This order of fish includes 27 species divided into two families: Halosauridae and Notacanthidae. These fish live in deep offshore waters around the world.

Fish in this group do not have an adipose fin—a small, fleshy fin found on some fish between the dorsal (back) fin and the caudal (tail) fin.

Members of the Halosauridae, or halosaur family, have long, eel-like bodies and are typically found in very deep ocean waters.

The Notacanthidae, also known as deep-sea spiny eels, have long, slightly tapered bodies. They live in offshore waters worldwide at depths ranging from about 400 to 11,000 feet (122 to 3,353 meters).