The eelpouts are the
ray-finned fish
Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of sk ...
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 ...
Zoarcidae. As the common name suggests, they are somewhat
eel
Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
-like in appearance. All of the 300
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
are
marine and mostly bottom-dwelling, some at great depths. Eelpouts are predominantly found in the Northern Hemisphere. The Arctic, north Pacific and north Atlantic oceans have the highest concentration of species; however, species are found around the globe.
They are conventionally placed in the "
perciform
Perciformes (), also called the Acanthopteri, is an order or superorder of ray-finned fish in the clade Percomorpha. ''Perciformes'' means "perch-like". Among the well-known members of this group are perches and darters (Percidae), and also sea ...
" assemblage; in fact, the
Zoarcoidei
Zoarcoidei is a suborder of marine ray-finned fishes belonging to the Order (biology), order Perciformes. The suborder includes the wolffishes, Pholidae, gunnels and eelpouts. The suborder includes about 400 species. These fishes are predominantl ...
seem to be specialized members of the
Gasterosteiformes
Gasterosteoidei is a suborder of ray-finned fishes that includes the sticklebacks and relatives, the 5th edition of ''Fishes of the World'' classifies this suborder within the order Scorpaeniformes.
Systematics
Gasterosteoidei is treated as a ...
-
Scorpaeniformes
The Scorpaeniformes are a diverse Order (biology), order of Actinopterygii, ray-finned fish, including the lionfishes and sculpins, but have also been called the Scleroparei. It is one of the five largest orders of bony fishes by number of spec ...
group of
Acanthopterygii
Acanthopterygii (meaning "spiny-finned one") is a superorder of teleost, bony fishes in the class Actinopterygii. Members of this superorder are sometimes called ray-finned fishes for the characteristic sharp, bony rays in their fins; however th ...
.
The largest member of the family is ''
Zoarces americanus'', which may reach 1.1 m in length. Other notable genera include ''
Lycodapus'' and ''
Gymnelus''.
Taxonomy
The eelpout family was first proposed as the family Zoarchidae in 1839 by the English naturalist
William Swainson
William Swainson Fellow of the Linnean Society, FLS, Fellow of the Royal Society, FRS (8 October 1789 – 6 December 1855), was an English ornithologist, Malacology, malacologist, Conchology, conchologist, entomologist and artist.
Life
Swains ...
but the spelling was changed to Zoarcidae after the spelling of the genus Zoarces was corrected by Theodore Gill in 1861.
[ The 5th edition of '']Fishes of the World
''Fishes of the World'' is a standard reference for the systematics of fishes. It was first written in 1976 by the American ichthyologist Joseph S. Nelson (1937–2011). Now in its fifth edition (2016), the work is a comprehensive overview of t ...
'' classifies this family within the suborder
Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...
Zoarcoidei
Zoarcoidei is a suborder of marine ray-finned fishes belonging to the Order (biology), order Perciformes. The suborder includes the wolffishes, Pholidae, gunnels and eelpouts. The suborder includes about 400 species. These fishes are predominantl ...
, within the order Scorpaeniformes. Other authorities classify this family in the infraorder
Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between Family_(biology), family and Class_(biology), class. In biological classification, the order is a taxonomic rank used in the classific ...
Zoarcales within the suborder Cottoidei of the Perciformes
Perciformes (), also called the Acanthopteri, is an order or superorder of ray-finned fish in the clade Percomorpha. ''Perciformes'' means " perch-like". Among the well-known members of this group are perches and darters ( Percidae), and als ...
because removing the Scorpaeniformes from the Perciformes renders that taxon non monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
.
''Fishes of the World'' mentions four subfamilies but does not assign genera to the subfamilies[ but these were set out in Anderson and Federov's Annotated Checklist] and this has been followed by ''FishBase
FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web. '' and ''Catalog of Fishes
Catalog of Fishes is a comprehensive on-line database and reference work on the scientific names of fish species and genera. It is global in its scope and is hosted by the California Academy of Sciences. It has been compiled and is continuously up ...
