The need for Osmoregulation in Animals:

Water is essential component of the animal's body; it is needed for maintenance of life and other metabolic processes. It forms main medium and most necessary nutrients in all animals. Water accounts for between 60% and 95% of animal's body weight. Water inside animals may be inside cells as intracellular fluid (ICF) or it may be outside cells in form of extracellular fluid (ECF). ECF itself may be distributed between many smaller compartments, like blood plasma and cerebrospinal fluid. The variety of solutes in form of ions and nutrients are dissolved in fluids. Animals require maintaining suitable and right amounts of water and solutes in the different body fluids. Skill to regulate water and solute concentrations in animals is referred as Osmoregulation. Osmoregulation and excretion are closely related together in animals as most animals utilize their excretory organs for osmoregulatory functions. Animals generally need Osmoregulation to make sure:

• Protection of animal's internal environment from harmful fluctuations
• Maintenance of balance in water loss and uptake in animal's body
• Proper control of animal's internal environment.

Principle of Osmosis:

Term osmosis is best described as movement of water across the selectively permeable membrane that divides two solutions, from the region of water high concentration (that is dilute solution) to the region of lower water concentration (that is a concentrated solution). When 2 aqueous solutions of different solute concentrations are divided by membrane permeable to water but impermeable to solute molecules, water diffuses through membrane from dilute solution to more concentrated solution. What occurs here is known as osmosis. This procedure will continue until equilibrium is established, at which point there is no further net movement of water and concentrations of solution on either side of selectively permeable membrane are equal. The selectively permeable membrane is one that permits only water to go through it and no other substances.

Osmotic Responses of Animals:

Animals may be categorized in two broad categories on the basis of the osmotic responses; they are either osmoconformers or Osmoregulators.

Osmoconformers:

Osmoconformers are animals whose body fluid concentration is accurately same as that of immediate environment in which they exist. Typical osmoconformers comprise marine invertebrates, whose body fluid concentration is same as that of salt water. This signifies that 2 solutions (body fluid /sea water) are isosmotic (i.e., with same osmotic pressure). Though these animals may be in osmotic equilibrium, they don't essentially have to hold same composition or be in ionic equilibrium. The great deal of energy is needed for ionic regulation. Therefore, for osmoconformers, there is requirement for corresponding change in osmotic concentration of the body as soon as external environment changes in osmotic concentration. Few osmoconformers are capable of tolerating broad changes in osmotic concentration of the immediate environment. These are referred to as euryhaline animals. In contrast, few osmoconformers can only tolerate much smaller changes in osmotic concentration of the immediate environment. They are called as stenohaline animals.

Osmoregulators:

Osmoregulators conversely are animals that maintain body fluid concentration which is different from of immediate environment. If osmotic concentration of body fluids is maintained at concentration greater than that of immediate environment they are termed as hyperosmotic regulators (like crabs); if they preserve their body fluid concentration below immediate environment they are termed as hypoosmotic regulators (like some crustaceans). Terrestrial animals comprising humans, fact is that they live on land, are generally in hyperosmotic condition to the environment, therefore they are osmoregulators.

Marine Invertebrates:

Marine environment is basically characterized by high acidity, mineral concentration, salinity, temperature, tidal action and density. Such physical and chemical characteristics stay quite constant through year except in few seasons where there are slight fluctuations. Animals found in this environment contain body fluid concentration similar to salt water where they live. They vary from seawater they inhabit on basis of the ionic composition. The organisms overcome the osmotic challenges either as osmoconformers or osmoregulators.

