchronizerhas not been found. Cyclic changes in estradiol-17ßaccompany the gonadal cycle, while spawning behavior is stimulatedby neurohypophyseal hormones. Both temperature and photoperiodhave been implicated in the control of seasonal reproductionin F. heteroclitus. Early workers focused on males and concludedthat increasing temperature was the stimulus that initiatedgonadal recrudescence in spring. These experiments were notrigorous tests of the effects of photoperiod. We have shownthat in females both warm temperatures and long photoperiodsare involved. In female F. heteroclitus, as in most mammalsand birds, it is the timing rather than the duration of thelight exposure which determines its effectiveness in stimulatinggonadal maturation. Maintenance of gonadal maturity in femaleF. heteroclitus depends on long photoperiods, but the ovarymay become refractory and regress even when stimulatory photoperiodsare sustained in the laboratory. Ovarian maturity can be stimulatedwith gonadotropin injection in refractory animals, indicatingthat the brain-pituitary axis is the site of refractoriness.The pathway by which light influences ovarian maturity may involvean encephalic photoreceptor, since neither the eyes nor thepineal gland is necessary.