Explain about osmoconformers and osmoregulators.

Osmoregulators are organisms that control the salt concentration of the body irrespective of the environmental salt concentration. For example fresh water fishes, vertebrates, birds, reptiles are osmoregulators.

Osmoconformers are the organisms that always maintain the solute concentration of their body just equal to the concentration of the surrounding. All marine invertebrates and some vertebrates are osmoconformers.

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Osmolarity measures the amount of dissolved ions in water. In animals with blood cells, osmolarity is a big deal. If the osmolarity of the animal's blood is too high (i.e., the blood is too salty), the blood cells tend to shrivel up as the water is drawn out of them. If the osmolarity of the animal's blood is too low (not enough salt), water is drawn into the blood cells until they explode. The blood cells of freshwater aquatic vertebrates tend to be more resistant to osmotic damage than their terrestrial counterparts, but for all animals' blood cells, there's a limit to the osmotic range they can tolerate. 

The body fluids of an osmoconformer are at the same osmolarity as the surrounding water. Echinoderms (sea stars, sea urchins, sand dollars, etc.), cnidarians (sea anemones, corals, jellyfish), and ctenophorans (comb jellies) are all osmoconformers, with internal osmolarity matching the surrounding seawater (~1700-2200 milliosmoles (mOs)). 
An osmoregulater maintains a constant internal osmolarity distinct from the surrounding environment. Arthropods, annelids, and vertebrates are osmoregulators. Osmoregulators have to use specialized organs to regulate ion levels: examples are the protonephridia of flatworms, the metanephridia of annelids and arthropods, the kidneys of vertebrates, and the salt glands found in some reptiles and birds. The difference in osmolarity between blood and environment can be quite large - a marine mammal's blood is around 300 mOs, about 1900 mOs less than the surrounding sea water. 

Each approach has advantages and disadvantages. Osmoconformers don't have to waste energy pumping ions in and out of their cells, and don't need specialized structures like kidneys or nephridia to maintain their internal salt balance, but they're very sensitive to environmental changes in osmolarity. Also, because they can't adapt easily to environmental changes in osmolarity, osmoconformers have trouble adapting to habitats with osmolarity different from their preferred environment. In contrast, osmoregulators can adapt to a wide variety of environments, and, because their internal osmolarity is constant, their metabolic and other enzymatic reactions can be optimized for a single osmotic condition. However, they need specialized excretory organs to maintain their internal osmolarity, and waste a lot of energy on osmoregulation.


Resistance of various species' erythrocytes to osmotic damage:
Aldrich, Saunders, Sievert, and Sievert. Comparison of erythrocyte osmotic fragility among amphibians, reptiles, birds and mammals.
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