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 concentr…
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.
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Q1: Why can't most fish survive in both saltwater and freshwater environments?
Fish evolved different osmoregulatory strategies for each environment because water and ion balance differ dramatically between them. Saltwater fish lose water through osmosis due to higher external osmolarity, while freshwater fish absorb excess water because their cells require higher ion concentrations than surrounding water. These opposing challenges require distinct physiological adaptations that fish cannot easily switch between.
Q2: What is the difference between osmoconformers and osmoregulators?
Osmoconformers, like sharks, maintain internal osmolarity equal to or higher than surrounding water, so they don't lose water through osmosis. Osmoregulators, which include most fish, maintain internal osmolarity independent of their environment by actively transporting ions and adjusting water and urine excretion. This makes osmoregulators adaptable to changing environments and capable of migration.
Q3: How do marine fish compensate for water loss in saltwater?
Marine osmoregulators drink large quantities of seawater to replace water lost through osmosis. They then excrete excess ions through their gills and produce concentrated urine to conserve water. This active ion transport and selective excretion maintains their internal osmolarity despite the hypertonic external environment.
Q4: What strategies do freshwater fish use to handle excess water absorption?
Freshwater osmoregulators drink little to no water and instead excrete dilute urine to eliminate excess water absorbed through osmosis. They actively take in ions through their gills to replenish those lost in dilute urine, maintaining higher internal ion concentrations than their hypotonic environment requires.
Q5: How does osmosis drive water movement across fish cell membranes?
Osmosis occurs when solute concentrations differ across a semipermeable membrane. Water passively diffuses toward the solution with higher solute concentration, creating net water movement. When external osmolarity is higher than internal osmolarity, water exits fish cells; when external osmolarity is lower, water enters cells, potentially causing them to swell or shrivel.
Q6: Why does osmoregulation require energy expenditure in fish?
Osmosis naturally tends to equalize ion concentrations across membranes, but fish require ion levels different from their environment. They must use active transport to maintain concentration gradients that optimize osmotic balance. The energy required depends on the difference between internal and external ion concentrations; larger differences demand more energy.
Q7: How do salmon change their osmoregulatory strategy during migration?
Salmon are euryhaline fish that undergo physiological changes when migrating from freshwater to ocean. They produce more cortisol to grow salt-secreting cells in their gills and shift to active ion transport out of the gills and concentrated urine excretion. This allows them to maintain osmotic balance in both hypotonic and hypertonic environments.