Size, speed and buoyancy adaptations in aquatic animals

Abstract

Animals are denser than either fresh water or sea water, and therefore tend to sink, unless they have adaptations that give buoyancy. Very small organisms sink slowly, reproduce rapidly and can be kept suspended by natural turbulence: individuals lost by sinking are replaced by reproduction. This is likely to be effective only for organisms of less than 150μm diameter. Larger animals will sink unless they swim or evolve buoyancy organs. Hovering is one of the options available to them, but the "hop and sink" technique used by some copepods is more economical than steady hovering. Another option is to use fins as hydrofoils, as sharks, tunnies and many squids do. This implies an energy cost because work has to be done against drag on the hydrofoils. Many animals are made buoyant by gas-filled floats, low-density organic compounds or body fluids of unusual ionic composition. Such buoyancy aids increase the energy cost of swimming at given speed because they increase the animal's bulk. Buoyancy aids are more economical than hydrofoils for animals that swim slowly but hydrofoils are more economical for those that swim fast. © 1990 by the American Society of Zoologists.