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Aquacultured northern quahogs, Mercenaria mercenaria, set in the environment for overwintering have experienced episodes of anomalous, highly variable and site-specific overwinter mortality. Prior research has revealed factors likely contributing to mortality during isothermal winter lows, when M. mercenaria do not feed, and the postwinter approach to spring. However, a synthesis of prior studies suggests juvenile M. mercenaria survival during the prewinter period of temperature may be subject to the timely dietary availability of two highly unsaturated fatty acids, 20:5n-3, eicosapentaneoic acid, and 22:6n-3, docosahexaneoic acid, and the biosynthesis of the nonmethylene interrupted fatty acids, 22:2Δ7,13 and 22:2Δ7,15.

In a preliminary investigation, first-year cultured northern quahogs, Mercenaria mercenaria (Linnaeus, 1758), were administered a matrix of dietary highly unsaturated n-3 fatty acids, 20:5n-3 and 22:6n-3 to observe their effect on homeoviscous adaptation to low temperature. The quahogs were subjected to high magnitude thermal fluctuations of the natural, declining temperature regime of a temperate estuary during the approach to winter. Inverse correlations emerged between mortality and dietary abundance of 20:5n-3 during the upper range of the temperature decline (18°–12 °C), and 22:6n-3 during the lower range (12°–6 °C), demonstrating their unique roles in supporting homeoviscous adaptation within each respective phase of thermal decline. High survival of a group whose diet was devoid of these n-3 fatty acids was associated with high molar % increases of endogenic non-methylene-interrupted fatty acids, 22:2Δ7,13 and 22:2Δ7,15. Conversely in the current investigation, similarly grouped juvenile quahogs all experienced reductions in incorporated 22:2Δ7,13 and 22:2Δ7,15 when subjected to a mild temperature decrease of 0.2 °C per day, suggesting that high magnitude thermal fluctuation is a signal for the synthesis of non-methylene-interrupted fatty acids. The unfavorable circumstance of this group of quahogs deprived of dietary 20:5n-3 and 22:6n-3 was exacerbated by diminishing incorporation of these compounds and suffered the earliest and highest overall mortality of all dietary treatments. High early-phase mortality of this same group was associated with high levels of previously incorporated 22:6n-3, considered to be homeoviscously unfavorable at the upper thermal range above 12 °C, and this direct relationship fell neatly within the 95% confidence interval of a linear regression model established in the preliminary study. Statement of relevance Results of this supplemental experiment provide novel correlations among thermal decline, diet and mortality of juvenile northern quahogs, distinct from the preliminary investigation cited in this paper. This work distinguishes the effects of absolute temperature decline and rapid decline on mortality of juvenile northern quahogs. This work also demonstrates the requirement of high magnitude thermal decline to signal the endogenic biosynthesis of EPA, DHA and 22:2 NMIs in juvenile northern quahogs in homeoviscous adaptation to falling temperature. Results of this work can be applied to tailor supplemental diets of northern quahog seed to prepare for forecasts of rapid thermal decline. Such timely dietary supplementation may reduce loss of bivalve seed which might improve the industries annual production and profitability.

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.

Estuarine organisms are exposed to periodic strong fluctuations in seawater pH driven by natural O2/CO2 biogeochemical cycles. The average pH of ocean surface waters has decreased by about 0.1 units from 8.2 to 8.1, since the beginning of the industrial revolution. This phenomenon, dubbed ocean acidification, may be exacerbated into the future by increasing atmospheric CO2 from fossil fuel combustion and associated land use changes. The direct biological effect of ocean acidification is not well understood and may vary among organisms. In a preliminary investigation to detect the sensitivity of Limulus polyphemus to pH changes, sets of 100 fertilized eggs were exposed to five isolated systems with pH values ranging from 6.0 to 8.0 for 4 weeks. This preliminary investigation chronicled the effects of pH on larval and/or juvenile carapace length and the development to the post-larval juvenile stage over time. Trends in carapace length (without telson) for larva and juveniles (with telson), measured by scanning electron microscopy (SEM), were positively correlated to pH within the experimental range. The long-term consequences of ocean acidification on early stage horseshoe crab development are unknown, and this work prompts further investigation.