EVALUATING THE INFLUENCES OF TEMPERATURE, PRODUCTIVITY, AND PHYLOGENETIC CONSTRAINT ON BIVALVE GROWTH RATES

One of the most striking features of the marine fossil record is the long-term increase in the mean body size and metabolic scope of fossil taxa. Numerous drivers, both biotic and abiotic, have been hypothesized to explain this trend, but evaluating them requires improved understanding of the controls on metabolic rates, growth rates, and longevities of marine invertebrates. Recent analyses have shown that among marine bivalves, lifespan increases, and growth rate decreases, with latitude. Here we dissect this pattern further by examining the influences of temperature and primary productivity on bivalve growth rates within a phylogenetically controlled framework. We integrated a global database of growth rates for 700 bivalve populations representing 202 species with satellite measurements of sea surface temperature (SST) and chlorophyll-a concentration. We extracted maximum and minimum monthly average SST and chlorophyll concentrations for 0.042-degree latitude-longitude cells based on satellite data from 2012 to 2016. Relationships between environmental variables and growth rates were evaluated using linear regression, quantile regression, and gradient boosting machines. In order to assess trends in a phylogenetically independent framework, we generated a phylogeny for the 96 species for which sequence data were available and tested for phylogenetic signal using Pagel’s lambda and Blomberg’s K. None of the evaluated predictors show a strong linear relationship to growth rate. The most important predictor is minimum temperature. Growth rates are uniformly low when winter temperatures are low, but vary widely when winter temperatures are high, implying that warm winter temperatures are a necessary but not sufficient condition for high growth rates. Maximum temperatures exhibit a narrower range than minimum temperatures, and have a weaker influence on growth rates. Chlorophyll concentration has little to no significant influence on growth rates at the global scale, but minimum chlorophyll concentration is a significant predictor of growth rates when considering only extratropical observations. Notably, growth rates exhibit no significant phylogenetic signal, suggesting that studies of growth rate variation in the fossil record should focus on potential environmental drivers.