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Marine bony fishes osmoregulate their extracellular fluids to maintain them at one-third to one-half the osmolarity of seawater. Thus marine bony fishes must conserve water, which they are constantly losing through osmosis, and must excrete excess solutes. Marine bony fishes cannot produce urine that is more concentrated than their extracellular fluids, so they minimize water loss by producing very little urine. In contrast, freshwater fishes produce lots of dilute urine.

How do marine bony fishes deal with the large salt loads they ingest with food? Marine bony fishes do not absorb from their gut some of the ions they take in, especially divalent ions such as Mg²⁺ and SO₄^2–. NaCl, the major salt ingested, is actively excreted across the gill membranes. As mentioned earlier, bony fishes can lose their nitrogenous waste, ammonia, by diffusion across their gill membranes.

Sharks and rays are osmoconformers but not ionic conformers. As mentioned earlier, they raise the osmolarity of their body fluids by retaining urea and TMAO making their extracellular fluids hyperosmotic to seawater. These species have adapted to a concentration of urea in the body fluids that would be toxic to other vertebrates. Sharks and rays still have the problem of excreting the large amount of salts they take in with their food. They solve this problem by having a gland in the rectum that actively secretes NaCl by a mechanism similar to that of the nasal salt glands of seabirds.