Seasonal and circannual rhythms in behavioral thermoregulation and their modifications by pinealectomy in the white sucker,Catostomus commersoni

Abstract

1. Sexually immature white suckers (Catostomus commersoni) were captured at monthly intervals for 20 months and individually placed in horizontal thermal gradients with seasonally appropriate photoperiods and temperature regimes and in a number of cases broad temperature ranges. Fish displayed annual variations in their diel rhythms of behavioral thermoregulation, selecting significantly higher temperatures in the scotophase (D) than the photophase (L), with maximum transitions between higher and lower preferred temperatures occurring during twilights. In the summer months fish selected significantly higher mean preferred temperatures and had significantly greater amplitudes in their diel rhythms of thermoregulation than did fish captured in the winter months. 2. There were significant seasonal differences in the effects of pinealectomy on behavioral thermoregulation. During the summer pinealectomy completely eliminated the diel rhythm of behavioral thermoregulation and resulted in preferred temperatures that were significantly higher than those chosen by intact or sham-pinealectomized fish during the scotophase. During the winter pinealectomy had minimal effects on the diel rhythm of thermoregulation, though the mean preferred temperature was higher than that chosen by intact or sham-pinealectomized individuals. These annual variations in the effects of pinealectomy on behavioral thermoregulation may arise from seasonal differences in the degree of pineal involvement in determining circadian organization, integration of the circadian processes involved in thermoregulation and photoperiodic control of thermoregulatory mechanisms. 3. In the absence of seasonal environmental cues there was an apparent ‘endogenous circannual rhythm’ in the thermoregulatory behavior of intact and pinealectomized fish. Intact fish held under a constant 12 h light: 12 h dark cycle and a thermal gradient of 4–20 °C displayed an annual rhythm of approximately 310 days in their diel rhythms of behavioral thermoregulation. Pinealectomized fish kept under these constant conditions displayed a corresponding circannual rhythm in the degree of disruption or elimination of their diel rhythm of behavioral thermoregulation.