Buoyancy regulation by the sticklebacks Culaea inconstans and Pungitius pungitius in response to different salinities and water densities

Abstract

The sticklebacks Culaea inconstans and Pungitius pungitius maintain buoyancy equilibrium in laboratory experiments by altering swim-bladder volume when held in a range of salinities (0–22.5 ppt). By holding C. inconstans in a Percoll solution in which water density is increased but tonicity remains similar to that of fresh water, we show that this species adjusts its buoyancy in response to a change in water density. When C. inconstans is transferred abruptly from fresh water to brackish water (10 ppt) buoyancy equilibrium is not regained until 96 h later. During this period of swim-bladder adjustment, hydrodynamic forces are used to provide the appropriate lift. Both species encounter variation in salinity in nature and the ability to respond to such changes could be highly adaptive.