Barreleyes, also known as spook fish (a name also applied to several species of chimaera), are small

deep-sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

comprising the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Opisthoproctidae found in tropical-to-temperate waters of the

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

, and

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...

s. These fish are named because of their barrel-shaped, tubular eyes, which are generally directed upwards to detect the silhouettes of available prey; however, the fish are capable of directing their eyes forward, as well. The family name Opisthoproctidae is derived from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

words ''opisthe'' 'behind' and ''proktos'' ' anus'.

Description

The

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

of the Opisthoproctidae varies between three main forms: the stout, deep-bodied barreleyes of the genera ''

Opisthoproctus ''Opisthoproctus soleatus'' is a species of fish in the family (biology), family Opisthoproctidae. It was first described in 1888 by Léon Vaillant. The species lives in most tropical seas, but is more common in the eastern Atlantic, from wester ...

'' and ''

Macropinna Pacific Barreleye Fish (''Macropinna'') is a genus of ray-finned fish belonging to Opisthoproctidae, the barreleye family. It contains one species, ''M. microstoma''. It is recognized for a highly unusual transparent, fluid-filled shield on its ...

'', the extremely slender and elongated spookfishes of the genera ''

Dolichopteryx ''Dolichopteryx'' is a genus of barreleyes. Species There are currently 9 recognized species in this genus: * '' Dolichopteryx anascopa'' A. B. Brauer, 1901 * '' Dolichopteryx andriashevi'' Parin, Belyanina & Evseenko, 2009 * ''Dolichopteryx ...

'' and ''

Bathylychnops ''Bathylychnops'' is a genus of barreleye Barreleyes, also known as spook fish (a name also applied to several species of chimaera), are small deep-sea argentiniform fish comprising the family Opisthoproctidae found in tropical-to-temper ...

'', and the intermediate fusiform spookfishes of the genera ''

Rhynchohyalus ''Rhynchohyalus natalensis'', the glasshead barreleye, is a species of barreleye found in oceans around the world at depths from . This species grows to a length of SL. It and the brownsnout spookfish are the only vertebrates known to employ mi ...

'' and '' Winteria''. All species have large, telescoping eyes, which dominate and protrude from the head, but are enclosed within a large transparent dome of soft tissue. These eyes generally gaze upwards, but can also be directed forwards. The opisthoproctid eye has a large

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

and a

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

with an exceptionally high complement of rod cells and a high density of rhodopsin (the "visual purple" pigment); no cone cells are present. To better serve their vision, barreleyes have large, dome-shaped, transparent heads; this presumably allows the eyes to collect even more incident light and likely protects the sensitive eyes from the nematocysts (stinging cells) of the

siphonophores Siphonophorae (from Greek ''siphōn'' 'tube' + ''pherein'' 'to bear') is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 specie ...

, from which the barreleye is believed to steal food. It may also serve as an accessory lens (modulated by intrinsic or peripheral muscles), or refract light with an index very close to seawater. ''

Dolichopteryx longipes The brownsnout spookfish or brown-snout spookfish (''Dolichopteryx longipes'') is a species of barreleye in the family Opisthoproctidae. It and the glasshead barreleye fish are the only vertebrates known to employ a mirror, in addition to a lens ...

'' is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images. The toothless mouth is small and terminal, ending in a pointed snout. As in related families (e.g.

Argentinidae The herring smelts or argentines are a family, Argentinidae, of marine smelts. They are similar in appearance to smelt (fish), smelts (family Smelt (fish), Osmeridae) but have much smaller mouths. They are found in oceans throughout the world. ...

), an epibranchial or

crumenal organ The Argentiniformes are an order (biology), order of ray-finned fish whose distinctness was recognized only fairly recently. In former times, they were included in the Osmeriformes (typical Smelt (fish), smelt and allies) as suborder Argentinoid ...

is present behind the fourth

gill arch Branchial arches, or gill arches, are a series of bony "loops" present in fish, which support the gills. As gills are the primitive condition of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arc ...

. This organ—analogous to the

gizzard The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including archosaurs (pterosaurs, crocodiles, alligators, dinosaurs, birds), earthworms, some gastropods, so ...

—consists of a small diverticulum wherein the

gill raker Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the ...

s insert and interdigitate for the purpose of grinding up ingested material. The living body of most species is a dark brown, covered in large, silvery imbricate scales, but these are absent in ''Dolichopteryx'', leaving the body itself a transparent white. In all species, a variable number of dark

melanophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in cont ...

s colour the muzzle, ventral surface, and midline. Also present in ''Dolichopteryx'', ''Opisthoproctus'', and ''Winteria'' species are a number of luminous organs; ''Dolichopteryx'' has several along the length of its belly, and ''Opisthoproctus'' has a single organ in the form of a rectal pouch. These organs glow with a weak light due to the presence of symbiotic bioluminescent

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

, specifically, '' Photobacterium phosphoreum'' (family

Vibrionaceae The Vibrionaceae are a family of Pseudomonadota given their own order, Vibrionales. Inhabitants of fresh or salt water, several species are pathogenic, including the type species ''Vibrio cholerae'', which is the agent responsible for cholera. ...

