The small size and slow movement of aquatic, microscopic organisms means that the viscosity of water has a predominant influence on their motion. Temperature, through its effects on physiological processes, also influences motion. Because water viscosity is physically coupled to temperature, changes in temperature can influence the activity of microscopic organisms through both physiological and physical means. To partition these effects, we artificially altered seawater viscosity and, at two temperatures, we measured swimming speed and water movement by larvae of the sand dollar Dendraster excentricus. Over an environmentally relevant, 10-degree drop in water temperature (22 to 12°C), swimming speed was reduced by approximately 40 % and water movement was reduced by 35 %. 40% of the decrease in swimming speed and 55 % of the decrease in water movement were accounted for by increases in viscosity alone. The physical effects of viscosity can therefore make up a large component of the effect of temperature on activity of microscopic organisms. If uncorrected for effects of viscosity, temperature coefficients such as Q10 values can overestimate the influence of temperature on the physiological processes that underlie the generation of motion at small spatial scales. These changes in viscosity may cause substantial reductions or increases in swimming and feeding rates that are biologically relevant. Environmental variation in viscosity due to temperature fluctuations could lead to temperature responses or adaptations that are nonphysiological.

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