ABSTRACT: Water viscosity is inversely related to water temperature. Because marine invertebrate larvae generally operate in a hydrodynamic environment dominated by viscous forces (i.e. Reynolds numbers <1), temperature-induced changes in water viscosity may exertprofound influence on the swimming and sinking velocity of larvae. Whilst the physiological effects of water temperature on larval locomotion have received considerable experimental investigation, the influence of temperature-induced changes in waterviscosity has received little attention. We investigated the relative physiological and viscosity-induced effects of water temperature at 25 and 15°C on the swimming and sinking velocity of larvae of the serpulid polychaete Galeolaria caespitosa(L.). Larvae of this species undergo considerable growth and development during their planktonic period such that the Reynolds number of the larval body (trochosphere) increases from 0.19 at hatching to 1.11 at 120 h post-hatching. Consequently we suggestthat inertial forces may exert an influence on swimming at the later stages of larval development and therefore the influence of viscosity may change over the course of larval development. We also suggest that a temperature-induced increase in waterviscosity will reduce the sinking velocity of larvae and may reduce the energy expenditure required to maintain location in the water column. Our results indicate that both physiological and viscosity components of water temperature influence the swimmingvelocity of G. caespitosa larvae. However, the influence of water viscosity did not change significantly over the course of larval development. The sinking velocity of G. caespitosa larvae was reduced with a temperature-induced increase inwater viscosity. The reduction in sinking velocity of the larvae was proportional to the increase in water viscosity. We estimated and compared the metabolic costs of swimming to counteract sinking at 25 and 15°C by estimating Q₁₀ valuesfrom the metabolic effects of temperature on swimming velocity. We suggest that the metabolic costs of swimming to counteract sinking in G. caespitosa larvae are similar at 25 and 15°C, but that the metabolic costs of swimming are slightly higherat 15°C.