Sharks, rays, and skates retain urea and trimethylamine oxide (TMAO) within their blood allowing them to become only slightly hyperosmotic thereby reducing the cost of osmoregulation.

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Sharks, rays, and skates are able to retain urea and TMAO in their blood, which allows them to maintain a slightly hyperosmotic state. This reduces the cost of osmoregulation, as they do not need to constantly expend energy to maintain their internal salt balance.

Sharks, Rays, and Skates Retain Urea and Trimethylamine Oxide (TMAO)