Acclimatory responses to environmental temperature in two species of marine crab : the stenothermal Cancer pagurus and the eurythermal Carcinus maenas
Temperature acclimation effects on leg muscle plasma membrane composition and biophysical state, neuromuscular function and whole organism thermal tolerance, and their dependence on the background seasonal acclimatization were investigated in the eurythermal Carcinus maenas and the stenothermal Cancer pagurus. Correlations between the changes observed at each of these levels are discussed. The average winter (5ºC), annual (8ºC), and summer (15ºC) sea-water temperatures and a warmer summer temperature (22ºC) were chosen as acclimation temperatures for their ecological significance. Warm-acclimation (22ºC) increased the thermal tolerance (CTMax) of both species and induced partial compensation of the neuromuscular function (assessed from leg nerve axonal conduction velocity and the amplitudes of excitatory junction potentials in dactylopodite closer muscle fibres). The time-course of acclimation showed it was complete after two weeks, but a longer exposure to 22ºC reduced neuromuscular performance, an effect that may not be acclimation related. At plasma membrane level, the response to warm-acclimation depended on acclimation temperature. Acclimation to 15ºC induced a significant increase in the S/U fatty acids ratio of PC and PE, but no increase in plasma membrane order. Acclimation to 22ºC induced a significant increase in the Ch/PL molar ratio and in plasma membrane order, but no major changes in overall fatty acid composition of membrane phospholipids, compared to cold-acclimated crabs. A seasonal background, determined by complex factors, underlined these responses to temperature-acclimation. At all acclimation temperatures, crabs acclimated in autumn and winter had lower S/U fatty acid ratios and Ch/PL molar ratios than in spring, and the temperature dependence of plasma membrane fluidity was reduced in summer, compared to spring and autumn. The response to 15ºC- acclimation was greater in winter than in other seasons. The CTMax and the magnitude or efficacy of the acclimation responses showed that the stenothermal species was more thermally sensitive than the eurythermal species.