Reynolds number (Re) has long been used to characterize hydrodynamic environments. Whether you are interested in objects moving through a fluid, or fluids moving through or past objects, Requantifies the ratio of inertial-to-viscous forces involved. In the past few decades, thanks in large part to Steve Vogel, more and more biologists have been seduced by the lure of understanding how fluid-dynamic forces impact living systems, and Re has played no small part in all of this. At high Re (>1000) inertial forces dominate, whereas at low Re (<1) viscous forces take the lead; thus, the physical worlds of a sperm whale (large Re) and sperm (low Re) are rather different. Interestingly, there is a range of intermediate or transitional Re (1000>Re>1) over which both inertial and viscous forces are potentially important. Many larval fish live in such transitional conditions, and the review by Ione Hunt von Herbing explores the effects that temperature can have on the hydrodynamic regime and physiology of these animals during swimming.

Like their parents, marine larval fish inhabit waters that differ widely in temperature, from those in polar regions, where water temperatures can drop below 0°C, to those in the tropics, where temperatures can rise above 30°C. In addition to the widespread effects of temperature on physiological processes, often quantified by Q10, temperature changes also affect the physical nature of fluids. For example, a shift in temperature from 0°C to 30°C leads to a twofold change in seawater's viscosity.

By varying the water viscosity independent of temperature with additives such as dextran or methyl cellulose, it has been possible to separate the physical versus physiological effects of temperature on larval fish swimming. For example, the swimming speed of small larval fish can be cut in half by relatively small changes in water viscosity, whereas a doubling of temperature has little effect on speed. By contrast, in larger larval fish,swimming speeds increase substantially with temperature but appear unphased by comparable changes in viscosity. Such results imply a size threshold for larval fish with respect to speed and temperature; namely, as fish approach 20 mm, the physiological effects of a change in temperature outweigh the viscous effects of such a change.

Temperature-related changes in viscosity can also be important for locomotor energetics in fish larvae. For example, the cost of transport for larval cod is 1.4 times higher at 5°C than at 10°C. This change in metabolic cost is not simply due to temperature per se but is probably related, in part, to temperature-induced shifts in viscosity, which the larvae must overcome while swimming. Indeed, Hunt von Herbing suggests that Q10 values for metabolism in small larval fish can overestimate the direct effects of temperature on swimming energetics by up to 60–70%.

Relatively few studies to date have tried to separate the effects of temperature and viscosity on aspects of behavior and performance in aquatic organisms. However, the clever work by Fuiman, Batty, Podolsky and Emlet,which Hunt von Herbing highlights, emphasizes the potential importance of viscosity, independent of temperature, for marine fish and invertebrate larvae of certain sizes. Moreover, this work illuminates how empirical studies can be used to test hydrodynamic theory and explore the physical world of organisms living at low and transitional Re.

Hunt von Herbing, I. (
). Effects of temperature on larval fish swimming performance: the importance of physics to physiology.
J. Fish Biol.