In a previous study, a major remodeling of lipids, consistent with the theory of homeoviscous adaptation, was observed in the juvenile hard clam Mercenaria mercenaria during a temperature reduction from 24 to -1 degrees C. In addition, the lipid remodeling varied between genetically distinct lines of hard clams. The present study examined whether adult hard clams originating from different locations and a selectively bred variety show differences related to their genetic characteristics in the remodeling of lipids that normally occur during decreasing temperatures. Wild hard clams from 4 locations in Atlantic Canada and the selectively bred M mercenaria var. notata were held at an aquaculture growout site located at the northern distribution limit of the species in the Gulf of St. Lawrence from August to December 2006. Gills were sampled monthly for lipid analyses. Hard clams from the 5 groups showed an increase in the unsaturation index, mainly attributable to 22:6n-3 and 20:5n-3 as temperature decreased during the fall, followed by an increase in the phospholipid to sterol ratio in December. Although hard clams from the wild showed a lower unsaturation index than the selectively bred clams, there was no effect of location on the lipid remodeling. This result coincided with low genetic differentiation among hard clams from the 5 groups. Interestingly, the levels of 20:5n-3, 20:4n-6 and 22:6n-3-3 fatty acids obtained from the diet-were generally lower in the environment compared to the levels in hard clams, thus suggesting some mechanisms for the selective incorporation of these fatty acids in hard clams.