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Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.

At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks and rays, though their last common ancestor with them lived nearly 400 million years ago. Today, they are largely confined to deep water.

Description and habits


They have elongated, soft bodies, with a bulky head and a single gill-opening. They grow up to 150 cm in length, although this includes the lengthy tail found in some species. In many species, the snout is modified into an elongated sensory organ.

Like other members of the class Chondrichthyes, chimaera skeletons are composed of cartilage. Their skin is smooth and naked, lacking placoid scales (except in the claspers), and their color can range from black to brownish gray. For defense, most chimaeras have a venomous spine in front of the dorsal fin. Chimeras move throughout the water with a combination of body undulations and pectoral fin flapping. They use these fins to "fly" through water.

Chimaeras resemble sharks in some ways: they employ claspers for internal fertilization of females and they lay eggs with leathery cases. They also use electroreception to find their prey. However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead (a type of tentaculum) and in front of the pelvic fins. The females lay eggs in spindle-shaped, leathery egg cases.

They also differ from sharks in that their upper jaws are fused with their skulls and they have separate anal and urogenital openings. They lack sharks' many sharp and replaceable teeth, having instead just three pairs of large permanent grinding tooth plates. They also have gill covers or opercula like bony fishes. '''Furthermore, they have true nostrils (along with gills). These nostrils are found under the snout, just above the mouth. Water can be drawn in through these nostrils and then pass over the gills .'''

Distribution
Chimaeras live in temperate ocean floors down to 2,600 m (8,500 ft) deep, with few occurring at depths shallower than 200 m (660 ft). Exceptions include the members of the genus Callorhinchus, the rabbit fish and the spotted ratfish, which locally or periodically can be found at shallower depths. Consequently, these are also among the few species from the chimaera order kept in public aquaria. They live in all the oceans except for the Arctic and Antarctic oceans. Chimeras have a high degree of endemism with about 37% of species only found in one area of the world '''. One third of the species documented are found in the Southwest Pacific. Younger chimaeras typically occupy deeper waters than their adult counterparts .'''

'''Currently, there are three extant families; Callorhinchidae, Rhinochimaeridae and Chimaeridae. The distribution of each family varies greatly, and may not completely capture all areas that inhabit due to the lack of research [Image showing the distribution of the three extant families would be included here] .'''

Growth and reproduction
'''Unlike 75% of the class of Chondrichthyes that give birth to live young (ovoviviparity and viviparity), Chimaera’s lay their young in eggs (Oviparous), often referred to as Mermaids Purses, which is a leathery case that will hatch in 6-12 months. Additionally, when females lay their eggs, they are paired and remained attached to the female for a period of time before attaching to the seabed . The process of egg-laying takes around 18-30 hours and can occur every 10-14 days throughout the breeding season (which is 6-8 months long), indicating an annual fecundity range of 19.5 - 28.9 . Chimaera’s exhibit sexual dimorphism, where males reach sexual maturity after reaching a length of 18.5 - 20 cm, and females reach sexual maturity when they reach a length of 24 - 25 cm . '''

'''The eggs of the chimera can reach up to 10 cm long. After incubation, the eggs will hatch and young chimera will appear at about roughly 14 cm in length. The chimaera does not have a larval stage as they are fully developed once they hatch . Unlike other sharks, it is hard to age ghost sharks. They lack the hard internal structures which are typically used to age other chondrichthyans. Estimates from various studies and observations place chimaeras with a maximum age estimate of about 40 years .'''

Diet
As mentioned in the description and habits section, chimaera, unlike sharks have three pairs of large permanent grinding tooth plates '''. In addition to this, Chimaeras are not strong swimmer, which further limits their diet to bottom dwelling invertebrates such as crabs, clams, shrimp, and small benthic fish. In addition to small invertebrates, they have been known to eat their own egg cases and even other chimaeras .'''

Phylogenetics
Tracing the evolution of these species has been problematic given the paucity of good fossils. DNA sequencing has become the preferred approach to understanding speciation.

