In a recent paper, Specht & Fuchs (2018; Mar Ecol Prog Ser 589:129-140) claim that 2 ciliary suspension-feeding bivalves, the blue mussel Mytilus edulis and the hard clam Mercenaria mercenaria, differ in how they respond to viscosity. For M. mercenaria, the authors found no change in filtration rate or beat frequency of water-pumping cilia in response to changes in viscosity at constant temperature. For M. edulis on the contrary, a previous study found these parameters to depend on manipulated viscosity at constant temperature in the same way as viscosity changes with temperature. To reconcile the opposing views, Specht & Fuchs suggested that the 2 bivalves may fundamentally differ in their responses to viscosity. But this suggestion is unwarranted. In addition to other shortcomings, we show for example that Specht & Fuchs likely misidentified compound laterofrontal cirri as lateral cilia, leading to erroneous conclusions. Furthermore, general fluid mechanical aspects were not considered by Specht & Fuchs, although many studies have shown that temperature-dependent viscosity of the ambient water controls or strongly affects bio-mechanical activity, such as beat frequency of water-pumping cilia in suspension-feeding bivalves, as well as water flows in general.