Do upper thermal tolerances differ in geographically separated populations of the beachflea Orchestia gammarellus (Crustacea: Amphipoda)?

Fundamental (as opposed to realised) between-population differences in maximum temperature tolerances were examined for the common beachflea of European shores, Orchestia gammarellus. Individuals were collected from two distinct populations, one in S.E. England and one in N. Scotland, and were acclimated to a number of different thermal regimes (5, 10, 15, 28 and 25 degrees C). Temperature tolerances increased with body size for virtually all combinations of population and acclimation temperature tested. Such a relationship complicates comparison between populations, although when body size differences were controlled for at two acclimation temperatures, individuals of the Southern population tended to have higher thermal tolerances. Controlling for body size tends, however, to underestimate the difference in temperature tolerances of the two populations. This is because the southern population consisted of larger individuals than the northern, giving rise to markedly different frequency distributions of tolerances. Clearly upper thermal tolerance limits do vary between these two geographically widely separated populations of O. gammarellus and this variance cannot be accounted for by acclimation. Such a finding were it to have general application, would have important implications for ecologists seeking to explain large-scale distribution patterns in terms of a 'fixed' species physiology. (C) 1998 Elsevier Science B.V. All rights reserved.