Thermal and viscous effects of temperature on Mercenaria mercenaria suspension feeding

Temperature can affect hard clam Mercenaria mercenaria growth and survival through its influence on suspension feeding. Warming raises metabolic rates and reduces dissolved oxygen in the surrounding fluid, and clams may compensate by increasing their pumping and ingestion rates. Warming also makes seawater less viscous and may affect ingestion rates through temperature-induced changes in viscous forces acting on beating gill cilia. To determine which physical property dominates suspension feeding dynamics, we conducted laboratory experiments to quantify the effects of temperature and viscosity on M. mercenaria clearance, ingestion and ciliary beat rates. Ingestion and clearance rates varied with temperature but not with viscosity, and unresponsiveness to viscosity was confirmed by separate ciliary beat measurements on isolated gill preparations at different viscosities. The lack of ciliary response to viscosity indicates that M. mercenaria ingestion rates are driven by physiological rather than biomechanical effects of temperature, a result that differs from previous findings for the mussel Mytilus edulis. A comparative analysis indicated that these different responses to viscosity cannot be attributed to differences in ciliary mechanics, suggesting that the 2 species differ in their ciliary control mechanisms. We also found that temperature and algal concentration are strong predictors of ingestion rate. These results have important implications for clams' ability to survive and grow in a warming ocean.