Barreleye

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Barreleyes
Opisthoproctus soleatus.png
Opisthoproctus soleatus . This image is drawn from a specimen taken to the surface; in a live specimen, the membrane over the top of the head forms a transparent dome.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Argentiniformes
Family: Opisthoproctidae
Schmidt, 1918
Genera
The eyes of Winteria telescopa differ slightly from those of other opisthoproctids by their more forward-pointing gaze. Winteria telescopa.png
The eyes of Winteria telescopa differ slightly from those of other opisthoproctids by their more forward-pointing gaze.

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-temperate waters of the Atlantic, Pacific, and Indian Oceans. [2] [3] [4] [5]

Contents

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

Description

The morphology of the Opisthoproctidae varies between three main forms: the stout, deep-bodied barreleyes of the genera Opisthoproctus and Macropinna , the extremely slender and elongated spookfishes of the genera Dolichopteryx and Bathylychnops , and the intermediate fusiform spookfishes of the genera Rhynchohyalus 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. [6] These eyes generally gaze upwards, but can also be directed forwards. [7] The opisthoproctid eye has a large lens and a retina 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, from which the barreleye is believed to steal food. [8] 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 is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images. [9]

The toothless mouth is small and terminal, ending in a pointed snout. As in related families (e.g. Argentinidae), an epibranchial or crumenal organ is present behind the fourth gill arch. This organ—analogous to the gizzard—consists of a small diverticulum wherein the gill rakers 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 melanophores 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, specifically, Photobacterium phosphoreum (family Vibrionaceae). The ventral surfaces of Opisthoproctus species are characterised by a flattened and projecting 'sole'; in the mirrorbelly (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 in the lab. Through restriction fragment length polymorphism analysis, the two strains have been shown to differ only slightly. [10] [11]

In all species, the fins are spineless 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. 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, and the caudal fin 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 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 rays (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 50 centimetres (20 in) standard length; most other species are under 20 centimetres (7.9 in).

Macropinna microstoma, showing the transparent membrane protecting the eyes Macropinna microstoma illustration.png
Macropinna microstoma , showing the transparent membrane protecting the eyes

Life cycle

Opisthoproctus grimaldii Opisthoproctus grimaldii.jpg

Barreleyes inhabit moderate depths, from the mesopelagic 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 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 such as hydroids, copepods, and other pelagic crustaceans. 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 8 °C (46 °F).

What little is known of barreleye reproduction indicates they are pelagic spawners; that is, eggs and sperm are released en masse directly into the water. The fertilized eggs are buoyant and planktonic; the larvae 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 (the retention of larval characteristics).

The bioluminescent organs of Dolichopteryx and Opisthoproctus, together with the reflective soles of the latter, may serve as camouflage 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 (Sternoptychidae). Also found in marine hatchetfish and other unrelated families are tubular eyes, such as telescopefish and tube-eye.

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<span class="mw-page-title-main">Marine hatchetfish</span> Subfamily of fishes

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<i>Monacoa grimaldii</i> Species of fish

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 and colleagues, it is known with certainty only from the Atlantic Ocean, with records from the Pacific Ocean representing other Monacoa species. However, FishBase and Catalog of Fishes include the Pacific, and in the case of the latter, the Indian Ocean in its range. It is typically mesopelagic, but it has been recorded from depths of 0 to 4,750 metres. It has a body length of 8 cm (3.1 in) SL.

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<span class="mw-page-title-main">Brownsnout spookfish</span> Species of barreleye in the family Opisthoproctidae

The brownsnout spookfish or brown-snout spookfish 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, to focus an image in its eyes. This species probably has a worldwide tropical and temperate distribution; in the Atlantic Ocean it is known from Bermuda, the Bahamas, the Greater Antilles, and the Gulf of Mexico, and in the Pacific Ocean it is known from the California Current region and the South China Sea. It is found in the mesopelagic and bathypelagic zones at a depth of 500–2,400 meters (1,600–7,900 ft), but usually occurs below 1,000 meters. In the Gulf of Mexico it is found shallower, at 310–460 meters (1,020–1,510 ft).

<i>Macropinna</i> Genus of ray-finned fishes

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 its head, through which the lenses of its eyes can be seen. It was originally believed that the tubular eyes of this fish were fixed in place and, therefore, only provided a tunnel vision view of what was seen above its head. However, in 2008, scientists discovered that its eyes were able to rotate both up and forward in its transparent shield. M. microstoma has a tiny mouth and most of its body is covered in large scales. The fish normally hangs nearly motionless in the water, at a depth of about 600 metres (2,000 ft) to 800 metres (2,600 ft), using its large fins for stability and with its eyes directed upward. In the low light conditions it is assumed the fish detects prey by its silhouette. This is because the eyes have no lens in the diverticulum, so images would most likely occur through reflection. Due to the eyes facing upwards, light is able to enter the eyes and through the illumination, silhouettes are formed. MBARI researchers Bruce Robison and Kim Reisenbichler observed that when prey such as small fish and jellyfish are spotted, the eyes rotate like binoculars, facing forward as it turns its body from a horizontal to a vertical position to feed. Robison speculates that M. microstoma steals food from siphonophores.

