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Osmoconformers are marine animals which, in contrast to osmoregulators, maintain the osmolarity of their body fluids such that it is always equal to the surrounding seawater. Osmoconformers decrease the net flux of water into or out of their bodies from diffusion. They maintain internal solute concentrations within their bodies at a level equal to the osmolarity of the surrounding medium.
Marine invertebrates (e.g., squid), including ascidians (sea squirts - primitive chordates), generally have body fluids that are isoosmotic with the same ionic composition as seawater, therefore they do not expend any energy for osmoregulation. The only vertebrate which does that is the Hagfish, which is a craniate (although not universally accepted as a vertebrate). However, its plasma differs from seawater, having a lower concentration of divalent ions (Ca2+, Mg2+, SO4 2-) and slightly higher concentrations of monovalent ions. Hagfish therefore have to expend some energy for osmoregulation.
Other vertebrate osmoconformers are the elasmobranchs - marine cartilaginous fish - such as sharks, rays and skates. Their body fluid is isoosmotic with seawater, but their high osmolarity is maintained by making the concentration of organic solutes unnaturally high. Sharks concentrate urea in their body, and since urea denatures proteins at high concentrations, they also accumulate TMAO to counter the effect.
- Dr. Marion McClary, Jr. (August 19, 2008). "Osmoconformer". Enclyclopedia of Earth. Retrieved 16 April 2012.
- Jørgensen, Jørgen Mørup (1998). "The Biology of Hagfishes". ISBN 9780412785306.
- Karleskint, George; Turner, Richard L; Small, James W (2009-01-02). "Introduction to Marine Biology". ISBN 9780495561972.
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