The group containing Chimeras and their close relatives (Holocephali) is thought to have originated about 420 million years ago during the Silurian. The oldest known Chimaeriform is Protochimaera from the Early Carboniferous (338-322 million years ago) of Russia, which is more closely related to modern chimeras (Chimaeroidei) than any other known extinct groups of Chimaeriformes. The earliest known remains attributable to modern chimaeras are known from the Early Jurassic (Pleinsbachian) of Europe, but egg cases from the Late Triassic of Yakutia, Russia and New Zealand that resemble those of rhinochimaerids and callorhinchids respectively indicates that they had a global distribution prior to the end of the Triassic. Unlike modern chimaeras, Mesozoic representatives are often found in shallow water settings. The extant species fall into three families—the Callorhinchidae, Rhinochimaeridae and Chimaeridae with the callorhinchids being the most basal clade.

Parasites
As other fish, chimaeras have a number of parasites. Chimaericola leptogaster (Chimaericolidae) is a monogenean parasite of the gills of Chimaera monstrosa; the species can attain 50 mm in length.

Climate Change
'''Climate change can have an effect on chimeras. Because they are deep-sea organisms, there is moderate exposure to climate change factors. This includes rising temperature as this affects their physicochemical environment. The rising sea temperatures could lead to a change in the distribution of the chimaeras. This change in distribution is not yet known . These changes could also affect currents and upwelling which will lead to a negative impact on the productivity in the deep ocean.'''

By-catch
'''In addition to climate change, another major threat to chimaeras is overfishing by the means of by-catch, through deep-sea and inshore trawling activities. These activities can threaten the stability of the species population because they're a deep-sea fish which often means they grow at a slower rate and reach reproductive maturity later in life .'''

Classification
In some classifications, the chimaeras are included (as subclass Holocephali) in the class Chondrichthyes of cartilaginous fishes; in other systems, this distinction may be raised to the level of class. Chimaeras also have some characteristics of bony fishes.

A renewed effort to explore deep water and to undertake taxonomic analysis of specimens in museum collections led to a boom during the first decade of the 21st century in the number of new species identified. A preliminary study found 8% of species to be threatened. There are 50 extant species in six genera and four families are described; an additional three genera and two families are only known from fossils):




