Osmoregulation in Fishes

Most fish live in either saltwater or freshwater but cannot survive in both. This is because fish in these two environments evolved different ways of balancing levels of water and ions in their bodily fluids. Too much water causes cells to swell and burst. Without enough water, cells wither and die. Ions are needed to support crucial life functions and must also be carefully balanced.

Fish maintain osmotic balance, regulation of water and ion levels, through concentration gradients. When the concentration of solutes, or dissolved substances such as ions, in surrounding water differs from that of bodily fluids, water enters or exits the body. This passive diffusion across membranes is an example of osmosis.

Fish are either osmoconformers or osmoregulators. Osmoconforming fish, such as sharks, maintain an internal osmolarity equal to, or even higher than, that of the surrounding water. Thus, they do not typically lose water. However, they must still maintain concentrations of specific solutes that differ from those in the external water.

Most fish are osmoregulators and maintain an internal osmolarity independent of the outside environment. Most marine fish lose water to osmosis since the higher external osmolarity drives water from their bodies. These osmoregulators, therefore, drink lots of seawater and excrete excess ions through their gills and in concentrated urine.

Freshwater fish face a different challenge because their cells require higher ion concentrations than those found in freshwater. Freshwater osmoregulators absorb water through osmosis, so they must expel excess water and replenish ions. Thus, they drink little water, excrete dilute urine, and actively take in ions.

A few fish species, like salmon, can actually change osmoregulatory status. Salmon undergo physiological changes when they migrate from freshwater to the ocean, including active transport of ions out of the gills and excretion of concentrated urine.