Many marine invertebrates are osmoconformers, i.e., osmotic concentration of the body fluids is same as that of seawater they live in. This signifies that they are in osmotic equilibrium (that is there is no net gain or loss of water). Though, this osmotic equilibrium doesn't essentially imply that they are in ionic equilibrium. Differences in ionic composition between seawater and body fluids will result in formation of concentration gradients. Resultant loss or gain of ions may challenge physiology of animal concerned and may also challenge osmotic equilibrium. For instance, animal may add ions from seawater if specific ion is at greater concentration in seawater than it is inside animal. This will produce in body fluids becoming hyperosmotic in relation to seawater and this in turn will yield in osmotic gain of water. Usually, osmotic concentrations of ions are not considerably different from equivalent concentration in seawater. Though, there are some exceptions, such as SO₄^2- and Ca²⁺ that in some species may be there in concentrations noticeably different from that discovered in seawater. This means that concentrations of such ions required to be physiologically regulated therefore; ions should be actively secreted or absorbed. In definite marine invertebrates like jellyfish, SO₄^2- ions are excreted to decrease density of animal, in order that its buoyancy can increase. SO₄^2- ion is comparatively heavy and removing it from animal will amount to decreasing animal's weight and increasing the buoyancy. Animal may also lose or gain ions by means of general body surface through ingestion of food and production of waste substances in form of urine. Though, there are some exceptions in which few other invertebrates such as octopus preserve body fluid concentrations which are more concentrated (hyperosmotic) to salt water; while others like brine shrimp and some crustaceans have less concentrated (hypoosmotic) to salt water.

Marine Vertebrates:

Marine vertebrates illustrate some remarkable differences in their osmotic responses when compared with saltwater invertebrates. The typical case of osmoconformers who are in osmotic and ionic equilibrium with seawater is hagfish. Hagfish (cysclostomes) are most ancient vertebrates. They illustrate few resemblances with marine invertebrates in osmotic response. Hagfish utilizes type of osmotic and ionic conformation which has been utilized as physiological evidence which vertebrates evolved in marine environment. Majority of other marine fish, though, illustrate varying degree of osmotic and ionic regulation. Osmotic concentration of plasma is approximately one-third that of seawater, consequently they are hypoosmotic regulators.

Marine Elasmobranchs:

Elasmobranchs (cartilaginous fishes) are very successful osmoregulators as they have evolved novel way of getting this regulation. Their plasma is only one-third as concentrated as seawater in which they live, they face 2 problems - loss of water and gain of ions. Loss of water is minimized by animals getting osmotic equilibrium by addition of solutes to plasma. Solutes added are urea and trimethylamine oxide (TMAO). Urea is made as outcome of protein metabolism, even as biosynthesis of TMAO is less clear. In several cases, more urea and TMAO is added to plasma than is essential to make osmotic equilibrium, therefore, making plasma hyperosmotic to seawater. Result of this is that animal gains water across surface of gills. Gills are generally composed of large surface area, thin walled and highly vascularised. Gain and loss of water and ions is beneficial to elasmobranchs as excess water can be utilized for production of urine and removal of waste products, like excess ions which diffuse in animal that happens across gills.

Potentially, biggest problem with addition of large amounts of urea to plasma is that urea inclined to denature and inactivate other plasma proteins. Though, these animals have overcome problem to such extent that proteins and enzymes are not capable to function properly without urea. Likewise, another problem faced by elasmobranchs is gain of ions. This is due to plasma has different solute composition to salt water; concentration gradient therefore exists which favors movement of ions into animals. For example, there is massive influx of Na+ ions across gills. Rectal gland helps in excretion of excess Na+ ions. It is specialized gland that opens out into rectum and secretes fluid which is rich in NaCl. Small osmotic influx of water in these animals allow for production of urine, that is another route by which excess NaCl may be excreted.

Marine Teleosts:

Marine teleosts (bony fishes) face similar problems as elasmobranchs as plasma is less concentrated than seawater. Loss of water, mainly across gills, is compensated for by drinking large volumes of seawater. This solves one problem, but exacerbates another by adding the further salt load to animal. This means that animal should somehow excrete large amounts of NaCl. As kidney of teleost fishes is not capable to create concentrated urine, there is some other organ which is able to excrete large amounts of NaCl. This organ is gill that has dual function in gas exchange and osmoregulation. Gills of marine teleosts have special cells called as chloridecells that are liable for active transport of NaCl from plasma to seawater. Cl^- ions are actively extruded form blood in chloride cells, accompanied by passive diffusion of Na⁺. Therefore, Cl^- moves passively out from gill in surrounding seawater.

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