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<i>Opisthoproctus</i> Species of fish

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 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 deep.

<i>Argyropelecus affinis</i> Species of fish

Argyropelecus affinis is a species of ray-finned fish in the family Sternoptychidae, described by Garman in 1899, found in the tropical and subtropical Atlantic, Indian and Pacific Oceans. Common names for this fish include Pacific hatchetfish, deepsea hatchetfish and slender hatchetfish. It inhabits the upper mesopelagic zone between 350 and 600m during the day, and between 100m and 350m during the night and is either non-migratory or performs short daily vertical migrations. They are distributed widely throughout all tropical and temperate seas. They are known for its laterally compressed body and characteristic photophores, which aid in bioluminescence. Argyropelecus affinis is closely related to Argyropelecus gigas.

<i>Monacoa</i> Genus of fishes

Monacoa is a genus of fish in the family Opisthoproctidae found in Atlantic and Pacific Oceans. They are also known as long-nosed mirrorbellies or simply mirrorbellies, in reference to the bioluminescent organ in their intestines. The largest species, Monacoa grimaldii, can grow to 8 cm (3.1 in) standard length.

<i>Neoscopelus macrolepidotus</i> Species of fish

Neoscopelus macrolepidotus, also known as a large-scaled lantern fish, is a species of small mesopelagic or bathypelagic fish of the family Neoscopelidae, which contains six species total along three genera. The family Neoscopelidae is one of the two families of the order Myctophiformes. Neoscopelidae can be classified by the presence of an adipose fin. The presence of photophores, or light-producing organs, further classify the species into the genus Neoscopelus. N. macrolepidotus tends to be mesopelagic until the individuals become large adults, which is when they settle down to the bathypelagic zone.

<i>Argyropelecus olfersii</i> Species of fish

Argyropelecus olfersii is a common species of marine hatchetfish, found in mesopelagic waters.

References

  1. ↑ Poulsen, Jan Yde; Sado, Tetsuya; Hahn, Christoph; Byrkjedal, Ingvar; Moku, Masatoshi; Miya, Masaki (2016). "Preservation obscures pelagic deep-sea fish diversity: doubling the number of sole-bearing opisthoproctids and resurrection of the genus Monacoa (Opisthoproctidae, Argentiniformes)". PLOS ONE. 11 (8): e0159762. Bibcode:2016PLoSO..1159762P. doi: 10.1371/journal.pone.0159762 .
  2. ↑ Froese, Rainer; Pauly, Daniel (eds.). "Family Opisthoproctidae". FishBase . February 2012 version.
  3. ↑ Nelson, J. S. (2006). Fishes of the World (4 ed.). Hoboken, NJ: John Wiley & Sons. p. 191. ISBN   978-0-471-25031-9.
  4. ↑ A. G. V. Salvanes; J. B. Kristofersen (2001). "Mesopelagic fishes". Encyclopedia of ocean sciences. Vol. 3. pp. 1711–1717.
  5. ↑ Peter B. Moyle; Joseph J. Cech, Jr (2004). Fishes: An introduction to ichthyology. Upper Saddle River, NJ: Prentice-Hall. p. 320. ISBN   978-0-13-100847-2.
  6. ↑ Weird Fish With Transparent Head National Geographic News. February 26, 2009 Photograph courtesy Monterey Bay Aquarium Research Institute
  7. ↑ "Fish with transparent head". Boing Boing. 25 February 2009.
  8. ↑ "Researchers solve mystery of deep-sea fish with tubular eyes and transparent head | MBARI". mbari.org. 23 February 2009. Retrieved September 27, 2019.
  9. ↑ Griggs, J. (2008-12-24). "First vertebrate eye to use mirror instead of lens". New Scientist . Archived from the original on 26 December 2008. Retrieved 2008-12-27.
  10. ↑ Connie J. Wolfe and Margo G. Haygood (August 1991). "Restriction Fragment Length Polymorphism Analysis Reveals High Levels of Genetic Divergence Among the Light Organ Symbionts of Flashlight Fish" (PDF). The Biological Bulletin. 181 (1): 135–143. doi:10.2307/1542496. JSTOR   1542496. PMID   29303659.
  11. ↑ Peter J. Herring (2000). "Bioluminescent signals and the role of reflectors" (abstract). Journal of Optics A: Pure and Applied Optics. 2 (6): R29 –R38. doi:10.1088/1464-4258/2/6/202.
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