Temporal changes in the fatty acid composition and fluidity of gill and hemocyte membranes during thermal acclimation of the sea scallop, Placopecten magellanicus

The fatty acid composition and fluidity of gill phospholipids and hemocyte membranes of a eurythermal bivalve, Placopecten magellanicus, were investigated following a 10°C reduction in acclimation temperature. The time course of the acclimation response from 15 to 5°C was also monitored over a 21-day period. Membrane physical properties were measured by means of an electron spin resonance (ESR) spin-labeling technique, using 5-doxyl stearic acid (5-SASL) as a probe. Changes in the fluidity of gill membranes were analyzed by incorporating 5-SASL into vesicles made from total gill phospholipid (PL). Membrane fluidity measurements showed that cold-acclimated scallop membranes were more fluid than those of warm-acclimated animals. However, the order parameter of 5-SASL was greatest during the first six days of the reduction in temperature, indicating a more rigid gill membrane structure. The fluidity of gill and hemocyte membranes were negatively correlated with the proportion of 20:5n-3 (r = −0.714, P < 0.001, n = 24) and the unsaturated to saturated fatty acid ratio (r = −0.775, P < 0.05, n = 8) respectively in the lipids of these tissues. Acclimation to reduced temperature for a prolonged period resulted in an increase in polyunsaturated fatty acid and a decrease in monounsaturated fatty acid content, in particular 20:5n-3 and C₂₀ monoenoic fatty acids. Maximum changes in PL fatty acid levels required 15–21 days following acclimation to 5°C. Scallop gill and hemocyte membranes underwent progressive changes in their membrane fluidity and fatty acid composition, an observation which is consistent with homeoviscous adaptation following a reduction in acclimation temperature.