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
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 relatively shallow 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.
They have elongated, soft bodies, with a bulky head and a single gill-opening. They grow up to 150 cm (4.9 ft) 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 constructed 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. 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.
Tracing the evolution of these species has been problematic given the scarcity of good fossils. DNA sequences have 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 earliest known remains attributable to modern chimaeras first appear in 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 39 extant species fall into three families—the Callorhinchidae, Rhinochimaeridae and Chimaeridae with the callorhinchids being the most basal clade.
16% of the species is considered to be threatened.
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 (2.0 in) in length.
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. 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 Chimaeroidei Patterson 1965
- †Eomanodon Ward and Duffin 1989 United Kingdom, Early Jurassic (Pleinsbachian)
- Family Callorhinchidae Garman, 1901
- 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 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
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... Perhaps the most intriguing feature of the newly described species, Hydrolagus melanophasma, is a presumed sexual organ that extends from its forehead called a tentaculum. ...
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