Osmoconformer

Osmoconformers allow their body fluid concentration to track the environmental osmolality rather than actively maintaining a constant internal concentration. Most marine invertebrates are osmoconformers. Among fish, elasmobranchs (sharks and rays) are unique in being nearly iso-osmotic to seawater, but achieve this by retaining organic solutes (urea and trimethylamine oxide, TMAO) rather than by conforming ionically — they still regulate ion concentrations. True ionic osmoconformers are extremely rare among fish. The advantage of osmoconforming is the elimination of the energy cost of active ion transport.