{{Short description|Genus of ray-finned fishes}} {{Multiple issues|{{Cleanup rewrite|date=July 2021}}{{More citations needed|date=March 2025}}{{Inline|date=March 2025}}}} {{Use dmy dates|date=August 2022}} {{Speciesbox | image = Macropinna_microstoma_illustration.png | image_caption = Illustration | image2 = Barreleye-fish GoK.jpg | parent_authority = W. M. Chapman, 1939 | taxon = Macropinna microstoma | authority = W. M. Chapman, 1939 }}
The '''Pacific barreleye fish'''<ref>{{Cite web|url=https://www.inaturalist.org/taxa/49886-Macropinna-microstoma|title=Pacific Barreleye (Macropinna microstoma)|website=iNaturalist}}</ref> ('''''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, offering only tunnel vision above its head. However, in 2008, scientists discovered that its eyes were able to rotate both up and forward with its transparent dome.<ref name=":1">{{Cite journal |last1=Robison |first1=Bruce H. |last2=Reisenbichler |first2=Kim R. |date=2008-12-18 |title=Macropinna microstoma and the Paradox of Its Tubular Eyes |journal=Copeia |volume=2008 |issue=4 |pages=780–784 |doi=10.1643/cg-07-082 |issn=0045-8511}}</ref><ref>{{Cite journal |date=2009 |title=SPIN News, February 2009 |url=https://doi.org/10.1037/e593452011-001 |access-date=2025-10-11 |website=PsycEXTRA Dataset |doi=10.1037/e593452011-001 |url-access=subscription }}</ref> This feature allows for ''M. microstoma'' to observe prey while keeping its eyes protected from stinging tentacles of jellyfish-like organisms it feeds on.<ref>{{Cite journal |last1=Johnsen |first1=Sönke |last2=Haddock |first2=Steven H. D. |date=2022-03-28 |title=Macropinna |url=https://www.cell.com/current-biology/abstract/S0960-9822(22)00242-1 |journal=Current Biology |language=English |volume=32 |issue=6 |pages=R256–R257 |doi=10.1016/j.cub.2022.02.020 |issn=0960-9822 |pmid=35349807|bibcode=2022CBio...32.R256J |url-access=subscription }}</ref>
''M. microstoma,'' a small fish of about 15 cm, has a tiny mouth and most of its body is covered in large scales.<ref>{{Cite web |title=Barreleye fish • MBARI |url=https://www.mbari.org/animal/barreleye-fish/ |access-date=2025-11-16 |website=MBARI |language=en-US}}</ref> The fish normally hangs nearly motionless in the water, at a depth of about {{convert|600|m|ft}} to {{convert|800|m|ft}} in the mesopelagic zone, 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.<ref>{{Cite journal |last1=Robison |first1=Bruce H. |last2=Reisenbichler |first2=Kim R. |date=2008-12-18 |title=Macropinna microstoma and the Paradox of Its Tubular Eyes |url=http://www.bioone.org/doi/abs/10.1643/CG-07-082 |journal=Copeia |language=en |volume=2008 |issue=4 |pages=780–784 |doi=10.1643/CG-07-082 |issn=0045-8511|url-access=subscription }}</ref> 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.<ref>{{cite web|url=http://www.mbari.org/barreleye-fish-with-tubular-eyes-and-transparent-head/|title=Researchers solve mystery of deep-sea fish with tubular eyes and transparent head|access-date=24 February 2009|first=Kim |last=Fulton-Bennett|date=23 February 2009}}</ref>
== Discovery and Evolutionary History == ''Macropinna microstoma'' was discovered in 1939 by marine biologist W. M. Chapman, in deep temperate waters off of the Pacific, Indian, and Atlantic oceans. It is restricted to deep oceanic water by its light-sensitive tubular eyes. The eyes are capped with bright green lenses and surrounded by a fluid-filled shield. The presence of this fish was mostly unknown until 2004, when researchers at Monterey Bay Aquarium Research Institute photographed ''M. microstoma'' off the coast of the north pacific.<ref>{{Cite journal |last1=Robison |first1=Bruce H. |last2=Reisenbichler |first2=Kim R. |date=2008-12-18 |title=Macropinna microstoma and the Paradox of Its Tubular Eyes |url=http://www.bioone.org/doi/abs/10.1643/CG-07-082 |journal=Copeia |language=en |volume=2008 |issue=4 |pages=780–784 |doi=10.1643/CG-07-082 |issn=0045-8511|url-access=subscription }}</ref>
