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.
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.
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.
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.
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