Physiology of Finfish and Shellfish (2+1)

2.4.2. Definitions

Most marine invertebrates have body fluids with the same osmotic pressure as the sea water; they are isosmotic or isosmotic with the medium in which they live (Greek isos = equal).

When there is a change in the concentration of the medium, an animal may respond in one of two ways. One way is to change the osmotic concentration of its body fluids to conform with that of the medium, thus remaining isosmotic with the medium; such an animal is an osmo-conformer. The other way is to maintain or regulate its osmotic concentration is spite of external concentration changes; such an animal is called an osmoregulator. For example, a marine crab that maintains a high concentration of salts in its body fluids after it has been moved to dilute brackish water is a typical osmoregulator.

Fresh-water animals have body fluids that are osmotically more concentrated than the medium; these animals are hyperosmotic. If an animal has a lower osmotic concentration than the medium, as does a marine teleost fish, it is said to be hypo-osmotic or hyposmotic.

The concentrations of the various individual solutes in the body fluids of an animal usually differ substantially from those in the medium, even if the animal is isosmotic with the medium. The differences are usually carefully regulated, a subject dealt with under the term ionic regulation. Some degree of ionic regulations seems to occur in all organisms, both in osmoregulators and in osmoconformers.

Some aquatic animals can tolerate wide variations in the concentration of salt in the water in which they live; they are called euryhaline animals (Greek eurys = wide, broad; halos = salt). Other animals have a limited tolerance to variations in the concentration of the medium; they are called stenohaline (Greek stenos = narrow, close).

A marine animal that can penetrate into brackish water and survive is euryhaline. An extremely euryhaline animal may even be able to tolerate shorter or longer periods in fresh water. The term euryhaline is used also for fresh-water animals that can withstand considerable increases in the salt content of the water. A strenohaline organism, whether marine or freshwater, can tolerate only a narrow range of changes in the salt concentration of the water in which it lives.

There is no sharp separation between euryhaline and stenohaline animals, and there is no commonly accepted definition that places a given animal in one or the other group.

The concentration of a dissolved substance is usually expressed in units of molarity: moles per liter solution. In a biological context, it is often convenient to use the unit millimole (mmol). For example, a solution of 0.5 mol per liter is equal to 500 mmol per liter.

The osmotic concentration of a solution can be expressed as the osmolarity (osmoles per liter). The osmolarity of a solution depends on the number of dissolved particles and can be stated without knowledge of which specific solutes are present. The osmolarity of a solution of a nonelectrolyte (e.g., sucrose or urea) equals the molar concentration.

A solution of an electrolyte (e.g., sodium chloride, which in solution dissociates into Na⁺ and Cl^-) has a higher osmotic concentration than is expressed by its molarity. However, it does not have exactly twice the osmotic concentration because the salt is not fully dissociated (see Appendix E). The degree of dissociated depends on the concentration of sodium chloride and on interaction with other ions. However, the total osmotic concentration can readily be determined. In biological work this is most commonly done by measuring the freezing point depression or the vapor pressure of the solution. It is the osmotic concentration, rather than a detailed list of all solutes, that is important in most considerations of osmotic regulation in animals.

Ordinary sea water, which contains about 470 mmol sodium and about 550 mmol chloride per liter plus substantial amounts of divalent ions (magnesium and sulfate), has an osmotic concentration of about 1000 milliosmoles (mOsm) per liter.

The term isotonic is used in a different sense. We say that a living cell is isotonic with a given solution if the cell neither swells nor shrinks in the solution. For example, if mammalian red blood cells are suspended in a sodium chloride solution of 150 mmol per liter (about 0.9%), the cells retain their size, shape, and volume. If, on the other hand, they are suspended in an isosmotic solution of urea (0.3 mol per liter), they rapidly swell and burst. The urea solution, although isosmotic, is not is isotonic because urea rapidly penetrates the red cell membrane so that the urea concentrations inside and outside the cell are equal. The electrolytes do not move out of the cell, which now behaves as if it were suspended in distilled water. Because there are salts inside and none outside, water flows into the cell, which swells and bursts.

Isosmotic is defined in terms of physical chemistry, isotonic is a descriptive word based on the behavior of cells in a given solution.