An osmoconformer is a marine invertebrate
Marine invertebrates
Marine invertebrates are animals that inhabit a marine environment and are invertebrates, lacking a vertebral column. In order to protect themselves, they may have evolved a shell or a hard exoskeleton, but this is not always the case....

 (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
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

. It however does have some differences: its plasma has a lower concentration of divalent
In chemistry, a divalent ion or molecule has a valence of two and thus can form two bonds with other ions or molecules. An older term for divalent is bivalent....

 ions (Ca2+, Mg2+, SO4 2+) than does seawater and slightly higher concentrations of monovalent
Monovalent may refer to:*In chemistry, valence is a measure of the number of chemical bonds formed by the atoms of a given element. Monovalent is a synonym of univalent.*Monovalent ions contain one valence electron....

 ions . They therefore have to expend some energy for osmoregulation.

Other vertebrate osmoconformers are the elasmobranchs - marine cartilaginous fish (sharks, rays, skate
Skates are cartilaginous fish belonging to the family Rajidae in the superorder Batoidea of rays. There are more than 200 described species in 27 genera. There are two subfamilies, Rajinae and Arhynchobatinae ....

). 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
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

 in their body, and since urea denatures proteins at high concentrations, they also accumulate TMAO, which opposes the effect.

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