Size, Speed and Buoyancy Adaptations in Aquatic Animals

Animals are denser than either fresh water or sea water, andtherefore tend to sink, unless they have adaptations that givebuoyancy. Very small organisms sink slowly, reproduce rapidlyand can be kept suspended by natural turbulence: individualslost by sinking are replaced by reproduction. This is likelyto be effective only for organisms of less than 150µmdiameter. Larger animals will sink unless they swim or evolvebuoyancy organs. Hovering is one of the options available tothem, but the "hop and sink" technique used by some copepodsis more economical than steady hovering. Another option is touse fins as hydrofoils, as sharks, tunnies and many squids do.This implies an energy cost because work has to be done againstdrag on the hydrofoils. Many animals are made buoyant by gas-filledfloats, low-density organic compounds or body fluids of unusualionic composition. Such buoyancy aids increase the energy costof swimming at given speed because they increase the animal'sbulk. Buoyancy aids are more economical than hydrofoils foranimals that swim slowly but hydrofoils are more economicalfor those that swim fast.