FANGUE, N.A.*; SCHULTE, P.M.; University of British Columbia; University of British Columbia: Performance as a Function of Temperature in Two Subspecies of Fundulus heteroclitus

Fundulus heteroclitus inhabits inshore bays and estuaries along the North American East Coast. At the extremes of the species� range, northern mummichogs, F.h.macrolepidotus, and southern mummichogs, F.h.heteroclitus, are exposed to substantially different environmental temperatures. Within this range, there is also large seasonal thermal variation. As a result, these subspecies provide an ideal model in which to examine the interplay between thermal adaptation and acclimation/acclimatization. We developed ecological thermal tolerance polygons for Fundulus acclimated in the laboratory to temperatures between 2 and 34°C by assessing the critical thermal maximum (CTMax) and minimum (CTMin); the temperatures above and below which the fish cannot maintain their equilibrium. CTMax and CTMin were higher in southern fish than in northern fish except at temperatures below 10°C where CTMin was equivalent to the freezing point of brackish water in both populations. The difference between populations in CTMax was greater at low acclimation temperatures (~2°C), than at high acclimation temperatures (~0.5°C), while the difference in CTMin was greatest at high temperatures (~2°C). To further explore the physiological differences between these two subspecies, we assessed critical swimming speed (Ucrit). At low and intermediate acclimation temperatures, northern fish performed better than southern fish. At higher temperatures Ucrit fell off rapidly for northern fish such that at 29°C the populations did not differ. Maximum Ucrit was achieved at acclimation temperatures of 15°C for northern fish and 22°C for southern fish. Taken together these data suggest that there is an intrinsic physiological difference between the two subspecies.