ENVIRONMENTAL CONTROLS ON BIVALVE SKELETAL GROWTH

Bivalves deposit growth bands following predictable patterns, and the measurement of the geochemical properties of those sequential growth bands can produce valuable proxies of an organism’s local environment and internal physiology. Many bivalves produce seasonal dark growth increments during times of growth cessation. Such “shutdowns” have been alternatively proposed to be dominantly caused by temperature, food availability, salinity fluctuations or seasonal spawning. We gathered more than 300 observations of seasonal growth, coded as binary variables with 1 representing winter shutdown and 0 representing continuous growth, and another binary 1/0 variable for summer shutdown. We then analyzed these binary response variables in comparison to continuous environmental variables using logistic regression. Environmental variables include latitude, temperature, seasonal productivity (remote sensed chlorophyll A concentration). Annual low temperature explains the greatest amount of variation in occurrence of winter shutdown, equating to a 12.8% increase in probability of a shutdown occurring in winter per decreased degree Celsius, followed by latitude with a 4.5% increase in probability of winter shutdown per additional degree poleward. Seasonal food supply shows no significant relationship, with many locations actually displaying a chlorophyll maximum during winter months. Winter calcification shutdowns (and therefore the creation of dark growth bands) across bivalves thus appear to be primarily driven by resource re-allocation in response to temperature stress rather than food scarcity. The summer shutdowns observed in some species are not significantly related to high temperature stress, instead largely appearing to be a function of local or species-specific conditions rather than a class-wide trait. Dark increments from fossil bivalves in the temperate zone or northward can be assumed to have formed during winter months rather than being the anomalous result of spawning, low food supply or other extraneous factors. Further investigation is needed to determine the relative contribution of such factors to determine what factors trigger summer cessation, possibly integrating soft tissue data and more granular environmental data into our model.