Effects of photoperiod and temperature on the regulation of the onset of maturation in the estuarine fish Menidia beryllina (Cope) (Atherinidae)

Menidia beryllina (Cope) is an annual species that inhabits coastal estuari es along the east coast of the United States from Cape Cod to the Gulf of M exico. In Rhode Island, USA, its peak spawning time and the duration of the spawning vary among years and estuaries. However, the onset of gonadal mat uration is more consistent (early May), suggesting that it may be regulated by more consistent cues than those regulating spawning. To determine the e ffects of photoperiod and temperature on the regulation of the onset of mat uration two laboratory experiments were conducted and the results compared to field observations. In the first experiment, fish collected from the fie ld in February were exposed to each of four treatments: increasing photoper iod/increasing temperature; increasing photoperiod/low temperature; low pho toperiod/increasing temperature; and low photoperiod/low temperature. Only fish exposed to both increasing photoperiod and increasing temperature were able to complete maturation. Fish exposed to low photoperiod and increasin g temperature responded by enlarging their livers, a response that was also observed in field fish collected in the fall. Fish exposed to the remainin g two treatments neither matured their gonads nor enlarged their livers. In the second experiment fish collected from the field in early March were ex posed to three treatments with different photoperiod regimes (daylight cons tant at 9.5 h, increasing up to 12 h, or increasing up to 15 h) and one inc reasing temperature regime. Fish in the 9.5-h treatment initiated maturatio n but were not able to complete the process, those exposed to the 12-h phot operiod matured and spawned for a short period of time before the gonads be gan to regress, and those exposed to the 15-h photoperiod matured and spawn ed large numbers of eggs throughout the remainder of the experiment. The 9. 5- and 12-h photoperiod exposures also resulted in accumulation of reserves in the liver in both females and males. The 15-h photoperiod treatment res ulted in liver enlargement in females, which were undergoing vitellogenesis , but not in males. Males exposed to the 9.5- and 12-h photoperiod accumula ted significantly more visceral fat than those exposed to the 15-h photoper iod. In females, the amount of visceral fat accumulated was inversely propo rtional to the hours of light. These findings suggest that this species has evolved mechanisms that enable it to anticipate the coming of winter as we ll as the coming of suitable breeding conditions and ensure that it exhibit s the appropriate response at the appropriate time (reserve accumulation fo r the winter or gonad maturation in the spring). (C) 1999 Elsevier Science B.V. All rights reserved.