Ophidiiformes is a genus of bony fishes. Previously belonged to the cod fishes. Fish with an elongated body covered with cycloid scales. Dorsal and anal fins are long, merged with a reduced caudal fin. The pelvic fins are located on the chin. Includes 4 families, 87 genera and about 300 species. Predominantly marine, benthic fish of tropical, subtropical and temperate waters. Reach a length of up to 1.5 meters. They live at depths of 100-500 m, some are found at depths up to 7500 m. Planktophagous, benthophagous and predators. Spawners and livebearers. Some species are commercial fish.

Contains many deep-sea marine species, including the deepest, Abyssobrotula galatheae, recorded at a depth of 8370 m off Puerto Rico. However, many other species live on coastal shoals, especially among coral reefs, with only a few freshwater species. Most species live in tropical and subtropical waters, but individual representatives are known from polar waters such as the Greenland coast and the Weddell Sea. Ophidiiformes have a generally narrow body with a small head. Scales are small or absent. The dorsal fin is long and the anal fin is usually combined with the caudal fin. The group includes pelagic, benthic and even parasitic species. Some representatives are viviparous. Body length ranges from 5 cm (Grammanoides opisthodon) to 2 m (Lamprogrammus shcherbachevi).

Marine, less often freshwater fish with more or less elongated and compressed from the sides of the body, noticeably tapered to the tail part. Dorsal and anal fins in these fish are very long and often merged with the caudal fin. Some species do not have pelvic fins, and in others they are located on the head: at the level of the gill cover or even closer to the snout - on the chin.

Most Ophidiiformes are benthic or near-bottom fishes as adults, and only some species live in the sea water column. Some species (Lucifuga) live in enclosed limestone caves in Cuba and the Bahamas, and others (Aphyonus) - in the ocean at depths of 4-5 thousand meters and are therefore known only from isolated finds. Some species of this order hatch eggs, while others are viviparous, and their males have a special external copulatory organ.

This group includes very interesting fish - carapus, the development of which is accompanied by the passage of two larval stages of metamorphosis. At the first stage in the larva increases the ray of the dorsal fin. It is equipped with blades and appendages, has a special name "vexillum" and serves as a special organ that allows the larva to float freely in the water column. At this stage of development, the larva is called vexillifer - "bearing vexillum". At a length of about 80-90 millimeters, the carapus larva loses the vexillum, its body lengthens to 20 centimeters and thins. This stage of development is called tenuis, which means "thin." This larva sinks to the bottom and embeds in the body cavity of a marine invertebrate animal - holothuria.

The larva lives in the body of the holothuria and feeds on its gonads and water lungs, but holothuria this cohabitation does not bring much harm, because the lost organs in her quickly recover. Outside the body of the host larva carapus at this stage can not exist. Turning into a fry, and then becoming an adult fish, carapus is often found in a free state, and holothuria uses only as a shelter.

Some species of Ophidiiformes live in the body cavity of starfish, mollusks or ascidians, and there are parasitic species among them.