experiments testing effects of viscosity

Small ciliated larvae of Dendraster provide a useful model organism for studying the interactive effects of temperature and viscosity because: (1) mechanisms of ciliary propulsion involve viscous forces ( Sleigh and Blake, 1977), (2) larvae can be tethered in place for measurements of water movement ( Emlet, 1990), (3) metabolic responses to temperature change have been examined ( McEdward, 1984, 1985), (4) effects of temperature on water movement are likely to have consequences for feeding and life-history characteristics related to feeding development ( Strathmann, 1971, 1985), and (5) these effects can be quantified and easily related to body form ( Hart, 1991). We measured swimming activity and water movement in larvae of the sand dollar Dendraster excentricus (Eschscholtz). To partition the effects of temperature and viscosity, we used a simple technique for artificially altering seawater viscosity. Environmental variation in viscosity due to temperature fluctuations could lead to temperature responses or adaptations that are nonphysiological. These changes in viscosity may cause substantial reductions or increases in swimming and feeding rates that are biologically relevant.

If uncorrected for effects of viscosity, temperature coefficients such as Q 10 values can overestimate the influence of temperature on the physiological processes that underlie the generation of motion at small spatial scales. The physical effects of viscosity can therefore make up a large component of the effect of temperature on activity of microscopic organisms. 40% of the decrease in swimming speed and 55% of the decrease in water movement were accounted for by increases in viscosity alone.

Over an environmentally relevant, 10-degree drop in water temperature (22 to 12☌), swimming speed was reduced by approximately 40% and water movement was reduced by 35%. 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. Because water viscosity is physically coupled to temperature, changes in temperature can influence the activity of microscopic organisms through both physiological and physical means. Temperature, through its effects on physiological processes, also influences motion. The small size and slow movement of aquatic, microscopic organisms means that the viscosity of water has a predominant influence on their motion.

Experiments testing effects of viscosity