Non-retractable mouth - characterized by fixed connection of the bones of the upper jaw with the skull. It is characteristic of most fish that feed on relatively large objects and in the process of capturing food forced to spend considerable effort to close the mouth. These are predators, as well as benthophagi, chewing shells of mollusks, hard shells of crustaceans and echinoderms.

For example, parrot fish are characterized by a non-retractable mouth and a different structure of jaw teeth. Their teeth are fused on the jaws to form a "parrot's beak", which consists of two suture-separated plates on each jaw.