Name: fish water and salt balance
Uploaded: 2019-08-01T07:40:27.000Z
Duration: 2 min 32 s
Channel: JoVE

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 sinc

osmoregulation in fishes

Osmoregulation in Fishes

When cells are placed in a hypotonic (low-salt) fluid, they can swell and burst. Meanwhile, cells in a hypertonic solution&mdash;with a higher salt concentration&mdash;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.

<h4>Fish Maintain Osmotic Balance by Osmoconforming or Osmoregulating</h4>

Osmoconformers maintain an internal solute concentration&mdash;or osmolarity&mdash;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.

<h4>Osmoregulation Requires Energy</h4>

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.

<h4>Alternative Osmotic Strategies</h4>

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&mdash;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.

When cells are placed in a hypotonic (low-salt) fluid, they can swell and burst. Meanwhile, cells in a hypertonic solution&mdash;with a higher salt ...

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fish water and salt balance

Fish water and salt balance helps keep their cells from swelling or shrinking in changing environments. Cells in a hypotonic, or low-salt, fluid can swell and burst. Cells in a hypertonic fluid, which has a higher salt concentration, can shrivel and die. Fish use osmoregulation to control water and dissolved ions such as sodium and chloride.

Osmosis is the movement of water across a membrane that lets water pass. Water moves in both directions, but the net flow goes toward the side with more solute. Fish must manage this movement to keep their internal osmolarity, or solute level, within a healthy range.

Some animals are osmoconformers. They keep their internal solute concentration equal to the surrounding water and do best in stable environments. All osmoconformers are marine animals, but many marine animals are not osmoconformers. Most fish are osmoregulators, which means they keep internal osmolarity separate from the environment. This helps them live in changing waters and migrate.

Osmoregulation takes energy because osmosis tends to equalize ion concentrations. Fish need ion levels that are different from the water around them, so they must spend energy to hold a solute gradient. The energy cost depends on how different the internal and external ion levels are. When those osmolarity differences are small, less energy is needed.

Some marine sharks and other cartilaginous fish use a different strategy. Their body fluids contain TMAO, which helps them store urea and keep internal osmolarity above the outside water. This lets them absorb water by osmosis. Most animals are stenohaline, so they cannot handle large changes in external osmolarity. Euryhaline species, such as salmon, can switch their osmoregulatory state. When salmon move from freshwater to the ocean, they produce more cortisol and grow salt-secreting cells.

Fish Water and Salt Balance

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Fish water and salt balance helps keep their cells from swelling or shrinking in changing environments. Cells in a hypotonic, or low-salt, fluid can swell and burst. Cells in a hypertonic fluid, which has a higher salt concentration, can shrivel and die. Fish use osmoregulation to control water and dissolved ions such as sodium and chloride. Osmosis is the movement of water across a membrane that lets water pass. Water moves in both directions, but the net flow goes toward the side with more...

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key terms and concepts

Explore how fish balance water and salt with osmosis, osmoconformers, osmoregulators, sharks, and salmon adapting to freshwater or seawater.

Fish Water and Salt Balance | JoVE K12

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Fish Water and Salt Balance

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