Opisthoproctus

Opisthoproctus soleatus is an argentiniform species of fish referred to as the barreleye, being the type genus and species of the family Opisthoproctidae. The species lives in most tropical seas, but is more common in the eastern Atlantic, from western Ireland to Mauritania and from Sierra Leone to Angola, and also in the South China Sea. O. soleatus can grow to a standard length of and usually live from about deep.

==Description== This species is a small fish, not exceeding in length. The body of Opisthoproctus soleatus is deep and laterally compressed. Scales are large, thin, and cycloid. The ventral side of the body was described by Vaillant as a "flattened, oval, elongate sole." The sole extends forwards below the head. It is covered in large thin scales that increase in pigmentation towards the distal parts. The back and sides of this fish are dark and the snout translucent, and there are several large melanophores behind and below the head. The caudal fin is large and forked, with 33 rays total. The dorsal fin is small and begins behind the middle of the body, and has 11 soft rays. The anal fin is inserted on the posterior of the body and has 6 rays. The pectoral fin has 13–15 rays, some of which are elongated and extend beyond the point of origin of the dorsal fin. The pelvic fin has 9–10 rays, and the adipose fin is present. thumb|right|300px|Diagram of the sole and light organ of the sister genus, [[Monacoa]] Opisthoproctus soleatus has a specialized adaptation of the intestine by the anus, termed the rectal bulb, that contains bioluminescent bacteria and produces light. A second specialized organ located in the ventral sole acts as a reflector for the rectal light-organ. Light generated by bioluminescent bacteria in the rectal bulb is projected into the tissue of the reflector organ, which has a reflective ventral wall that reflects the light downward. The reflector can be contracted or expanded, controlling the amount of light allowed to pass through the thin part of the scales and into the environment. Their genus name Opisthoproctus is presumably derived from Ancient Greek ὄπισθεν (ópisthen), meaning "back", and πρωκτός (prōktós), meaning "anus", as a reference to this intestinal light organ.