Monognathus, or Onejaws, is the only genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of the family Monognathidae of deep sea

Deep sea

The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...

 eel

Eel

Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

-like fishes. The name comes from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 monos meaning one and gnathos meaning jaw; a reference to the large mouth in comparison with the rest of the fish, and also the absence of an upper jaw (maxilla

Maxilla

The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

 and premaxilla

Premaxilla

The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

 bones are absent). The dorsal

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 and anal fins lack bony supports, the pectoral fins are missing. The snout has a fang which is connected to glands.

Typical lengths are from 4 to 10 centimeters (1.5-4 inches), the maximum length recorded is 15.9 centimeters (6.3 inches).

They are generally black in color, and are found at depths of over 2,000 meters (6,600 feet).

Species

There are 15 species:

• Monognathus ahlstromi Raju, 1974 (Paddletail onejaw)
• Monognathus berteli Nielsen & Hartel, 1996.
• Monognathus bertini Bertelsen & Nielsen, 1987.
• Monognathus boehlkei Bertelsen & Nielsen, 1987.
• Monognathus bruuni Bertin, 1936.
• Monognathus herringi Bertelsen & Nielsen, 1987.
• Monognathus isaacsi Raju, 1974.
• Monognathus jesperseni Bertin, 1936.
• Monognathus jesse Raju, 1974.
• Monognathus nigeli Bertelsen & Nielsen, 1987.
• Monognathus ozawai Bertelsen & Nielsen, 1987.
• Monognathus rajui Bertelsen & Nielsen, 1987.
• Monognathus rosenblatti Bertelsen & Nielsen, 1987.
• Monognathus smithi Bertelsen & Nielsen, 1987.
• Monognathus taningi Bertin, 1936.