ABSTRACT: In Specht & Fuchs (2018; Mar Ecol Prog Ser 589:129-140), we found that hard clams Mercenaria mercenaria modify their feeding rates in response to changes in temperature but not to changes in viscosity alone. These results differed from previous findings in the blue mussel Mytilus edulis, and we concluded that whereas M. edulis pumping rates appear to be driven by biomechanical effects of viscosity, M. mercenaria pumping rates appear to be driven by physiological effects of changing temperature, perhaps in response to changes in dissolved oxygen concentration. Riisgård & Larsen (2018; Mar Ecol Prog Ser 596:263-265) criticize several methodological details in our study and argue that no new explanation is needed for bivalve responses to temperature. Further investigation revealed that we did incorrectly identify laterofrontal cirri as lateral cilia in experiments on ciliary beat rate in Specht & Fuchs (2018). However, despite this error, our general conclusions are unchanged and are strongly supported by feeding experiments. Riisgård & Larsen (2018) overlook differences that make our feeding experiments on M. mercenaria more statistically robust than prior studies on M. edulis. Riisgård & Larsen (2018) also ignore the diversity of responses to viscosity in other members of the animal kingdom. Therefore, we stand by our previous conclusion that hard clams and mussels fundamentally differ in their responses to viscosity.