Seasonal-, tidal-cycle- and microhabitat-related variation in membrane order of phospholipid vesicles from gills of the intertidal mussel Mytilus californianus

Alterations in the order, or fluidity, of cellular membranes in response to variations in environmental temperature are well known. The mussel Mytilus californianus, a common inhabitant of mid-intertidal regions along the Pacific coast of North America, can experience large (20 °C or more) and cyclic (every 6 h) changes in body temperature (Tb ) during tidal cycles. In the present study, we explore membrane order during seasonal and tidal temperature cycles and find that vesicles prepared from gill phospholipids exhibit significant seasonal differences in order that are consistent with homeoviscous adaptation and suggest winter-acclimatization to mean Tb values and summer-acclimatization to upper extreme Tb values or to large cyclic thermal fluctuations, despite repeated resubmergence in sea water at 10 °C during both seasons. Phospholipid vesicles prepared from the gills of mussels acclimated for 6 weeks to constant high or low temperatures in the laboratory fail to exhibit temperature-compensatory differences in order. In addition, during the summer, mussels inhabiting high intertidal sites, but not those from low sites, possess the ability to alter membrane order rapidly (within hours). This alteration of order appears to represent a mechanism designed to offset the thermal variations encountered during the tidal cycle. Thus, M. californianus have the ability to adjust membrane order on seasonal as well as hourly time scales and do so on the basis of their height in the intertidal region.