Thermal tolerances of the desert pupfish, Cyprinodon macularius: A case For rapid evolution

Authors: Allan A. Schoenherr

Year: 2009 (xli)

Abstract

Tolerances among pupfishes for environmental extremes are legendary. Cyprinodon macularius is no exception. Published data for critical thermal maxima and minima of fish acclimated to field conditions showed a range of extremes from 7°C to 44.6°C. Bob Feldmeth (now deceased) and I asked the question, ‘How long do pupfish populations have to be isolated in order to change their inherited thermal tolerances?’ We tested fish from three populations by acclimating them to 29°C in the laboratory. By raising and lowering their temperatures 1°C per 2 minutes we determined Ctmax and Ctmin for these populations. Fish from constant-temperature Oasis Spring (28°C) were tested twice. Results showed a Ctmax of 42.7°C and 42.6°C and a Ctmin of 5.3°C and 6.3°C. We also tested fish from native habitat at variable-temperature Salt Creek and found a Ctmax of 41.9°C and Ctmin of 4.6°C. For comparison, a population of fish transferred from Salt Creek to variable-temperature habitat at Thousand Palms also were tested and showed values similar to the parental population with a Ctmax of 42.4°C and a Ctmin of 4.4°C. The latter temperature is the lowest ever recorded for the species. The Oasis Spring population originally transferred from native habitat was isolated for 13 years. During that time their Ctmax remained the same, but it appears that they lost a bit of their tolerance for cold water, clearly a rapid case of change. Other cases of changes in thermal tolerances for pupfishes have been recorded for populations that have been separated for thousands to millions of years. However, the question still remains, ‘Are thermal tolerances inherited or is this a case of inherited plasticity?’