3 Saltwater organisms: adaptation to high osmotic pressure

• Most marine bony fish lose water through their bodies by osmosis. To compensate they swallow large amounts of sea water; the gut absorbs water, then the excess salts are excreted through the kidneys (producing small volumes of concentrated urine) and by specialised cells in the gills (chloride cells) which pump salt out against the concentration gradient.
• Sharks use a different strategy: they retain a high concentration of urea (a nitrogenous waste of many animals, excreted in urine) in the blood (together with another compound, TMAO, that protects proteins from urea). This raises the osmotic pressure of their body fluids until it is close to that of the surrounding sea water, so there is hardly any net loss of water by osmosis.

• Problem. A paramecium in pond water gains water by osmosis at a rate of 4.0 pL (picolitres) per minute.
• Its contractile vacuole expels 0.5 pL each time it contracts.
• (a) How many contractions per minute are needed to keep the cell volume constant?
• (b) How long is the interval between contractions?

Solution. (a) 4.0 ÷ 0.5 = 8 contractions per minute. (b) 60 s ÷ 8 = 7.5 s. If the pond water is made saltier, the water entry slows and the contraction rate falls; in sea water the vacuole stops pulsing. This is a practical way to verify that the vacuole is an osmoregulation organelle.

• Problem. A marine bony fish has body fluid with a total dissolved-particle concentration of 0.35 osmol/L.
• Sea water is 1.0 osmol/L and river water is 0.002 osmol/L.
• (a) In which direction does water move in each case?
• (b) What would happen to a marine fish moved to river water?
• (c) How does salmon cope?

• Solution. (a) In sea water (1.0 > 0.35) water moves out of the fish; in river water (0.002 < 0.35) water moves into the fish.
• (b) A strictly marine fish placed in river water would take in water and lose salts and die (it cannot produce large volumes of dilute urine).
• (c) Salmon change their physiology: in the sea they drink and excrete salt through the gills; when they enter the river their gills take up salt and their kidneys produce large amounts of dilute urine.
• The change is controlled by hormones and takes some days, so they stay in the estuary for some time while adjusting.

• Problem. A shark's blood contains 0.40 osmol/L of salts, 0.35 osmol/L of urea and 0.25 osmol/L of TMAO.
• Sea water is 1.00 osmol/L.
• Is the shark's body fluid isotonic with sea water?
• Why does this avoid water loss?

Solution. Total = 0.40 + 0.35 + 0.25 = 1.00 osmol/L = same as sea water, so the blood is (almost) isotonic, and there is no net osmotic water loss. If the shark had only its 0.40 osmol/L of salts it would lose water like a bony fish.