Hymenocephalus lethonemus is a species of rattail.

It occurs at depths of up to 485 m (1591 ft) in the waters off southern Japan, the Philippines and northern Taiwan.

[1] This is a small, slender rattail with a total length of up to 14 cm (5.1 in).

It has a fairly long, sharp snout, small eyes, a large mouth and no chin barbel.

There is a long bioluminescent organ with two external lenses just in front of the anus.