The allometry of swimming speed and predation

Summary

Among motile aquatic organisms, ranging in size from bacteria to whales, average swimming velocity (V, in μm · s^-1) varies non-linearly with body length (L, in μm) as log₁₀(V) = 1.93 + 0.19(log₁₀L)² + 0.19(log₁₀L)³ — 0.014 (r² = 0.90; S_xy = 0.45; n = 146).

This is about half an order of magnitude slower than the maximum aerobically sustained speed. Quantitative description of swimming speed allows comparison of the costs of motility in terms of higher metabolism and of mortality, and its benefits as measured by calculated rates of prey encounter and ingestion. Relative to standard metabolic rate, metabolic costs of swimming are substantial only for big animals, but predation is likely a major source of mortality for organisms of all sizes. Predator swimming speeds are likely important in determining the feeeding rates of unicells and small poikilotherms, but larger swimmers regularly encounter more than enough prey to satiate their food demands. At food levels below 1 ppm (vol./vol.), small predators risk starvation unless food demands are lowered by reducing metabolic rates or encounter rates increased. Unicells and minute poikilotherms(<300μm) may necessarily depend on reduced metabolic demand but, for animals the size of larval fish (3–20mm), small scale turbulence may increase prey encounter rates enough to allow larval survival and growth at the low food levels found in nature.