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. "O. soleatus" is found in all the world's tropical and temperate oceans. It has been recorded in the Atlantic, Pacific, and Indian Oceans. In the eastern Atlantic, it is most frequently encountered between western Ireland and Mauritania, and between Sierra Leone and Angola. Its range in the Atlantic Ocean extends from 20° N to 10-12° S, . Its depth range is but it commonly frequents the range, often limited by the isotherm which often occurs at about . This range falls within the mesopelagic region, which receives dim light from above. This region contains resident fauna, as well as transitional fauna that migrate vertically in response to changes in light. "Opisthoproctus soleatus" has been found to primarily prey on bioluminescent siphonophores. The bioluminescence of these organisms makes them more difficult to detect against the dim incoming light, however, the structure of the lens and retina of "O. soleatus" increases the optics and resolution of the eye, allowing them to detect these cases of camouflage. "Opisthoproctus soleatus" have been found in the stomachs of beached Soweby’s beaked whales ("Mesoplodon bidens"). Other species of Opisthoproctidae have also been found in the stomachs of Pygmy Sperm Whales and some seabirds. It has been proposed that the direction of emitted light from the rectal bulb and reflecting organ in the downwards direction decreases the chances of that light being detected by predators with forward facing eyes. Like other deepwater fish, "Opisthoproctus soleatus" needs to find its prey in a very poorly-lit environment and avoid being detected itself by a larger predatory species. Fishes with large upward-facing eyes likely hunt by detecting the silhouettes of prey above them in contrast to the low amounts of light coming in. At the depths at which this fish lives, light is still directional, and many fish species have photophores (luminous organs) on their underside which provide them with camouflage by replicating the scintillations on the surface of the water above.