Bailey, Reeve M. Division of Fishes, Museum of Zoology, University of Michigan, Ann Arbor, Michigan.
Boschung, Herbert T. Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama.
Last reviewed:August 2020
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A suborder of deep-sea, actinopterygian (ray-finned) fishes belonging to the order Albuliformes and consisting of two families, Halosauridae (halosaurs) and Notacanthidae (spiny eels). The two families Halosauridae and Notacanthidae within the suborder Notacanthoidei are also known as Lyopomi and Heteromi, respectively. Some taxonomists do not consider the two families as part of the Albuliformes, instead assigning them to a provisional order known as Notacanthiformes. Regardless of the exact classification scheme for members of the Notacanthoidei, the body of these fishes is elongate and tapers posteriorly. The caudal fin is absent or essentially absent, pectoral fins are high on the body, pelvic fins are abdominal, and the anal fin is long (see illustration). There is no duct to the swim bladder (physoclistous); the orbitosphenoid, pterosphenoid, intercalary, and basisphenoid bones are absent; the posttemporal is simple or ligamentous; the transverse processes are not suturally joined to vertebral centra; and there is no mesocoracoid arch. Some species have photophores (light-emitting organs), which is characteristic of many deep-sea fishes. In addition, the halosaurs differ from the spiny eels in that they lack spines in the dorsal fin and have gill membranes that are completely separate rather than joined or partly joined. See also: Albuliformes; Deep-sea fauna; Photophore gland
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