- Osmoconformer
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An osmoconformer is a marine invertebrate (with the exception of the Hagfish) that maintains the osmolarity of its body fluids such that it is always equal to the surrounding seawater. By maintaining their internal solute concentration the same as their environment, osmoconformers avoid water diffusing into their bodies.
Marine invertebrates (e.g. Squids) and ascidians (sea squirts - primitive chordates) generally have body fluids that are isoosmotic and also have the same ionic composition as water and therefore need not expend any energy at all for osmoregulation. The only vertebrate which does that is the Hagfish, which is a craniate, but is not universally accepted to be a vertebrate. It however does have some differences: its plasma has a lower concentration of divalent ions (Ca2+, Mg2+, SO4 2+) than does seawater and slightly higher concentrations of monovalent ions [1]. They therefore have to expend some energy for osmoregulation.
Other vertebrate osmoconformers are the elasmobranchs - marine cartilaginous fish (sharks, rays, skate). Their body fluid is isoosmotic with seawater, but this 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, which opposes the effect.
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