Barreleyes, also known as spook fish (a name also applied to several species of

chimaera Chimaeras are Chondrichthyes, cartilaginous fish in the order (biology), order Chimaeriformes (), known informally as ghost sharks, rat fish (not to be confused with rattails), spookfish, or rabbit fish; the last two names are also applied to B ...

), are small

deep-sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combination of low tempe ...

argentiniform

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

comprising the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Opisthoproctidae found in tropical-to-temperate waters of the

Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...

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 the definition, to Antarctica in the south, and is bounded by the cont ...

, and

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

s. These fish are named because of their barrel-shaped, tubular

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

s, 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: 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 of all kno ...

words ''opisthe'' 'behind' and ''proktos'' '

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

', which may be a reference to the intestinal light organ of the

type genus In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearin ...

, ''

Opisthoproctus ''Opisthoproctus soleatus'' is a 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 Irel ...

''.

Description

Mitogenomic phylogeny of Argentiniformes and sole-bearing opisthoproctids

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 ''

'' and ''

Macropinna The 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 ...

'', 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'' August Bernhard Brauer, A. B. Brauer, 1901 * ''Dolichopteryx andriashevi'' Nikolai Vasilyevich Parin, Parin, ...

'' and ''

Bathylychnops ''Bathylychnops'' is a genus of barreleyes. Species There are currently three recognized species in this genus: * '' Bathylychnops brachyrhynchus'' ( A. E. Parr, 1937) * '' Bathylychnops chilensis'' Parin, Belyanina & Evseenko, 2009 * '' Bath ...

'', 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 m ...

'' 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 that 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 (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

with an exceptionally high complement of

rod cell Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in ...

s and a high density of

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...

(the "visual purple" pigment); no

cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...

s 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

nematocyst A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...

s (stinging cells) of the

siphonophores Siphonophorae (from Ancient Greek σίφων (siphōn), meaning "tube" and -φόρος (-phóros), meaning "bearing") is an Order (biology), order within Hydrozoa, a class of marine organisms within the phylum Cnidaria. According to the World Re ...

, 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 smelts (family Osmeridae) but have much smaller mouths. They are found in oceans throughout the world. They are small fishes, gro ...

), an epibranchial or crumenal organ is present behind the fourth

gill arch Branchial arches or gill arches are a series of paired bony/ cartilaginous "loops" behind the throat ( pharyngeal cavity) of fish, which support the fish gills. As chordates, all vertebrate embryos develop pharyngeal arches, though the event ...

. 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 (birds and other dinosaurs, crocodiles, alligators, pterosaurs), earthworms, some gast ...

—consists of a small

diverticulum In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false. In medicine, t ...

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 th ...

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 contrast ...

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 Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...

bioluminescent Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some fungi, microorganisms inc ...

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

, specifically, ''

Photobacterium phosphoreum ''Photobacterium phosphoreum'' is a Gram-negative, bioluminescent bacterium living in symbiosis with deep-sea marine organisms, such as anglerfish. It can emit bluish-green light (490 nm) due to a chemical reaction between FMN, luciferin an ...

'' (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. Mo ...

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

mirrorbelly ''Monacoa grimaldii'' is a species of fish in the family Opisthoproctidae, the barreleyed fish or spookfish. It has a body length of SL. Taxonomy The species was included in the genus ''Opisthoproctus'' from its description in 1911 until a 20 ...

(''Monacoa grimaldii'') and ''

Opisthoproctus soleatus ''Opisthoproctus soleatus'' is a species of fish referred to as the barreleye, being the type genus and Type species, species of the family (biology), family Opisthoproctidae. The species lives in most tropical seas, but is more common in the eas ...

'', 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 Culture ( ) is a concept that encompasses the social behavior, institutions, and norms found in human societies, as well as the knowledge, beliefs, arts, laws, customs, capabilities, attitudes, and habits of the individuals in these groups ...

in the lab. Through

restriction fragment length polymorphism In molecular biology, restriction fragment length polymorphism (RFLP) is a technique that exploits variations in homologous DNA sequences, known as polymorphisms, populations, or species or to pinpoint the locations of genes within a sequence. T ...

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 moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

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 fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...

s are inserted ventrolaterally rather than strictly ventrally. Several species also possess either a ventral or dorsal

adipose fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only b ...

, and the

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

is forked to emarginated. The

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

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 on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

originates slightly before or directly over the anal fin. A perceptible hump in the back begins just behind the head. The

gas bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift via swimming, ...

is absent in most species, and the

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelia ...

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'', the Javelin spookfish, is a species of barreleye found in the northern Pacific and in the eastern Atlantic Ocean near the Azores The Azores ( , , ; , ), officially the Autonomous Region of the Azores (), is one of the ...

(''Bathylychnops exilis'') is by far the largest species at

standard length Fish measurement is the measuring of individual fish and various parts of fish anatomy, their anatomies, for data used in many areas of ichthyology, including Taxonomy (biology), taxonomy and fishery biology. Overall length Standard length (SL) is ...

; most other species are under . File:Opisthoproctus soleatus.png, ''

'', the sole member of the

File:Opisthoproctus grimaldii.jpg, '' Monacoa grimaldii'', split out of ''Opisthoproctus'' in 2016 File:Winteria telescopa.png, The eyes of '' Winteria telescopa'' differ slightly from those of other opisthoproctids by their more forward-pointing gaze. File:Macropinna_microstoma_illustration.png, ''

Macropinna microstoma The 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 ...

'', showing the transparent membrane protecting the eyes File:Bathylychnops exilis.png, ''

Bathylychnops exilis ''Bathylychnops exilis'', the Javelin spookfish, is a species of barreleye found in the northern Pacific and in the eastern Atlantic Ocean near the Azores The Azores ( , , ; , ), officially the Autonomous Region of the Azores (), is one of the ...

Life cycle

Barreleyes inhabit moderate depths, from the

mesopelagic The mesopelagic zone (Greek language, 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 ...

to bathypelagic zone, ''circa'' 400–2,500 m deep. They are presumably solitary and do not undergo

diel vertical migration Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The adjective "diel" ( IPA: , ) comes from , and refers to a 24-hour ...

s; instead, barreleyes remain just below the limit of light penetration and use their sensitive, upward-pointing tubular eyes—adapted for enhanced

binocular vision Binocular vision is seeing with two eyes. The Field_of_view, field of view that can be surveyed with two eyes is greater than with one eye. To the extent that the visual fields of the two eyes overlap, #Depth, binocular depth can be perceived. Th ...

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 heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

, 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, ...

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s, and other pelagic

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s. The distribution of some species coincides with the

isohaline Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal to ...

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 Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

indicates they are pelagic spawners; that is,

eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

and

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...

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

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

ic; the

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e and juveniles drift with the currents—likely at much shallower depths than the adults—and upon

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

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 in modern humans is more signi ...

features, the result of

neoteny Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the Physiology, physiological, or Somatic (biology), somatic, development of an organism, typically an animal. Neoteny i ...

(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 Predation, prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, na ...

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 relate ...

(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 ''Stylephorus chordatus'', the tube-eye or thread-tail, is a deep-sea ray-finned fish, the only species in the genus ''Stylephorus'' and family Stylephoridae. It is found in deep subtropical and tropical oceans around the world, living at depths ...

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 Taxa named by Johannes Schmidt (biologist)