Magnitude of Seasonal Effects on Heat Tolerance in Fundulus heteroclitus

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Abstract
The heat tolerance of Fundulus heteroclitus collected at different times of year were evaluated by the critical thermal maximum (CTMax) technique, following a period of laboratory acclimation to standardize thermal history within collections during which contemporaneous field conditions of temperature and photoperiod were simulated. Mean midday CTMax values showed a strong seasonal effect, ranging from 32.20 C to 44.11 C over nine acclimation temperatures from 7 to 36 C. These values are among the highest recorded for fishes. A daily rhythm in CTMax occurred throughout the range of seasonal conditions employed, but its amplitude varied seasonally, with a maximum under spring/autumn conditions. Fish collected in autumn were transferred to summer conditions in the laboratory, and the effects of the transfer on the daily rhythm and midday CTMax were evaluated 6 and 12 wk later. A surprising result was that the fish did not recover summer thermal tolerance within 12 wk, and also that during this time the daily pattern of heat tolerance was severely disrupted. One of the implications of this is that routine acclimation protocols involving temperature and photoperiod shifts can alter or mask daily rhythms for months.