Barreleyes, also known as spook fish (a name also applied to several species of chimaera), are small deep-sea osmeriform fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...

 comprising the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Opisthoproctidae. Found in tropical-to-temperate waters of the Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

, Pacific
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

, and Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

s, the family contains thirteen species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 in six genera
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 (four of which are monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...


These fish are named for their barrel-shaped, tubular eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

s which are generally directed upwards to detect the silhouettes of available prey; however, according to Robison and Reisenbichler these fish are capable of directing their eyes forward as well. The family name Opisthoproctidae is derived from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 words opisthe ("behind") and proktos ("anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...


Physical description

The morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 of the Opisthoproctidae varies between three main forms: the stout, deep-bodied barreleyes of the genera Opisthoproctus and Macropinna; the extremely slender and elongate spookfishes of the genera Dolichopteryx and Bathylychnops; and the intermediate fusiform spookfishes of the genera Rhynchohyalus and Winteria
Winteria may refer to:* Winteria, a synonym of the genus Cleistocactus of cactuses.* Winteria, a genus of barreleye fishes containing only the species Winteria telescopa....


All species have large, telescoping
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 eyes which dominate and protrude from the skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

, 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
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 and a retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 with an exceptionally high complement of rod cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

s and a high density of rhodopsin
Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...

 (the "visual purple" pigment); there are no cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

s. 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 (stinging cells) of the siphonophores from which it is believed the Barreleye steals food. It may also serve as an accessory lens (modulated by intrinsic or peripheral muscles), or refracts light with an index very close to seawater. A recent study disclosed that Dolichopteryx longipes
Dolichopteryx longipes
The brownsnout spookfish, Dolichopteryx longipes, is a species of barreleye in the family Opisthoproctidae. It is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes...

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), there is an epibranchial or crumenal organ present behind the fourth gill arch. 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 birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

—consists of a small diverticulum
A diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....

 (pouch) wherein the gill raker
Gill raker
Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch and are involved with filter feeding tiny prey. They are not to be confused with the gill filaments that compose the bony part of the gill. Rakers are usually present in two rows, projecting from both 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 scale
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

s; but these are absent in Dolichopteryx, leaving the body itself a transparent white. In all species a variable number of dark melanophores colour the muzzle, ventral surface, and midline.
Also present in Dolichopteryx, Opisthoproctus, and Winteria species are a number of luminous organs; in Dolichopteryx there are several along the length of the belly, and in Opisthoproctus there is a single organ in the form of a rectal pouch. These organs glow with a weak light due to the presence of symbiotic
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...

 bacteria; specifically, Photobacterium phosphoreum
Photobacterium phosphoreum
Photobacterium phosphoreum or Vibrio phosphoreum is a Gram-negative bioluminescent bacterium living in symbiosis with marine organisms. It can emit bluish-green light thanks to a chemical reaction between FMN, luciferin and molecular oxygen catalysed by an enzyme called Luciferase.- In Popular...

(family Vibrionaceae
The Vibrionaceae are a family of Proteobacteria, given their own order. 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 surface of Opisthoproctus species is characterised by a flattened and projecting sole; in the mirrorbelly
The mirrorbelly or barreleye is a barreleye of the genus Opisthoproctus, is found in most deep tropical and subtropical oceans and seas, from depths of 200 to 2,000 m...

 (Opisthoproctus grimaldii) and Opisthoproctus soleatus 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
Microbiological culture
A microbiological culture, or microbial culture, is a method of multiplying microbial organisms by letting them reproduce in predetermined culture media under controlled laboratory conditions. Microbial cultures are used to determine the type of organism, its abundance in the sample being tested,...

 in the lab. Through restriction fragment length polymorphism
Restriction fragment length polymorphism
In molecular biology, restriction fragment length polymorphism, or RFLP , is a technique that exploits variations in homologous DNA sequences. It refers to a difference between samples of homologous DNA molecules that come from differing locations of restriction enzyme sites, and to a related...

 analysis, the two strains have been shown to differ only slightly.

In all species the fins are spineless
Spine (zoology)
A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

 and fairly small; in Dolichopteryx however, the pectoral fins are greatly elongated and wing-like, extending about half the body's length, and are apparently used for stationkeeping in the water column. In all species the pectoral fins are inserted low on the body, and in some the pelvic fins are inserted ventrolaterally rather than strictly ventrally. Several species also possess either a ventral or dorsal adipose fin, and the caudal fin is forked to emarginate. The anal fin is either present or greatly reduced, and may not be externally visible; it is strongly retrorse in Opisthoproctus. There is a single dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 originating slightly before or directly over the anal fin. There is a perceptible hump in the back, beginning just behind the head. The gas bladder
Gas bladder
The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...

 is absent in most species, and the lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

 is uninterrupted. The branchiostegal rays number 2–4. The javelin spookfish (Bathylychnops exilis) is by far the largest species at 50 centimetres standard length (SL; a measurement excluding the caudal fin); most other species are under 20 centimetres SL.