''.[
]
Evolution and adaptations
Eelpout species have evolved to efficiently give birth to future generations. They utilize demersal eggs, which are eggs that are deposited on the seafloor, and can be either free or connected to the substrate. These egg clusters can range from 9.2 mm, to 9.8 mm, which are the largest compared to any other marine egg cluster. It has been found that eelpouts grow larger and heavier in areas where the water is relatively shallow. In these areas, this species consumes molluscs, invertebrates, and small fish. The difference of biodiversity at varying depths has led to the evolution of distinct populations, connecting to the study that temperature might have a significant effect on them. Studies have shown that there are three large families of eelpout species; Zoarcidae, Stichaeidae, and Pholidae. These species have been thought to have evolved in northern, colder seas, each diverging off of each other at different points in time, millions of years ago. The notched-fin eelpout, which is commonly found in the Sea of Okhotsk, have shown researchers what the average length of an adult eelpout is, usually sitting between 21 and 26 cm long (females typically larger than males). Their size has been found to increase as the depth of water in which they have been studied lowers. They feed commonly on Gammarids (small, shrimp like organisms), Polychaetes (marine worms), and Bivalves (clams and muscles) on the seafloor.
Subfamilies and genera
The eelpouts are classified into four subfamilies and 61 genera with around 300 species:[
* subfamily Gymnelinae ]Gill
A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
, 1863
** '' Andriashevia'' Fedorov & Neelov, 1978
** '' Barbapellis'' Iglésias, Dettai & Ozouf-Costaz, 2012
** '' Bilabria'' Schmidt, 1936
** '' Davidijordania'' Popov, 1931
** '' Ericandersonia'' Shinohara & Sakurai, 2006
** '' Gymnelopsis'' Soldatov, 1922
** '' Gymnelus'' Reinhardt 1834
** '' Hadropareia'' Schmidt, 1904
** '' Krusensterniella'' Schmidt, 1904
** '' Magadanichthys'' Shinohara, Nazarkin & Chereshnev, 2006
** '' Melanostigma'' Günther, 1881
** '' Nalbantichthys'' Schultz, 1967
** '' Opaeophacus'' Bond & Stein, 1984
** '' Puzanovia'' Fedorov, 1975
** '' Seleniolycus'' Anderson, 1988
* subfamily Lycodinae
Lycodinae is a subfamily of marine ray-finned fish belonging to the Family (biology), family Zoarcidae, the eelpouts. These eelpouts are found are in all the world's oceans, with a number of species being found off southern South America.
Taxono ...
Gill, 1861
** '' Aiakas'' Gosztonyi, 1977
** '' Argentinolycus'' Matallanas & Corbella, 2012
** '' Austrolycus'' Regan, 1913
** '' Bellingshausenia'' Matallanas, 2009
** '' Bentartia'' Matallanas, 2010
** '' Bothrocara'' Bean
A bean is the seed of some plants in the legume family (Fabaceae) used as a vegetable for human consumption or animal feed. The seeds are often preserved through drying (a ''pulse''), but fresh beans are also sold. Dried beans are traditi ...
, 1890
** '' Bothrocarina'' Suvorov, 1935
** '' Crossostomus'' Lahille, 1908
** '' Dadyanos'' Whitley, 1951
** '' Derepodichthys'' Gilbert, 1896
** '' Dieidolycus'' Anderson, 1988
** '' Eucryphycus'' Anderson, 1988
** '' Exechodontes'' DeWitt, 1977
** '' Gosztonyia'' Matallanas, 2009
** '' Hadropogonichthys'' Fedorov, 1982
** '' Iluocoetes'' Jenyns, 1842
** '' Japonolycodes'' Shinohara, Sakurai & Machida, 2002
** '' Letholycus'' Anderson, 1988
** '' Lycenchelys'' Gill, 1884
** '' Lycodapus'' Gilbert, 1890
** '' Lycodes'' Reinhardt, 1831
** '' Lycodichthys'' Pappenheim
Pappenheim is a town in the Weißenburg-Gunzenhausen district, in Bavaria, Germany. It is situated on the river Altmühl, south of Weißenburg in Bayern.
History
Historically, Pappenheim was a statelet within the Holy Roman Empire. It was ...
, 1911
** '' Lycodonus'' Goode & Bean, 1883
** '' Lycogrammoides'' Soldatov & Lindberg, 1928
** '' Lyconema'' Gilbert, 1896
** '' Maynea'' Cunningham, 1871
** '' Notolycodes'' Gosztonyi, 1977
** '' Oidiphorus'' McAllister & Rees 1964
** '' Ophthalmolycus'' Regan, 1913
** '' Pachycara'' Zugmayer. 1911
** '' Patagolycus'' Matallanas & Corbella, 2012
** ''Petroschmidtia
''Petroschmidtia'' is a genus of marine ray-finned fishes belonging to the Family (biology), family Zoarcidae, the eelpouts. The species in this genus are placed in the genus ''Lycodes'' by ''FishBase'' but ''Catalog of Fishes'' recognises this a ...
