Water viscosity is inversely related to temperature. This simple physical relation couples two potential influences on organism performance. Seawater viscosity was manipulated, with and without temperature, to distinguish the physiological and mechanical effects of temperature on suspension feeding by ciliated echinoderm larvae. Change in viscosity alone accounted for half of the decline in the feeding rate at lower temperature. High viscosity shifted ingestion toward larger particles, which suggests that viscosity affects particle capture as well as rates of water processing. Temperature-induced change in viscosity, therefore, impacts suspension feeding independently of physiology and has implications for many small-scale biological processes.