The bluntsnout smooth-head, black slickhead, Cope's bluntsnout smooth-head, or Atlantic gymnast, Xenodermichthys copei, is a slickhead of the genus Xenodermichthys, found in the Atlantic, Indian, and Pacific oceans, and the Tasman Sea, at depths of 100 to 2,600 m. This species grows to a length of 31cm (12inches) TL.
Xenodermichthys copei is a dark-coloured fish that is scaleless, aside from a series of ~60 small, easily missed ring-shaped scales along the lateral line[1] . It possess 27-30 dorsal fin rays, 28-29 anal fin rays, 7 pectoral fin rays, 6 pelvic fin rays, and 55 caudal rays. It has 48-49 vertebrae.
The skin is studded with numerous nodular photophores (>500) and are more visible in preserved specimens where skin pigmentation has degraded over time. The skin of Xenodermichthys possess an unusual system of compartmented sub-dermal spaces filled with a dilute fluid (possibly lymph), thought to serve as a buoyancy aid, alongside scale loss and poor skeleton calcification[2] . This species lacks a swim bladder.
The photophores of Xenodermichthys have a distinctive reddish-violet hue in fresh specimens under white light, and are underlain with a greenish-blue reflector that lacks guanine. While the greater size of the ventral photophores may indicate their use in counterillumination, the arrangement and structure of the photophores, as well as difficulties in observing bioluminescence in live individuals, means that the purpose of these photophores is poorly understood.