Preliminary investigation into the effects of two dietary fatty acids, 20:5n-3 and 22:6n-3, on mortality of juvenile Mercenaria mercenaria during the approach to winter

Changes in the molar percentage of two omega-3 fatty acids, 20:5n-3 (EPA) and 22:6n-3 (DHA), occur in juvenile northern quahogs, Mercenaria mercenaria, as descending autumn temperatures approach winter lows in the temperate zone. Incorporation of these highly unsaturated fatty acids compensates for the effect of low temperature on cell membrane viscosity by lowering the phase transition of membrane phospholipids. Prior studies have demonstrated that the ability of bivalve mollusks, like M. mercenaria, to manufacture these compounds de novo does not exist or may be limited at best. Insufficient exogenous supply of EPA and DHA, therefore, may restrict their ability to overcome the inhibiting effect of high viscosity on cell membrane phospholipids during this interval. In the present study, a matrix of cultured algae containing various quantities of these compounds was administered to four groups of juvenile M. mercenaria as they adapted to low temperature. Each group displayed a unique response to diet in relative fatty acid profile and mortality. The group receiving abundant EPA adapted well during the early-phase temperature decline, and the one receiving abundant DHA adapted well to the late-phase temperature decline. The group receiving abundant DHA also enjoyed best overall survival following a severe 5-day drop in temperature. The strain native to the local region suffered between 31 and 117 % higher mortality than the notata strain during the 72-day period of temperature decline from 18 to 6 °C.