Effect of Pineal Extirpation on Daily and Long-Term Variations in Thermal Tolerance in a Tropical Catfish,Clarias batrachus

Thermal tolerance in laboratory acclimated Clarias batrachus was assessed by determining the critical thermal maximum (CTM) every 4 hours for 2 days in intact and pinealectomized (Px) animals during January and in sham-pinealectomized (sham-Px) and Px animals during April. In the intact and sham-Px group a statistically significant (P <0.005) diurnal cycle of thermal tolerance was validated which was asynchronous with that of the daily fluctuations in ambient temperature (Ta). A nocturnal elevation of thermal tolerance was observed in all the groups. Pineal removal resulted in a highly significant (P <0.001) increase of CTM. Additionally, pinealectomy abolished the normal diurnal cycle in CTM or dampened its amplitude considerably. In another experiment the long-term effect of pinealectomy on thermal tolerance was determined in sham-Px and Px fishes. CTM was significantly (P <0.05) elevated during the first three weeks following pinealectomy, pointing to a short-term control from the pineal on thermal tolerance. This study provides the first demonstration of a direct pineal influence on thermal tolerance and its temporal organization.