A method for the study of the control of the attainment of thermal acclimation has been applied to the crabs, Cancer pagurus and Carcinus maenas. Crabs were heterothermally acclimated by using an anterior-posterior partition between two compartments, one at 8°C and the other at 22°C. One compartment held a three-quarter section of the crab including the central nervous system (CNS), eye stalks, and ipsilateral legs; the other held a quarter section including the contralateral legs. Criteria used to assess the acclimation responses were comparisons of muscle plasma membrane fatty acid composition and ''f luidity.'' In both species, the major fatty acids of phosphatidylcholine were 16:0, 18:1, 20:5, and 22:6, whereas phosphatidylethanolamine contained significantly less 16:0 but more 18:0; these fatty acids comprised 80% of the total. Differences in fatty acid composition were demonstrated between fractions obtained from the ipsilateral and contralateral legs from the same heterothermally acclimated individual. In all acclimation states (except 22CNS, phosphatidylcholine fraction), membrane lipid saturation was significantly increased with acclimation at 22°as compared with 8°C. Membrane f luidity was determined by using 1,3-diphenyl-1,3,5 hexatriene (DPH) f luorescence polarization. In both species, membranes from legs held at 8°were more f luid than from legs held at 22°C irrespective of the acclimation temperature of the CNS. Heterothermal acclimation demonstrated that leg muscle membrane composition and f luidity respond primarily to local temperature and were not predominately under central direction. The responses between 8°C-and 22°C-acclimated legs were more pronounced when the CNS was cold-acclimated, so a central inf luence cannot be excluded.Poikilotherms generally inhabit environments subject to daily and seasonal temperature change, and thus, their body temperatures will vary with time. Many poikilotherms have been shown to possess a suite of behavioral, structural, physiological, and biochemical responses that enable them to compensate for the changes in functioning that follow from such fluctuations in ambient temperature (1).One clear outcome of a change in body temperature of a poikilotherm is the perturbation of cell membrane physical state. A rise in temperature will increase the rate and extent of phospholipid acyl chain motion, whereas a decrease in temperature will increase the ordering of membrane phospholipids. Changes in phase may also occur, altering the distribution of microdomains. Such temperature-induced changes in the organization of the lipid bilayer may have consequences for function.Poikilotherms have the ability to remodel membrane lipids in response to a change in prevailing temperature so that membrane physical properties remain appropriate. This remodeling has been shown to involve phospholipid head group composition (2), acyl chain unsaturation (3, 4), molecular species composition (5-7), and cholesterol content (8), a field recently reviewed (9). It is generally cons...
