The muddy arrowtooth eel, Ilyophis brunneus, is a cutthroat eel, the only species in the genus Ilyophis. It is found around the world at depths below 1,000 m. Its length is up to 160 cm. The muddy arrowtooth eel is an elongate eel, with the dorsal fin beginning above the small pectoral fin, and the anal fin's origin about a third of the way back along the body. The mouth is relatively small and has small teeth except for a few large teeth in a single row on the roof of the mouth. The gill apertures are almost horizontal and are low down under the throat. Scale arrangement is the same as the basketwork eel.
The colour is brown with darker patches on the gill region and darker fins.
Very little else is known.