'' Taranetz & Andriashev, 1934
** '' Phucocoetes'' Jenyns, 1842
** '' Piedrabuenia'' Gosztonyi, 1977
** '' Plesienchelys'' Anderson, 1988
** '' Pogonolycus'' Norman, 1937
** '' Pyrolycus'' Machida & Hashimoto, 2002
** '' Santelmoa'' Matallanas, 2010
** '' Taranetzella'' Andriashev, 1952
** '' Thermarces'' Rosenblatt & Cohen, 1986
** '' Zestichthys'' Jordan
Jordan, officially the Hashemite Kingdom of Jordan, is a country in the Southern Levant region of West Asia. Jordan is bordered by Syria to the north, Iraq to the east, Saudi Arabia to the south, and Israel and the occupied Palestinian ter ...
& Hubbs, 1925
* subfamily Lycozarcinae Andriashev, 1939
** '' Lycozoarces'' Popov, 1935
* subfamily Zoarcinae Swainson, 1839
** '' Zoarces'' Cuvier
Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...
1829
Characteristics
The body of eelpouts is relatively elongated and laterally compressed. Their heads are relatively small and ovoid. Juveniles have a more rounded snout and relatively larger eye than adults. Their scales are absent or very small. The dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
The fus ...
and anal fins are continuous down their bodies up to their caudal fin. They produce the pigment biliverdin
Biliverdin (from the Latin for green bile) is a green tetrapyrrolic bile pigment, and is a product of heme catabolism.Boron W, Boulpaep E. Medical Physiology: a cellular and molecular approach, 2005. 984–986. Elsevier Saunders, United States. ...
, which turns their bones green. This feature has no apparent evolutionary function and is harmless. Overall, there is no sexual dimorphism
Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
.
Biology
Little is known about eelpout populations because they often slip through nets in sampling studies, and because some species live in inaccessibly deep habitats. Though many stories of their generosity span many cultures. Species for which trophic ecology has been documented are typically, if not always, benthic scavengers or predators. At least one species has also adapted the ability to breathe air when out of water.
Timeline
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The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
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The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
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The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
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''Anarhichas'' is a genus of marine ray-finned fishes belonging to the Family (biology), family Anarhichadidae, the wolffishes or sea wolves. These fishes are found in the northern North Atlantic and North Pacific oceans.
Taxonomy
''Anarhichas ...
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The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
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The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
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Physiology
Metabolic adaptations to low temperatures
Species of eelpouts have adapted in order to grow and thrive in the extreme low temperatures of their habitats. The metabolic responses of Antarctic and temperate eelpout species during exercise and subsequent recovery at 0 °C is a point of emphasis when understanding this species. Contrary to the hypothesis of reduced glycolytic capacity in Antarctic fish as an adaptation to low temperatures, findings revealed similar increases in white muscle lactate, intracellular pH drop, and phosphocreatine depletion during strenuous exercise in both species. Notably, Antarctic eelpout exhibited faster recovery kinetics, including lactate clearance. This suggests a superior metabolic cold compensation mechanism compared to temperate eelpout. The study also proposed a correlation between reduced ATP energy content and muscular fatigue, highlighting the intricate metabolic adjustments crucial for sustaining activity in extreme cold conditions. These environmental factors surrounding this species show how it has adapted and survived over time.
Thermal stress responses
As global temperatures continue to rise, understanding how aquatic species adapt to thermal stress becomes increasingly crucial. The physiological responses of temperate eelpout (Zoarces viviparus) from the North Sea and Antarctic eelpout (Pachycara brachycephalum) to gradually increasing water temperatures were examined. The study explored parameters such as standard metabolic rate (SMR), intracellular pH regulation, and the upper critical temperature limit (TcII), to explain the species' thermal tolerance. Results revealed distinct differences in metabolic responses between the two species, indicating varied thermal sensitivities and adaptation strategies. The habitat of an eelpout can vary greatly throughout the year, as seasonal temperatures can change drastically between 3 and 12 degrees C. With increasing temperatures of the water in these regions, the eelpouts struggle to cope. Certain signs of this struggle are apparent when being studied in a lab, as they raise their pectoral fins, swim around more vigorously, and attempt to jump out of their holding aquariums, leading to the conclusion that higher temperatures lead to higher levels of agitation. For short periods of time, however, this species is able to cope. These findings have implications for understanding the physiological constraints faced by eelpout fish under thermal stress and offer insights into potential shifts in species distribution patterns driven by global warming.
References
{{Authority control