Osmoregulation in Different Environments

Osmoregulation is the process by which organisms maintain the balance of water and solutes in their bodies. Different organisms have adapted to different environments and face varying challenges when it comes to regulating their osmotic balance.

For example, marine organisms live in a highly salty environment and must prevent dehydration and maintain a proper salt concentration within their cells. On the other hand, freshwater organisms live in an environment where there is a lower concentration of solutes, so they need to prevent the uptake of excess water and eliminate the excess solutes.

Similarly, organisms that live in arid environments, such as deserts, must conserve water and prevent excessive water loss through transpiration or evaporation. They have evolved adaptations such as reduced leaf surface area, thick cuticles, and water storage structures to help them survive in these conditions.

In contrast, organisms that live in wet or humid environments may face the opposite problem of excess water uptake, which can lead to swelling and lysis of their cells. They have adapted mechanisms to prevent this, such as waterproofing their surfaces or actively pumping out excess water.

Osmoregulation is a critical process for the survival of all living organisms, and the mechanisms used to regulate water and solute balance can vary greatly depending on the organism's environment.

 

Marine Environment

The sea, where animals first evolved, is the only environment that supports osmoconformers. The total solute concentration in jellyfish, scallops (a mollusk), lobsters, and most marine invertebrates conforms to that of seawater, and thus these animals do not expend energy regulating their water content.

Among marine vertebrates cartilaginous fishes, such as sharks and rays have body fluids that are isotonic to seawater. It is surprising, though, that while they are isotonic, the body fluids of cartilaginous fishes do not contain the same amount of mineral ions as sea water. The answer to this paradox is that their blood contains a concentration of urea high enough to match the tonicity of the sea. Because urea in high concentration is harmful, so these fishes retain a chemical called trimethylamine oxide for protection against urea. Most cartilaginous fishes have lower internal salt concentration than that of sea’s water. Salts are excreted through gills; the fishes also have salt excretory organs such as rectal gland, (glands on the rectum).

Osmoregulation in freshwater fish and marine fish

Bony fishes

The body fluids of all bony fishes normally have only a moderate amount of salts. Apparently their common ancestors evolved in fresh water, and only later did some group invaded the sea. Marine bony fishes are therefore prone to water loss and could become dehydrated. To counteract this, they drink seawater constantly. The areas of the body permeable to water are the gills and lining of the buccal cavity and pharynx. While they get water by drinking, they also get salt. To get rid of excess salt, the fishes actively transport sodium (Na) and chloride (Cl) ions into the surrounding seawater through the gills. This causes a passive loss of water through gills. The fish also excrete concentrated urine to excrete maximum salt and minimum water.

Freshwater Environment

All freshwater species of protozoans have a lower water potential than their surroundings, i.e. more concentrated solution in their cells. There is therefore a constant tendency for water to enter the cell by osmosis through the cell surface membrane. They have contractile vacuoles which remove water that enters the cell by osmosis, therefore preventing the cell from increasing in size and bursting e.g. Amoeba, Paramecium.

Freshwater fish

The freshwater fish has concentration of solutes in its internal fluids much higher than that of fresh water. Freshwater fish gains water through its body surface especially through its gills and lining of the buccal cavity and pharynx and also in its food. It does not drink water except with its food. The freshwater fish also loses some solutes in its urine. It takes the work of three organ systems to achieve the proper water and solute balance in a freshwater fish. (a) The digestive system of the animal takes up ions from the food. (b) Its respiratory system i.e. gills also take up ions, i.e. specially Na and Cl. (c) the excretory system i.e. kidneys of the fish, work constantly to produce large amount of dilute urine.

Terrestrial Environment

Land animals are osmoregulators. They need to take water occasionally to make up for the water loss by excretions and respiration. A great majority of insects are adapted for life on land. Adaptation for preventing water loss include: (a) Body is covered by impermeable waxy layer to reduce water loss from the body surface. (b) Spiracles are the only openings in the body for gaseous exchange. Spiracles have valves to reduce water loss. (c) The excretory product is semisolid i.e. uric acid. (d) The embryo develops inside an egg with a relatively impermeable shell that prevents water loss.

Most terrestrial vertebrates, including humans, have an outer skin formed of multiple layers of dead water resistant cells. Also key to survival on land are adaptations that protect fertilized eggs and developing embryo from drying out.

To prevent loss of water during the process of breathing, certain animals, like the camel and kangaroo rat, have a nasal passage that has a highly convoluted mucous membrane surface. This surface allows them to capture moisture from exhaled air, which they use to humidify air that is being inhaled. Humans mainly conserve water by producing hypertonic urine, and its faecal material is almost completely dry. Terrestrial animals can tolerate dehydration and it differs in various animals. This characteristic is known as anhydrobiosis. Behavioural patterns also serve to help many animals avoid situations in which water loss is greatest. For example, many desert animals are active during the cooler night time hours and rest in their cool underground burrows during the hottest daytime hours. Some desert animals e.g. kangaroo rat survives without drinking water by feeding on seeds of desert plants containing more carbohydrates which produce water of metabolism.