Influence of photoperiod and temperature on serum melatonin in the diamondback water snake, Nerodia rhombifera.

Serum melatonin concentrations in the diamondback water snake, Nerodia rhombifera, were measured with a radioimmunoassay to determine the influence of photoperiod and temperature on melatonin levels. A diel cycle of serum melatonin, with elevated concentrations during scotophase, occurred in snakes acclimatized for 2 weeks to a 12L:12D photoperiod and 25 degrees. This cycle persisted in snakes acclimatized to a reverse photoperiod, with a longer duration of elevated scotophase levels. In snakes acclimatized to a normal 12L:12D photoperiod and 10 degrees, photophase serum melatonin concentrations were lower than at 25 degrees, and there was no increase during scotophase. Snakes acclimatized to a normal 12L:12D photoperiod and 35 degrees had lower serum melatonin levels than those at 25 degrees, but this difference was not statistically significant. Photoperiod and temperature interact to influence serum melatonin in N. rhombifera, with photoperiod affecting the phase and temperature affecting the amplitude of the diel melatonin cycle.