Simple SummaryDetermining the physiological and biochemical causes of aging in animals is important both because of the potential health utility for humans and because aging is related to growth, reproduction processes, and the response of organisms to environmental conditions and stress. It is assumed that the peculiarities of the fatty acid composition of mitochondrial membranes (“membrane-pacemaker” theory of aging) can influence the rate of oxidative damage in cells, as well as the rate of the aging process. This property, in turn, may be fundamental for all living organisms. In this study, the fatty acid composition of gill tissues’ mitochondrial membranes, in marine bivalves, was determined and analyzed. The observed features in the lipid composition of mollusk mitochondria correlate well with the longevity of these animals.AbstractMarine bivalves belonging to theMytilidaeandPectinidaeFamilies were used in this research. The specific objectives of this study were: to determine the Fatty Acids (FAs) of mitochondrial gill membranes in bivalves with different lifespans, belonging to the same family, and to calculate their peroxidation index; to compare the levels of ROS generation, malondialdehyde (MDA), and protein carbonyls in the mitochondria of gills, in vitro, during the initiation of free-radical oxation; to investigate whether the FAs of mitochondria gill membranes affect the degree of their oxidative damage and the maximum lifespan of species (MLS). The qualitative membrane lipid composition was uniform in the studied marine bivalves, regardless of their MLS. In terms of the quantitative content of individual FAs, the mitochondrial lipids differed significantly. It is shown that lipid matrix membranes of the mitochondria of long-lived species are less sensitive to in vitro-initiated peroxidation compared with the medium and short-lived species. The differences in MLS are related to the peculiarities of FAs of mitochondrial membrane lipids.