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 asses sed by determining the critical thermal maximum (CTM) every 4 hours fo r 2 days in intact and pinealectomized (Pr) animals during January and in sham-pinealectomized (sham-Px) and Pr animals during April. In the intact and sham-Px group a statistically significant (P<0.005) diurna l cycle of thermal tolerance was validated which was asynchronous with that of the daily fluctuations in ambient temperature (Ta). A nocturn al elevation of thermal tolerance was observed in all the groups. Pine al 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 sh am-Px and Pr fishes. CTM was significantly (P<0.05) elevated during th e first three weeks following pinealectomy, pointing to a short-term c ontrol from the pineal on thermal tolerance. This study provides the f irst demonstration of a direct pineal influence on thermal tolerance a nd its temporal organization.