). The ventral surfaces of ''Opisthoproctus'' species are characterised by a flattened and projecting 'sole'; in the

mirrorbelly ''Monacoa grimaldii'', also known as the mirrorbelly, barreleye, Grimaldi's barreleye, and flatiron fish, is a species of fish in the family Opisthoproctidae. Different sources express different views on its distribution. According to Poulsen an ...

(''Opisthoproctus grimaldii'') and ''

Opisthoproctus soleatus ''Opisthoproctus soleatus'' is a species of fish in the family Opisthoproctidae. It was first described in 1888 by Léon Vaillant. The species lives in most tropical seas, but is more common in the eastern Atlantic, from western Ireland to Maur ...

'', this sole may act as a reflector, by directing the emitted light downwards. The strains of ''P. phosphoreum'' present in the two ''Opisthoproctus'' species have been isolated and cultured in the lab. Through restriction fragment length polymorphism analysis, the two strains have been shown to differ only slightly. In all species, the fins are spineless and fairly small; in ''Dolichopteryx'' however, the

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

s are greatly elongated and wing-like, extending about half the body's length, and are apparently used for stationkeeping in the water column. The pectoral fins are inserted low on the body, and in some species, the pelvic fins are inserted ventrolaterally rather than strictly ventrally. Several species also possess either a ventral or dorsal

adipose fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

, and the

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

is forked to emarginated. The anal fin is either present or greatly reduced, and may not be externally visible; it is strongly retrorse in ''Opisthoproctus''. A single

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

originates slightly before or directly over the anal fin. A perceptible hump in the back begins just behind the head. The gas bladder is absent in most species, and the lateral line is uninterrupted. The

branchiostegal ray This glossary of ichthyology is a list of definitions of terms and concepts used in ichthyology, the study of fishes. A B C ...

s (bony rays supporting the gill membranes behind the lower jaw) number two to four. The javelin spookfish (''Bathylychnops exilis'') is by far the largest species at standard length; most other species are under . Macropinna Microstoma

Lifecycle

Barreleyes inhabit moderate depths, from the

mesopelagic The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at ...

to bathypelagic zone, ''circa'' 400–2,500 m deep. They are presumably solitary and do not undergo diel vertical migrations; instead, barreleyes remain just below the limit of light penetration and use their sensitive, upward-pointing tubular eyes—adapted for enhanced

binocular vision In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...

at the expense of lateral vision—to survey the waters above. The high number of rods in their eyes' retinae allows barreleyes to resolve the silhouettes of objects overhead in the faintest of ambient light (and to accurately distinguish bioluminescent light from ambient light), and their binocular vision allows the fish to accurately track and home in on small

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

such as

hydroids Hydroids are a life stage for most animals of the class Hydrozoa, small predators related to jellyfish. Some hydroids such as the freshwater '' Hydra'' are solitary, with the polyp attached directly to the substrate. When these produce buds ...

, copepods, and other pelagic

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s. The distribution of some species coincides with the isohaline and isotherm layers of the ocean; for example, in ''Opisthoproctus soleatus'', upper distribution limits coincide with the 400-m isotherm for . What little is known of barreleye reproduction indicates they are pelagic spawners; that is,

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

and sperm are released ''en masse'' directly into the water. The fertilized eggs are buoyant and

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

ic; the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e and juveniles drift with the currents—likely at much shallower depths than the adults—and upon metamorphosis into adult form, they descend to deeper waters. ''Dolichopteryx'' species are noted for their

paedomorphic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...

features, the result of

neoteny Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compa ...

(the retention of larval characteristics). The bioluminescent organs of ''Dolichopteryx'' and ''Opisthoproctus'', together with the reflective soles of the latter, may serve as

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

in the form of '' counterillumination''. This

predator avoidance Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avo ...

strategy involves the use of ventral light to break up the fishes' silhouettes, so that (when viewed from below) they blend in with the ambient light from above. Counterillumination is also seen in several other unrelated deep-sea families, which include the

marine hatchetfish Marine hatchetfishes or deep-sea hatchetfishes are small deep-sea mesopelagic ray-finned fish of the stomiiform subfamily Sternoptychinae. They should not be confused with the freshwater hatchetfishes, which are not particularly closely related T ...

(Sternoptychidae). Also found in marine hatchetfish and other unrelated families are tubular eyes, such as

telescopefish Telescopefish are small, deep-sea aulopiform fish comprising the small family Giganturidae. The two known species are within the genus ''Gigantura''. Though rarely captured, they are found in cold, deep tropical to subtropical waters worldwide ...

and tube-eye.

References

External links

*MBARI press release
Researchers solve mystery of deep-sea fish with tubular eyes and transparent head
23 February 2009. Includes photographs and video of ''Macropinna microstoma''.
Guardian newspaper article on the barreleye, with photo of live specimen
{{Taxonbar, from=Q720515 Opisthoproctidae