Osmoregulation in Freshwater and Marine Fish

How freshwater and marine teleosts balance water and salt through gill ionocytes, kidney and gut, why sudden salinity change kills, and the physiology behind therapeutic salt.

Two opposite problems

Most bony fish (teleosts) are osmoregulators: they hold the salt concentration of their blood roughly constant whatever the surrounding water does. Osmoconformers, by contrast, let body fluids follow the environment. In fresh water a fish is saltier than its surroundings, so water floods in by osmosis and salts leak out. In sea water the fish is less salty than its surroundings, so it loses water and gains salt. The gills, kidney and intestine work together to cancel both effects, and the direction of every process reverses between the two habitats.

Hyper- and hypo-osmotic regulation

A freshwater teleost is a hyper-osmotic regulator: it keeps its plasma far more concentrated than the water, absorbs salt actively at the gills and excretes surplus water as dilute urine. A marine teleost is a hypo-osmotic regulator: it keeps its plasma well below the concentration of sea water, drinks sea water to replace the water lost by osmosis, absorbs that water in the gut and pumps the extra sodium and chloride out through the gills. Drinking is the only way a marine teleost can make good its osmotic water loss, but each sip adds more Na+ and Cl- that must then be excreted. Some popular texts swap the terms hyper- and hypo-osmotic; the physiology literature uses them as described here.

Fresh water: about 1 mOsm/kg; plasma of freshwater teleosts about 274 mOsm/kg (review table in Takvam et al., 2021).
Sea water: about 1050 mOsm/kg; plasma of marine teleosts about 360 mOsm/kg (same table).

Gill ionocytes (chloride cells)

The gill epithelium contains pavement cells, mucus cells and several types of mitochondria-rich ion-transporting cells called ionocytes, the classic "chloride cells". Their engine is Na+/K+-ATPase on the inner (basolateral) membrane. In sea water this pump drives the Na+-K+-2Cl- cotransporter (NKCC1), which loads chloride into the cell; chloride then leaves through the apical CFTR channel, while sodium escapes between cells along an electrical gradient. In fresh water the same energy supply is used the other way round, to pull sodium and chloride in from very dilute water against their gradients. In larval fish whose gills are not yet developed, ionocytes sit on the skin and fins.

The kidney: a lot of dilute urine or a little concentrated urine

In fresh water the kidney filters blood at a high rate, reabsorbs almost all filtered ions and lets the water go, so the fish produces large volumes of dilute urine. In sea water filtration drops sharply, the tubules reclaim as much water as possible, and the kidney becomes the main exit for divalent ions (magnesium, sulfate, calcium) that enter with swallowed sea water. Monovalent ions are handled mainly by the gills and gut. Nephrons of freshwater teleosts have a distal tubule; those of marine teleosts typically lack it.

Freshwater teleosts: glomerular filtration about 4-16 ml/kg/h, urine flow about 1-6 ml/kg/h, urine about 20-50 mOsm/L.
Marine teleosts: glomerular filtration about 0.5-2 ml/kg/h, urine flow about 0.2-0.3 ml/kg/h, urine nearly isosmotic with plasma and rich in Mg2+ and SO4 2-.

Why a sudden salinity change kills

Osmotic shock is the dysfunction caused when the solute concentration around cells changes faster than they can adjust. In a hypertonic medium cells lose water and shrink; in a hypotonic medium water rushes in and cells swell, sometimes until they burst or self-destruct. A fish moved abruptly between fresh and salt water faces the same mismatch at the scale of the whole body: its ionocytes, kidney and gut are still set for the old habitat and keep moving salt and water in the wrong direction. Stenohaline species, which make up most teleosts, cannot reverse these systems at all and cannot survive outside their normal salinity. The effect is used deliberately against parasites: marine protozoa burst when a marine fish is briefly dipped in fresh water, because the single-celled parasite cannot regulate its volume.

Euryhaline and brackish-water species

Only about 3-5% of teleosts are euryhaline, able to acclimate to both strongly hypotonic and hypertonic water. Anguillid eels breed in the sea and grow in fresh or coastal water, so they must fully reverse their osmoregulatory machinery during migration; salmon do the same in the opposite direction. The switch is under endocrine control. Cortisol, released from the interrenal tissue, rises when euryhaline fish are transferred from fresh water to sea water and supports the seawater-type transport pattern. Prolactin is known as the freshwater-adapting hormone: it is essential for acclimation to fresh water and its plasma level correlates with salt uptake at the gill and water excretion by the kidney. Growth hormone also takes part.

Euryhaline fish named by Wikipedia: short-finned molly (Poecilia sphenops), salmon, tilapia, killifish, desert pupfish, barramundi, striped bass, sturgeon, round goby, bull shark.
Fish commonly kept in brackish aquaria: monos, scats, archerfish, pufferfish, gobies, chromides, mollies and the Siamese tigerfish (Datnioides pulcher).

The logic of therapeutic salt

Sodium chloride helps freshwater fish because it narrows the gradient they must fight. A handled or transported fish in fresh water keeps taking on water across the gills and spends energy expelling it; raising the salinity of the water slows the influx and spares energy reserves. Salt also stimulates mucus production on the skin and gills. At high concentration for a short time it acts against external protozoa, which suffer osmotic damage. Its chloride also competes with nitrite for uptake at the gill, the basis of chloride protection against nitrite, covered in a separate guide. Non-iodised table salt or rock salt fit for human or livestock use is suitable for freshwater treatments; marine fish need complete sea salt with its minor ions.

Transport and handling: 0.1-0.3% salt (1-3 g/L) to reduce osmoregulatory stress (UF/IFAS).
Bath against some freshwater parasites: 0.5-1.0% for several hours, or about 1% for 30 minutes up to several hours (UF/IFAS).
Dip: 3% (30 g/L) for 30 seconds to 10 minutes depending on species, removing the fish as soon as it loses equilibrium (UF/IFAS).
Marine fish: a freshwater dip of no more than 10 minutes, then return to clean sea water (UF/IFAS).