Atrophacanthus japonicus

Atrophacanthus has the first three spines in the dorsalfin are well developed and clearly exposed, the fourth spine is clearly shorter than the first three but is still visible above the skin but the fifth and sixth spines very poorly developed and either just protrude on the surface of the skin or so not protrude at all. The first spine is rather short, not extending far beyond the fourth spine when folded down. There are 13 or 14 soft rays in the dorsal fin and between 11 and 13 in the anal fin. The short conical teeth are arranged in a single row on each jaw with between 19 and 23 teeth in upper jaw and 22–27 in the lower jaw. the dorsal profile of the head is neither convex or concave. The mouth is at the end of the short snout. The gill opening is relatively long, typically reaching down to around one-half to four-fifths of the base of the pectoral fin. This species has a maximum published standard length of 4.4 cm (1.7 in).

Atrophacanthus is found in the Indo-West Pacific region where it has been recorded from Japan, the Philippines, Sulawesi Sea, off Tanzania, and probably from South Africa, with records as far east as the Mariana Islands. This is a bathypelagic zone and benthic fish found at depths between 300 and 2,000 m (980 and 6,560 ft).

Atrophacanthus combines atrophia, meaning "rudimentary", and acanthus, which means "thorn" or "spine", an allusion to the poorly developed rearmost 3 spines in the dorsal fin. The specific name Japonicus refers to the type locality of Tydemania japonica.