What is Osmoregulation
The regulation of solute movement, and hence, water movement, which follows solutes by osmosis, is known as osmoregulation. Osmosis may be defined as a type of diffusion where the movement of water occurs selectively across a semipermeable membrane. It occurs whenever two solutions, separated by semipermeable membrane (the membrane that allows water molecules to pass but not the solutes) differ in total solute concentrations, or osmolarity. The total solute concentration is expressed as molarity or moles of solute per litre of solution. The unit of measurement for osmolarity is milliosmole per litre (mosm L–1). If two solutions have the same osmolarity, they are said to be isotonic. When two solutions differ in osmolarity, the solution with higher concentration of solute is called hypertonic, while the more dilute solution is called hypotonic. If a semipermeable membrane separates such solutions, the flow of water (osmosis) takes place from a hypotonic solution to a hypertonic one.
Osmoconformers vs Osmoregulators
Osmoconformers are the animals that do not actively control the osmotic condition of their body fluids. They rather change the osmolarity of body fluids according to the osmolarity of the ambient medium. All marine invertebrates and some freshwater invertebrates are strictly osmoconformer. Osmoconformers show an excellent ability to tolerate a wide range of cellular osmotic environments.
Osmoregulators, on the other hand, are the animlas that maintain internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited osmoregulators. Most vertebrates are strict osmoregulators, i.e. they maintain the composition of the body fluids within a narrow osmotic range. The notable exception, however, are the hagfish (Myxine sp., a marine cyclostome fish) and elasmobranch fish (sharks and rays).
Osmoregulators must either eliminate excess water if they are in hypotonic medium or continuously take in water to compensate for water loss if they are in a hypertonic situation. Therefore, osmoregulators have to spent energy to move water in or out and maintain osmotic gradients by manipulating solute concentrations in their body fluids.
Water and solute Osmoregulation in freshwater environment
Osmolarity of freshwater is generally much less than 50 mosm L–1 while the freshwater vertebrates have blood osmolarities in the range 200 to 300 mosm L–1. The body fluids of freshwater animals are generally hypertonic to their surrounding environment. Therefore, freshwater animals constantly face two kinds of osmoregulatory problems : they gain water passively due to osmotic gradient, and continuously lose body salts to the surrounding medium of much lower salt content.
However, the freshwater animals prevent the net gain of water and net loss of body salts by several means, Protozoa (Amoeba, Paramoecium) have contractile vacuoles that pump out excess water. Many others eliminate water from the body by excreting large volume of very dilute urine. As a general rule, animals do not drink water, including freshwater fish do not drink water to reduce the need to expel and salt loss are minimised by a specialised body covering (subcutaneous fat layer of scaleless fish and scales over the body of fish or crocodile). Freshwater animals have remarkable ability to take up salts from the environment. The active transport of ions takes place against the concentration gradient. Specialised cells, called ionocytes or chloride cells in the gill membrane of fresh water fish can import Na+ and Cl– from the surrounding water containing less than 1mM NaCl, when their plasma concentration of NaCl exceeds 100 mM.
Water and solute Osmoregulation in marine environment
Sea water usually has an osmolarity of about 1000 mosm L–1. Osmolarity of human blood is about 300 mosm L–1. The osmoregulatory problems in marine situation are opposite to those in freshwater environment. Marine bony fish have the body fluids hypotonic to seawater, and thereby, they tend to lose water from the body through permeable surfaces (gill membranes, oral and anal membranes). To compensate for the water loss, marine bony fish drink seawater. However, drinking seawater results in a gain of excess salts. The ionocytes or chloride cells of the gill membrane of marine bony fish help to eliminate excess monovalent ions from the body fluid to the seawater. Divalent cations are generally eliminated with faeces. Hilsa, salmon and other fish that migrate between seawater and freshwater, when in ocean, drink and excrete excess salt through the gill membrane. A number of hormones play a key role in this switching over process.
In general, the body fluids of marine invertebrates, ascidians and the hagfish are isotonic to seawater. In elasmobranch fish (sharks and rays) and coelocanths (lobefin fish), osmolarity of the body fluids is raised by accumulating certain organic substances (osmolytes). Retention of osmolytes in body fluids reduces the osmoregulatory challenges. The best known examples of such organic osmolytes are urea and trimethylamine oxide (TMAO). Body fluids of sharks and coelocanths are slightly hyperosmotic to seawater due to retention of urea and TMAO while hypoionic to seawater as they maintain far lower concentration of inorganic ions in the body fluids.
Water and solute Osmoregulation in terrestrial environment
Land animals are always subject to osmotic desiccation, like the marine animals. Air-breathing animals constantly lose water through their respiratory surfaces. However, animals utilise various means to minimise this water loss. Good examples are the waxy coatings of the exoskeletons of insects, the shell of the land snails and the multiple layers of dead, keratinised skin cells covering most terrestrial vertebrates. Despite such protective measures, a considerable amount of water is lost through oral, nasal and respiratory surfaces. This may even be fatal for the animal concerned. Humans, for examples, die if they lose around 12 per cent of the body water. Therefore, water loss must be compensated by drinking and eating moist food. Desert mammals are well adapted to minimise water loss. Kangaroos rats, for example, lose so little water that they can recover 90 percent of the loss by using metabolic water (water derived from different cellular metabolic processes.) The nasal countercurrent mechanism for conserving respiratory moisture is also important. Behavioural adaptations, such as nervous and hormonal mechanisms that control thirst, are important osmoregulatory mechanisms in terrestrial animals. Many desert animals are nocturnal to avoid the heat of day-time, another important behavioural adaptation that minimises dehydration. The camels, however, reduce the chance of overheating by orienting to give minimal surface exposure to direct sunlight. They produce dry faeces and concentrated urine. When water is not available, the camels do not produce urine but store urea in tissues and solely depend on metabolic water. When water is available, they rehydrate themselves by drinking up to 80 litres of water in 10 minutes.