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This article is about the cartilaginous fish order. For the namesake genus, see Chimaera (genus). For other uses, see Chimera (disambiguation).
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish (not to be confused with the rattails), or rabbitfishes (not to be confused with the "true" rabbitfishes of the family Siganidae). They may be the "oldest and most enigmatic groups of fishes alive today."[2] At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though in evolutionary terms they branched off from sharks nearly 400 million years ago and have remained isolated ever since, typically confined to deep water.[2]
[edit] Description and habitsChimaeras live in temperate ocean floors down to 2,600 metres (8,500 ft) depth, and most do not occur at depths shallower than 200 metres (660 ft). Exception include the members of the genus Callorhinchus, the rabbit fish and the spotted ratfish, which locally/periodically can be found at relatively shallow depths. Consequently, these are also among the only species from the Chimaera order that are kept in public aquaria.[3] They have elongated, soft bodies, with a bulky head and a single gill-opening. They grow up to 150 centimetres (4.9 ft) in length, although this includes the – in some species – lengthy tail. In many species, the snout is modified into an elongated sensory organ.[4] Like other members of the class Chondrichthyes, chimaeras have a skeleton constructed of cartilage. Their skin is smooth and lacks scales, and their color can range from black to brownish gray. For defense, most chimaeras have a venomous spine located in front of the dorsal fin. Chimaeras resemble sharks in some ways: they employ claspers for internal fertilization of females and they lay eggs with leathery cases. However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead (a type of tentaculum)[5] and in front of the pelvic fins.[4] The females lay eggs in spindle-shaped leathery cases.[1] 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 the many sharp and replaceable teeth of sharks, having instead just three pairs of large permanent grinding tooth plates.[4] [edit] ClassificationIn 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 have led to a boom during the first decade of the 21st century in the number of new species identified.[2] There are about forty extant species in six genera and three families (an additional three genera and two family are only known from fossils):  Family Callorhinchidae
Family Chimaeridae
Family Rhinochimaeridae
Family †Squalorajidae
Family †Echinochimaeridae
[edit] See also[edit] ReferencesSpecific references:
General references:
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