23.5: Osmoregulation in Fishes

When cells are placed in a hypotonic (low-salt) fluid, they can swell and burst. Meanwhile, cells in a hypertonic solution—with a higher salt concentration—can shrivel and die. How do fish cells avoid these gruesome fates in hypotonic freshwater or hypertonic seawater environments?

Fish employ osmoregulatory strategies to balance bodily levels of water and dissolved ions (i.e., solutes), such as sodium and chloride.

Imagine two solutions separated by a membrane that is permeable to water. Although water crosses the membrane in both directions, more water flows (i.e., there is net water movement) into the solution with a higher solute concentration; this is the essential part of osmosis.

Fish Maintain Osmotic Balance by Osmoconforming or Osmoregulating

Osmoconformers maintain an internal solute concentration—or osmolarity—equal to that of their surroundings, and so they thrive in environments without frequent fluctuations. All osmoconformers are marine animals, although many marine animals are not osmoconformers.

Most fish are osmoregulators. Osmoregulators maintain internal osmolarity independent of the environment, making them adaptable to changing environments and equipped for migration.

Osmoregulation Requires Energy

Osmosis tends to equalize ion concentrations. Since fish require ion levels different from environmental concentrations, they need energy to maintain a solute gradient that optimizes their osmotic balance.

The energy required for osmotic balance depends on multiple factors, including the difference between internal and external ion concentrations. When osmolarity differences are minimal, less energy is required.

Alternative Osmotic Strategies

The bodily fluids of marine sharks and most other cartilaginous fish contain TMAO; this enables them to store urea and internally surpass the external osmolarity, allowing them to absorb water through osmosis.

Most animals are stenohaline—unable to tolerate large external osmolarity fluctuations. Euryhaline species, like salmon, can change osmoregulatory status. When salmon migrate from freshwater to the ocean, they undergo physiological changes, such as producing more cortisol to grow salt-secreting cells.

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.