Question

Water and ionic regulations in freshwater and marine animals

Answer 1

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Osmosis – passive regulation

Cells of living organisms contain a lot of water and different solutes (ions, proteins, polysaccharides), creating a specific concentration inside the cell membrane. This membrane is semi-permeable, meaning that it only allows the solvent (water) to move across, but not the solutes. When cells are submerged into a solution of a different concentration, the law of osmosis comes into play. Osmotic pressure is a tendency of water to move into one solution from another by osmosis – passing a membrane from a high water concentration to a low water concentration. The higher the osmotic pressure (difference between the solutions on either side of a membrane), the more water tends to move across in order to balance the concentration.

Osmoregulation – active regulation

Keeping the homeostasis in balance is a big challenge for freshwater and marine fishes, because metabolic processes can only take place in very specific physical and chemical environment. In order to keep the “internal environment” constant, continuous adaptations with regard to temperature, pH and the concentrations of Na+, K+, Ca2+, glucose, CO2 and O2, take place. The key to their problem is osmoregulation – active regulation of the osmotic pressure to maintain the fluid balance and concentration of salts.

Let first take a look at freshwater fishes. Because the salt concentration inside their body is higher as in the surrounding water, water enters the body due to osmosis. Without any active regulation of this process, fishes would swell and get bigger and bigger. To compensate, the kidney produces a large amount of urine, which at the same time means loss of salts. In order to maintain a sufficient salt level, special cells in the gills (chloride cells) take up ions from the water, which are then directly transported into the blood.

In contrast, marine fishes face the opposite challenge – since the salt content in their blood is much lower than that of seawater, they constantly tend to lose water and build up salt. To replace the water loss, they continually need to drink seawater. Since their small kidney can only excrete relatively small amount of urine, the excretion of salt additionally takes place in the gills where chloride cells work in reverse as in freshwater fishes.