A halosaur is a thin, elongated fish resembling an eel. The largest of halosaurs grows to about 20 in (51 cm) long. Unlike the eel, the halosaur has a backbone composed of many vertebrae. It has somewhat large scales, numbering fewer than 30 horizontal rows on each of its sides. This fish lives close to or on the bottom of the sea and is thus referred to as a benthic fish. It feeds on the ocean floor; like many bottom-feeding fish, its mouth is inferior, meaning that its jaw is positioned under its projecting snout. The halosaur's eyes, like those of the eel, are covered with transparent skin, called spectacles. It is believed that this membrane serves to protect the fish's eyes while it feeds on the bottom. This fish has a single dorsal fin composed of 9-13 soft rays, pelvic fins on its abdomen, and a long anal fin that extends to the tip of its tail. It has no caudal (tail) fin.
Scientists differ in their classification of the halosaur. Some scientists classify halosaurs in the order Albuliformes, the suborder Notacanthoidei, and the family Halosauridae. However, other scientists classify halosaurs in the order Notacanthiformes. All fish in this order have pectoral fins placed high on their sides, pelvic fins positioned on their abdomens, and anal fins that are long and tapering into their tails. All are deep water fish, inhabiting depths of between 656-17,062 ft (200-5,200 m). The order is distributed world wide and contains 20 species and six genera. According to this classification, the order Notacanthiformes has three families; the most notable of which are the Halosauridae (halosaurs) and the Notacanthidae (spiny eels).
Within the family Halosauridae, there are three genera with 15 species. The eight species in the genus Halosaurus live in the Atlantic, Indian, and Pacific Oceans, usually near the continental shelves. The six species in the genus Aldrovandia occur throughout the Atlantic and Indian Oceans as well as in the central and western Pacific Ocean. There is only one species in the third genus, referred to as Halosauropsis macrochir, and it lives in the western Pacific, Atlantic, and Indian Oceans.
Like its close relative, the spiny eel, the halosaur commonly moves slowly over the ooze covering the deep-sea floor in search of food. Because it has a long tapering tail which ends without a fin, it has modified its mode of locomotion. Like other deep-sea fish with its body type, it is believed that the halosaur moves by rolling its long anal fin or by using quick strokes of its pectoral fins. Furthermore, it may accomplish locomotion by undulating its long body.
This fish has been caught swimming at up to 5,200 ft (1,585 m) below the ocean's surface. Because it lives at such extreme depths, it is rarely seen, and little is known about its habits.