Marine Osmoregulation — Salt & Water Balance in Fish

Species

Environment

Water salinity 35 ppt

Water temperature 18 °C

Live Physiology

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Osmotic gradient

0 mOsm/L

Net water flux

0 µL/g/h

Active-transport ATP cost

0 rel. units/h

How it works

Water crosses the semi-permeable gill and skin membrane by osmosis, moving toward the side with the higher total solute concentration. The driving force is the osmotic gradient predicted by van 't Hoff's law:

π = i · C · R · T
ΔOsm = C_internal − C_external   (mOsm/L)
J_water = k_p(T) · ΔOsm          (µL of water per g body mass per hour)

k_p(T) is gill permeability, which rises with temperature (a Q10 effect on membrane diffusion). A marine teleost keeps its blood near 330 mOsm/L — far below seawater (~1000 mOsm/L at 35 ppt) — so it constantly loses water outward and must drink seawater, then use gill chloride cells (Na⁺/K⁺‑ATPase) to actively pump excess Na⁺/Cl⁻ out, against the gradient. A freshwater teleost is the mirror image: blood (~280 mOsm/L) is far saltier than the water around it, so it gains water passively and must actively pump salt in through the gills while excreting large volumes of dilute urine. A shark or ray retains urea and TMAO in its blood to stay close to isosmotic with seawater, so passive water flux is small — but salt still leaks in, and a rectal gland actively secretes it back out.

The ATP cost of active transport scales with |ΔOsm|, because moving ions against their concentration gradient requires free energy (ΔG = R·T·ln(C₂/C₁) per mole). Toggle transport off to see the unregulated flux a real fish would face — and why it would not survive for long.

Water molecules (osmosis) Active ion pump (ATP) Blood / body fluid

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