Life cycle

Barreleyes inhabit moderate depths, from the mesopelagic
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...

 to bathypelagic zone, ca. 400–2,500 metres down. They are presumably solitary and do not undergo diel vertical migration
Diel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement that some organisms living in the ocean and in lakes undertake each day. Usually organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone...

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
Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...

 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 heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

 such as hydroid
-Marine biology:Hydroids are a life stage for most animals of class Hydrozoa, small predators related to jellyfish.-Botany:In mosses, hydroids form the innermost layer of the stem of long, colourless, thin walled cells of small diameter.The cells are dead and lack protoplasm.They function as water...

s, copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s, and other pelagic crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

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-metre isotherm for 8°C.

What little is known of barreleye reproduction indicates they are pelagic spawner
Spawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...

s; that is, eggs
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 and sperm
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

 are released en masse directly into the water. The fertilized eggs are buoyant and plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic; the larva
A larva 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 features, the result of neoteny
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...

 (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 a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

 in the form of counterillumination. This predator avoidance 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 hatchetfish
Marine hatchetfishes or deep-sea hatchetfishes are small deep-sea mesopelagic ray-finned fish of the stomiiform subfamily Sternoptychinae...

 (Sternoptychidae). Also found in marine hatchetfish and other unrelated families are tubular eyes; cf. telescopefish
Telescopefish are small, deep-sea aulopiform fish comprising the small family Giganturidae. There are just two known species, both within the genus Gigantura. Though rarely captured, they are found in cold, deep tropical to subtropical waters worldwide.The common name of these fish are...

, tube-eye
The tube-eye or thread-tail, Stylephorus chordatus, is a deep-sea Stylephoriformes fish, the only fish in the genus Stylephorus and family Stylephoridae....



There are fourteen species in six genera:
  • Genus Bathylychnops
    • Javelin spookfish, Bathylychnops exilis
      Bathylychnops exilis
      Bathylychnops exilis is a translucent, mesopelagic fish and the only known species in the genus Bathylychnops...

      Cohen, 1958.
  • Genus Dolichopteryx
    • Dolichopteryx anascopa Brauer, 1901.
    • Dolichopteryx binocularis Beebe, 1932.
    • Dolichopteryx brachyrhynchus Parr, 1937.
    • Brownsnout spookfish, Dolichopteryx longipes
      Dolichopteryx longipes
      The brownsnout spookfish, Dolichopteryx longipes, is a species of barreleye in the family Opisthoproctidae. It is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes...

      Léon Vaillant
      Léon Louis Vaillant was a French zoologist. He is most famous for his work in the areas of herpetology, malacology, and ichthyology...

      , 1888)
    • Dolichopteryx minuscula Fukui & Kitagawa, 2006.
    • Winged spookfish, Dolichopteryx parini Kobyliansky & Fedorov, 2001.
    • Dolichopteryx pseudolongipes Fukui, Kitagawa & Parin, 2008.
    • Dolichopteryx rostrata Fukui & Kitagawa, 2006.
  • Genus Macropinna
    • Macropinna microstoma
      Macropinna microstoma
      Macropinna microstoma is the only species of fish in the genus Macropinna, belonging to Opisthoproctidae, the barreleye family. It is recognized for a highly unusual transparent, fluid-filled dome on its head, through which the lenses of its eyes can be seen...

      Chapman, 1939.
  • Genus Opisthoproctus
    • Mirrorbelly
      The mirrorbelly or barreleye is a barreleye of the genus Opisthoproctus, is found in most deep tropical and subtropical oceans and seas, from depths of 200 to 2,000 m...

       or Barreleye, Opisthoproctus grimaldii Zugmayer
      Erich Zugmayer
      Erich Johann Georg Zugmayer was an Austrian zoologist and ichthyologist-Species described by Zugmayer:* Benthalbella infans Zugmayer, 1911* Cetostoma regani Zugmayer, 1914...

      , 1911
    • Opisthoproctus soleatus Vaillant
      Léon Vaillant
      Léon Louis Vaillant was a French zoologist. He is most famous for his work in the areas of herpetology, malacology, and ichthyology...

      , 1888
  • Genus Rhynchohyalus
    • Rhynchohyalus natalensis (Gilchrist & von Bonde, 1924).
  • Genus Winteria
    • Winteria telescopa Brauer, 1901.

External links

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