TEMPERATURE-DEPENDENT VARIATION IN EGG SIZE IN MARINE PROTOBRANCH BIVALVES

Coastal ecosystems around the world are rapidly changing due to anthropogenic nutrient loading, deoxygenation, and the uptake of atmospheric CO₂. These global-scale processes have the potential to alter the life histories of many marine animals, including bivalve mollusks. Aspects of bivalve life history are recorded in the morphology of their biomineralized tissues. For example, larval shell size is positively correlated with egg size, and because bivalves exhibit incremental growth these larval features can be preserved on shells throughout adulthood. Previous studies have suggested that egg size varies inversely with temperature; smaller eggs are expected in warmer waters. To test this hypothesis, we sourced live-collected and historical shells from two protobranch bivalve families (Nuculidae and Nuculanidae) from 48 sites around the world. Using scanning electron microscopy, we measured the diameter of the first stage of larval shell growth (Prodissoconch I) of each specimen, and assigned it a taphonomic grade. Our dataset consists of measurements of 29 well-preserved specimens. We then compared these larval shell measurements with mean annual sea surface temperatures at the sampling sites. We found that Nucula from sites with higher sea-surface temperatures had significantly smaller larval shells than Nucula sampled from cool waters, consistent with previous work (linear regression model; p < 0.05). Nuculana qualitatively appeared to follow the same trend, although this relationship was not statistically significant (p = 0.10). We are currently compiling data on mean primary productivity at these sites in order to determine the relative impacts of temperature and nutrients on bivalve life histories.