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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 10.5 centimetres (4.1 in) and usually live from about 500 to 700 metres (1,600 to 2,300 ft) deep.
This species is a small fish, not exceeding 10.5 cm (4.1 in) 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.
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.[citation needed]
The head is compressed,[clarification needed] with the snout being about 3⁄8 the length of the head. The mouth is small and extends to half the length of the snout. The upper part of the snout and the area of the head above the brain are semi-transparent. Small teeth are present in the mouth on the lower jaw and the head of the vomer.
The eyes are particularly distinctive, being tubular in shape and directed dorsally (upwards); they have a sideways-oriented diverticulum in the front wall of the eye, though it is unclear what the precise function of this pouch-like pocket is. The eye is large, equal in length to the snout, with the suborbital space covering the entire cheek and an extremely large lens. The suborbital bone is extended and covers the eye laterally.
Opisthoproctus soleatus is found in all the world's tropical and temperate oceans. It has been recorded in the Atlantic, Pacific, and Indian Oceans.[verification needed] 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 300 to 800 m (1,000 to 2,600 ft) but it commonly frequents the 500 to 700 m (1,600 to 2,300 ft) range, often limited by the 8 °C (46 °F) isotherm which often occurs at about 400 m (1,300 ft). 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.
Like other deepwater fish, Opisthoproctus soleatus needs to find its prey in a very dark environment, and avoid being detected itself by larger predators. Fishes with large upward-facing eyes likely hunt by detecting the silhouettes of prey above them which contrast with the low amounts of light coming in. At the depths at which this fish lives, light is still directional, and many species of fish and invertebrates have photophores (light organs) on their underside which camouflage them against the incoming sunlight, which at these depths can still be measured (hence the term "twilight zone"). O. soleatus itself does not have typical photophores, instead having the luminous organ in its intestines. The light produced is shined onto a reflector and projected downward between the ventral scales of the sole, creating a counter-illumination effect similar to that of the ventral photophores of other species. While the exact purpose of this is unknown, it has been proposed that O. soleatus uses this apparatus as a method of camouflage or to communicate with members of their species.
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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 10.5 centimetres (4.1 in) and usually live from about 500 to 700 metres (1,600 to 2,300 ft) deep.
This species is a small fish, not exceeding 10.5 cm (4.1 in) 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.
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.[citation needed]
The head is compressed,[clarification needed] with the snout being about 3⁄8 the length of the head. The mouth is small and extends to half the length of the snout. The upper part of the snout and the area of the head above the brain are semi-transparent. Small teeth are present in the mouth on the lower jaw and the head of the vomer.
The eyes are particularly distinctive, being tubular in shape and directed dorsally (upwards); they have a sideways-oriented diverticulum in the front wall of the eye, though it is unclear what the precise function of this pouch-like pocket is. The eye is large, equal in length to the snout, with the suborbital space covering the entire cheek and an extremely large lens. The suborbital bone is extended and covers the eye laterally.
Opisthoproctus soleatus is found in all the world's tropical and temperate oceans. It has been recorded in the Atlantic, Pacific, and Indian Oceans.[verification needed] 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 300 to 800 m (1,000 to 2,600 ft) but it commonly frequents the 500 to 700 m (1,600 to 2,300 ft) range, often limited by the 8 °C (46 °F) isotherm which often occurs at about 400 m (1,300 ft). 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.
Like other deepwater fish, Opisthoproctus soleatus needs to find its prey in a very dark environment, and avoid being detected itself by larger predators. Fishes with large upward-facing eyes likely hunt by detecting the silhouettes of prey above them which contrast with the low amounts of light coming in. At the depths at which this fish lives, light is still directional, and many species of fish and invertebrates have photophores (light organs) on their underside which camouflage them against the incoming sunlight, which at these depths can still be measured (hence the term "twilight zone"). O. soleatus itself does not have typical photophores, instead having the luminous organ in its intestines. The light produced is shined onto a reflector and projected downward between the ventral scales of the sole, creating a counter-illumination effect similar to that of the ventral photophores of other species. While the exact purpose of this is unknown, it has been proposed that O. soleatus uses this apparatus as a method of camouflage or to communicate with members of their species.