Fossil evidence has also shown to be especially important when discovering ''M. microstoma'' history. The genus ''M. microstoma'' has existed for millions of years that can be traced back to the Miocene epoch, with similar tubular-eye features preserved in a fossil species. Fossil evidence suggests that the barreleye's distinct features are not recent adaptations but are ancient traits precise for deep-sea life.<ref name=":0">{{Cite journal |last=Nazarkin |first=Mikhail V. |date=2016-06-15 |title=Barreleye <i>Macropinna</i> sp. (Argentiniformes, Opisthoproctidae) from the Miocene of Sakhalin Island, Russia |journal=Journal of Vertebrate Paleontology |volume=36 |issue=5 |article-number=e1187158 |doi=10.1080/02724634.2016.1187158 |bibcode=2016JVPal..36E7158N |issn=0272-4634}}</ref>
Another important aspect of the ''M. microstoma'' is that they represent an extreme example of deep-sea fish traits such as their eyes. Over evolutionary time, these traits have allowed ''M. microstoma'' to acquire an ecological niche, where the precise vision and protection from stinging prey are essential for species survival.<ref>{{Cite journal |last1=Johnsen |first1=Sönke |last2=Haddock |first2=Steven H. D. |date=2022-03-28 |title=Macropinna |url=https://www.cell.com/current-biology/abstract/S0960-9822(22)00242-1 |journal=Current Biology |language=English |volume=32 |issue=6 |pages=R256–R257 |doi=10.1016/j.cub.2022.02.020 |issn=0960-9822 |pmid=35349807|bibcode=2022CBio...32.R256J |url-access=subscription }}</ref>
== Environment == ''M. microstoma'' is a deep-sea fish, living around 1015 meters (about 3330.05 ft) below the water's surface off of California's coast and in the North Pacific Ocean. Because the barreleye's habitat is deep water, the species has enhanced sensitivity to faint light and bioluminescence. The evolution of enhanced light sensitivity is provided through small protrusions in their eyes called ocular diverticulum. With the use of their unique eyes and eyesight, the barreleye is able to utilize upward ocular viewing allowing the fish to detect silhouettes of prey that are upwards and towards the light.''<ref>{{Cite journal |last1=Wagner |first1=Hans-Joachim |last2=Genner |first2=Martin J. |last3=Partridge |first3=Julian C. |last4=Chung |first4=Wen-Sung |last5=Marshall |first5=N. Justin |last6=Robison |first6=Bruce H. |last7=Douglas |first7=Ronald H. |date=2022-12-01 |title=Diversity and evolution of optically complex eyes in a family of deep-sea fish: Ocular diverticula in barreleye spookfish (Opisthoproctidae) |journal=Frontiers in Ecology and Evolution |volume=10 |article-number=1044565 |doi=10.3389/fevo.2022.1044565 |bibcode=2022FrEEv..1044565W |doi-access=free |issn=2296-701X}}</ref>''
''M. microstoma'' can get as long as {{Convert|6|in|cm}}. It has a large, dome-shaped, transparent head. This protects its sensitive eyes from the nematocysts (stinging cells) of the siphonophores, from which it is believed to steal food. Through the dome, the entire inner part of the head can be seen, that is, their eyes, brain and all the nerve endings that make up their head. At first glance it seems that its eyes are at the front of their head in the form of two black holes; these are its olfactory organs. Marine biologists used to believe that the barreleye's eyes were fixed in its head, which would only allow it to look upward.
