Question

How have freshwater and saltwater fish adapted to deal with osmosis in their respective environments?

Answer 1

The problem
- Osmosis moves water from low solute concentration to high solute concentration. Freshwater and marine environments impose opposite osmotic pressures on fish, so fish have evolved very different ways to keep body fluids in balance (osmoregulation).

Freshwater fish (body fluids are hyperosmotic to their environment)
- Tend to gain water and lose salts by diffusion.
- Drinking: generally do not drink appreciable water.
- Urine: produce large volumes of very dilute urine to shed excess water (high glomerular filtration and many nephrons adapted for dilution).
- Ion conservation: actively take up Na+, Cl–, and other ions across the gills using mitochondria-rich (chloride/ionocyte) cells and Na+/K+-ATPase–driven transporters; also absorb ions from food.
- Low permeability: skin, scales and mucus help reduce passive ion loss.
- Hormonal control: hormones such as prolactin promote ion uptake and reduce water permeability in freshwater acclimation.
- Example: many freshwater teleosts (carp, goldfish).

Saltwater teleosts (body fluids are hypoosmotic to seawater)
- Tend to lose water and gain salts by diffusion.
- Drinking: actively drink seawater to replace water lost.
- Desalination: gut and kidneys remove much of the ingested salt; intestinal epithelia absorb water while pumping ions out.
- Gill salt excretion: gill ionocytes actively secrete excess Na+ and Cl– to the seawater (Na+/K+-ATPase, NKCC cotransporters, apical Cl– channels such as CFTR); Na+ often leaves paracellularly.
- Urine: produce small volumes of urine that is relatively concentrated and conserves water; kidneys excrete divalent ions (Mg2+, SO42–).
- Hormonal control: cortisol and other hormones promote seawater-type ion transport and drinking.
- Example: most marine bony fishes (tuna, cod).

Special case — elasmobranchs (sharks, rays)
- Many are roughly isosmotic to seawater by retaining high concentrations of urea and trimethylamine N-oxide (TMAO) in their blood, so they don’t lose as much water by osmosis.
- Still must manage salt ions: rectal gland and gill/renal processes actively excrete excess NaCl and regulate divalent ions.

Summary
- Freshwater fish avoid drinking, make lots of dilute urine, and actively uptake ions. Marine teleosts drink seawater, excrete salts mainly via gills, and make small volumes of concentrated urine. Both strategies require active transport (ATP) and are hormonally regulated; some groups (e.g., sharks) use distinct chemistry (urea retention) to reduce osmotic stress.