 * †Suborder Myriacanthoidei Patterson 1965 (Late Triassic-Late Jurassic)
 * †Family Chimaeropsidae
 * †Chimaeropsis Zittel 1887 Belgium, Early Jurassic (Sinemurian)
 * †Family Myriacanthidae Woodward 1889
 * †Acanthorhina Fraas 1910 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
 * †Agkistracanthus Duffin and Furrer 1981 Austria, England and Switzerland, Late Triassic-Early Jurassic (Rhaetian-Sinemurian)
 * †Alethodontus Duffin 1983 Germany, Early Jurassic (Sinemurian)
 * †Halonodon Duffin 1984 Belgium and Luxembourg, Early Jurassic (Sinemurian)
 * †Metopacanthus Zittel 1887 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
 * †Oblidens Duffin and Milàn 2017 Hasle Formation, Denmark, Early Jurassic (Pliensbachian)
 * †Myriacanthus Agassiz 1837 United Kingdom, Late Triassic-Early Jurassic (Rhaetian-Sinemurian)
 * †Recurvacanthus Duffin 1981 United Kingdom, Early Jurassic (Sinemurian)
 * †Suborder Protochimaeroidei Lebedev & Popov, 2021
 * †Family Protochimaeridae Lebedev & Popov, 2021
 * †Protochimaera Lebedev & Popov, 2021 Moscow Region, Russia, Lower Carboniferous (Viséan–Serpukhovian)
 * Suborder Chimaeroidei Patterson 1965
 * †Eomanodon Ward and Duffin 1989 United Kingdom, Early Jurassic (Pleinsbachian)
 * Family Callorhinchidae Garman, 1901
 * Genus Callorhinchus Lacépède, 1798
 * Callorhinchus callorynchus Linnaeus, 1758 (ploughnose chimaera)
 * Callorhinchus capensis A. H. A. Duméril, 1865 (Cape elephantfish)
 * Callorhinchus milii Bory de Saint-Vincent, 1823 (Australian ghost shark)
 * †Brachymylus A. S. Woodward 1894 Germany, Early Jurassic (Pleinsbachian)
 * †Bathytheristes Duffin 1995 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
 * †Ottangodus Popov, Delsate & Felten, 2019 France, Middle Jurassic (Bajocian)
 * Family Chimaeridae Bonaparte, 1831
 * Genus Chimaera Linnaeus, 1758
 * Chimaera argiloba Last, W. T. White & Pogonoski, 2008 (whitefin chimaera)
 * Chimaera bahamaensis Kemper, Ebert, Didier & Compagno, 2010 (Bahamas ghost shark)
 * Chimaera compacta Iglésias, Kemper & Naylor, 2022
 * Chimaera cubana Howell-Rivero, 1936
 * Chimaera fulva Didier, Last & W. T. White, 2008 (southern chimaera)
 * Chimaera jordani S. Tanaka (I), 1905 (Jordan's chimaera)
 * Chimaera lignaria Didier, 2002 (carpenter's chimaera)
 * Chimaera macrospina Didier, Last & W. T. White, 2008 (longspine chimaera)
 * Chimaera monstrosa Linnaeus, 1758 (rabbit fish)
 * Chimaera notafricana Kemper, Ebert, Compagno & Didier, 2010 Cape chimaera
 * Chimaera obscura Didier, Last & W. T. White, 2008 (shortspine chimaera)
 * Chimaera opalescens Luchetti, Iglésias & Sellos, 2011
 * Chimaera owstoni S. Tanaka (I), 1905 (Owston's chimaera)
 * Chimaera panthera Didier, 1998 (leopard chimaera)
 * Chimaera phantasma Jordan & Snyder, 1900 (silver chimaera)
 * Genus Hydrolagus Gill, 1863
 * Hydrolagus affinis Brito Capello, 1868 (smalleyed rabbitfish)
 * Hydrolagus africanus Gilchrist, 1922 (African chimaera)
 * Hydrolagus alberti Bigelow & Schroeder, 1951
 * Hydrolagus alphus Quaranta, Didier, Long & Ebert, 2006 (whitespot ghost shark)
 * Hydrolagus barbouri Garman, 1908
 * Hydrolagus bemisi Didier, 2002 (pale ghost shark)
 * Hydrolagus colliei Lay & E. T. Bennett, 1839 (spotted ratfish)
 * Hydrolagus deani H. M. Smith & Radcliffe, 1912 (Philippine chimaera)
 * Hydrolagus eidolon Jordan & Hubbs, 1925
 * Hydrolagus homonycteris Didier, 2008 (black ghostshark)
 * Hydrolagus lemures Whitley, 1939 (blackfin ghostshark)
 * Hydrolagus lusitanicus Moura, Figueiredo, Bordalo-Machado, Almeida & Gordo, 2005
 * Hydrolagus macrophthalmus de Buen, 1959
 * Hydrolagus marmoratus Didier, 2008 marbled ghostshark
 * Hydrolagus matallanasi Soto & Vooren, 2004 (striped rabbitfish)
 * Hydrolagus mccoskeri Barnett, Didier, Long & Ebert, 2006 (Galápagos ghostshark)
 * Hydrolagus melanophasma K. C. James, Ebert, Long & Didier, 2009 (Eastern Pacific black ghostshark)
 * Hydrolagus mirabilis Collett, 1904 (large-eyed rabbitfish)
 * Hydrolagus mitsukurii Jordan & Snyder, 1904 (spookfish)
 * Hydrolagus novaezealandiae Fowler, 1911 (dark ghostshark)
 * Hydrolagus ogilbyi Waite, 1898
 * Hydrolagus pallidus Hardy & Stehmann, 1990
 * Hydrolagus purpurescens Gilbert, 1905 (purple chimaera)
 * Hydrolagus trolli Didier & Séret, 2002 (pointy-nosed blue chimaera)
 * Hydrolagus waitei Fowler, 1907
 * Family Rhinochimaeridae Garman, 1901
 * Genus Harriotta Goode & Bean, 1895
 * Harriotta haeckeli Karrer, 1972 (smallspine spookfish)
 * Harriotta raleighana Goode & Bean, 1895 (Pacific longnose chimaera)
 * Genus Neoharriotta Bigelow & Schroeder, 1950
 * Neoharriotta carri Bullis & J. S. Carpenter, 1966 (dwarf sicklefin chimaera)
 * Neoharriotta pinnata Schnakenbeck, 1931 (sicklefin chimaera)
 * Neoharriotta pumila Didier & Stehmann, 1996 (Arabian sicklefin chimaera)
 * Genus Rhinochimaera Garman, 1901
 * Rhinochimaera africana Compagno, Stehmann & Ebert, 1990 (paddle-nose chimaera)
 * Rhinochimaera atlantica Holt & Byrne, 1909 (straightnose rabbitfish)
 * Rhinochimaera pacifica Mitsukuri, 1895 (Pacific spookfish)