''M. microstoma's'' large, flat fins allow it to remain nearly motionless in the water, and to maneuver very precisely. Most of the time, the fish hangs motionless in the water, with its body in a horizontal position and its eyes looking upward. The green pigments in its eyes may filter out sunlight coming directly from the sea surface, helping the barreleye spot the bioluminescent glow of jellyfish or other animals directly overhead. Recent studies have suggested that the transparent dome of Macropinna microstoma may help reduce light distortion, allowing the fish to better detect prey and bioluminescent signals in the deep sea.<ref>{{cite journal |last1=Robison |first1=Bruce H. |last2=Reisenbichler |first2=Kim R. |date=2008-12-18 |title=Macropinna microstoma and the Paradox of Its Tubular Eyes |url=http://www.bioone.org/doi/abs/10.1643/CG-07-082 |journal=Copeia |volume=2008 |issue=4 |pages=780–784 |doi=10.1643/CG-07-082 |issn=0045-8511|url-access=subscription }}</ref>
=== Diet === ''M. microstoma'' shares its deep-sea environment with many different types of cnidarians. Some of the most common are siphonophores (which can reach length of 30 feet). The barreleye has a strong digestive system and it usually feeds on cnidarians and small drifting animals. The small marine creatures trapped on the tentacles of the cnidarians are also targeted by the barreleye fish.<ref name=":1" />
=== Reproduction === ''M. microstoma'' is thought to be a pelagic spawner (the eggs are coated with a layer of oil that allows them to float on the closest surface of the sea until they hatch). The female lays eggs in water and male releases sperms in water. After hatching, the larvae begin to descend to the depths as they grow, feeding on zooplankton and other small particles of organic material.<ref>{{cite web |url=http://ourmarinespecies.com/c-fishes/barreleye-fish/ |title=Barreleye Fish: Characteristics, types, reproduction and more… |website=ourmarinespecies.com |date=18 December 2019 |access-date=31 August 2022}}</ref> It is believed that there is no sexual dimorphism between the male and the female and that they do not care for their young.
''M. microstoma'' has been known to science since 1939, but is not known to have been photographed alive until 2004. Old drawings do not show the transparent dome, as it is usually destroyed when brought up from the depths. It is broadly distributed across the northern Pacific Ocean, from the Bering Sea to Japan and Baja California.
== References == {{Reflist}}
==Further reading== * {{FishBase |genus= Macropinna|species= microstoma| month = February | year = 2012}} *{{cite journal |jstor=25512162 |doi=10.1643/CG-07-082 |title=''Macropinna microstoma'' and the Paradox of Its Tubular Eyes |year=2008 |last1=Robison |first1=Bruce H. |last2=Reisenbichler |first2=Kim R. |journal=Copeia |volume=2008 |issue=4 |pages=780–4|s2cid=85768623 }} *{{cite journal |first1=Margaret |last1=McFall-Ngai |first2=Lin |last2=Ding |first3=James |last3=Childress |first4=Joseph |last4=Horwitz |year=1988 |title=Biochemical Characteristics of the Pigmentation of Mesopelagic Fish Lenses |journal=Biological Bulletin |pages=397–402 |jstor=1541731 |url=http://biostor.org/reference/10229 |volume=175 |issue=3 |doi=10.2307/1541731}} *{{cite journal |doi=10.1007/BF00618122 |title=The visual pigment sensitivity hypothesis: Further evidence from fishes of varying habitats |year=1985 |last1=Crescitelli |first1=Frederick |last2=McFall-Ngai |first2=Margaret |last3=Horwitz |first3=Joseph |journal=Journal of Comparative Physiology A |volume=157 |issue=3 |pages=323–33|pmid=3837092 |s2cid=24345329 }} *{{cite journal |first1=Ivan R. |last1=Schwab |first2=Viet |last2=Ho |first3=Alan |last3=Roth |first4=Thomas N. |last4=Blankenship |first5=Paul G. |last5=Fitzgerald |pmid=11797302 |year=2001 |title=Evolutionary attempts at 4 eyes in vertebrates |volume=99 |pages=145–56; discussion 156–7 |pmc=1359005 |journal=Transactions of the American Ophthalmological Society}} *{{cite book |first1=Ivan R. |last1=Schwab |year=2011 |chapter=Barreleye, ''Macropinna microstoma'' |chapter-url=https://books.google.com/books?id=ftpQB_GjtJgC&pg=PA101 |pages=101–2 |title=Evolution's Witness: How Eyes Evolved |publisher=Oxford University Press |isbn=978-0-19-536974-8}}
== External links == *[https://web.archive.org/web/20150208062518/http://www.mbari.org:80/midwater/macropinna/ Photos] (archived from the original) *[http://www.mbari.org/barreleye-fish-with-tubular-eyes-and-transparent-head/ MBARI Article with ROV Video footage] *[http://www.educatedearth.net/video.php?id=3903 ROV Video footage from above article] *{{YouTube|id=Akv36y8WLYA|title=The barreleye fish, from Monterey Bay Aquarium Research Institute}}
{{Taxonbar|from=Q131722}}
Category:Opisthoproctidae Category:Monotypic fish genera Category:Fish of the North Pacific Category:Fish described in 1939 Category:Taxa named by Wilbert McLeod Chapman