Osmoconformers

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.

Description
Marine invertebrates (e.g. Squids) and ascidians (sea squirts - primitive chordates) generally have body fluids that are isoosmotic and have the same ionic composition as water, therefore they do not expend any energy 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. They therefore have to expend some energy for osmoregulation.

Examples
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.