Wikipedia
Monognathus
Monognathus, or onejaw, is the only genus of the family Monognathidae of deep-sea, eel-like fishes. The name comes from the Greek 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 and premaxilla bones are absent). The dorsal 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 cm (1.5–4 in), the maximum length recorded is 15.9 cm (6.3 in).
They are generally black in color, and are found at depths of over 2,000 m (6,600 ft).