The effects of melatonin treatment on gonosomatic index in the teleost, Fundulus similis, and the tree frog, Hyla cinerea

Melatonin-treated Fundulus similis collected in January or May and maintained on a long photoperiod had significantly smaller gonads than control animals. The effect of melatonin was independent of the time of day that it was administered. Melatonin treatment of F. similis maintained on a short photoperiod in May retarded testicular enlargement; melatonin treatment of animals on a short photoperiod in January was without effect. The gonadal enlargement seen in control Hyla cinerea maintained on a long photoperiod in December was inhibited by melatonin.