| Chimaeras | |
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| Hydrolagus colliei (Chimaeridae) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Holocephali |
| Order: | Chimaeriformes Obruchev, 1953 |
| Subgroups | |
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Chimaeras[1] arecartilaginous fish in theorderChimaeriformes (/kɪˈmɛrɪfɔːrmiːz/), known informally asghost sharks,rat fish (not to be confused withrattails),spookfish, orrabbit fish; the last two names are also applied toOpisthoproctidae andSiganidae, respectively.
At one time a "diverse and abundant" group (based on thefossil record), their closest living relatives aresharks andrays, though their last common ancestor with them lived nearly 400 million years ago.[2] Living species (aside fromplough-nose chimaeras) are largely confined to deep water.[3]
Chimaeras are soft-bodied, shark-like fish with bulky heads and long, tapered tails; measured from the tail, they can grow up to 150 cm (4.9 ft) in length. Like other members of the classChondrichthyes, chimaera skeletons are entirely cartilaginous, or composed ofcartilage. Males use forehead denticles to grasp a female by a fin during copulation.[4] Thegill arches are condensed into a pouch-like bundle covered by a sheet of skin (anoperculum), with a singlegill-opening in front of thepectoral fins.[5]
The pectoral fins are large enough to generate lift at a relaxed forward momentum, giving the chimaera the appearance of "flying" through the water. Further back on the body are also a pair of smallerpelvic fins, and some genera bear ananal fin in front of the tail. Inchimaerids andrhinochimaerids, the tail isleptocercal, meaning that it is thin and whip-like, edged from above and below by fins of similar size. Incallorhinchids, the tail is insteadheterocercal, with a larger upper lobe inclined upwards, similar to many sharks. There are two dorsal fins: a large triangular first dorsal fin and a low rectangular or depressed second dorsal fin. For defense, some chimaeras have avenomousspine on the front edge of thedorsal fin.[4]
In many species, the bulbous snout is modified into an elongated sensory organ, capable ofelectroreception to find prey.[5][6] The cartilaginous skull isholostylic, meaning that thepalatoquadrate (upper jaw cartilage) is completely fused to theneurocranium (cranial cartilage). This contrasts with modern sharks, where the palatoquadrate is movable and detachable, a trait known ashyostyly. The back of the head is supported by a complex of fused vertebrae called the synarcual, which also connects to the dorsal fin spine.[4]
Instead of sharks' many sharp, consistently-replaced teeth, chimaeras have just six large, permanent tooth-plates, which grow continuously throughout their entire life. These tooth-plates are arranged in three pairs, with one pair at the tip of the lower jaws and two pairs along the upper jaws. They together form a protruding, beak-like crushing and grinding mechanism, comparable to theincisor teeth ofrodents andlagomorphs (hence the name "rabbit fish").[4] Chimaera teeth are unique among vertebrates, due to their mode of mineralization. Most of each plate is formed by relatively softosteodentin, but the active edges are supplemented by a unique hypermineralizedtissue calledpleromin. Pleromin is an extremely hardenamel-like tissue, arranged into sheets or beaded rods, but it is deposited bymesenchyme-derived cells similar to those that formbone. In addition, pleromin's hardness is due to the mineralwhitlockite, which crystalizes within the teeth as the animal matures. Other vertebrates with hypermineralized teeth rely on enamel, which is derived fromameloblasts and encases round crystals of the mineralapatite.[7]
Chimaeras also differ from sharks in that they have separateanal andurogenital openings.
Chimaeras live in temperate ocean floors, with some species inhabiting depths exceeding 2,000 m (6,600 ft),[8] with relatively few modern species regularly inhabiting shallow water. Exceptions include the members of thegenusCallorhinchus, therabbit fish and thespotted ratfish, which locally or periodically can be found at shallower depths. Consequently, these are also among the few species kept inpublic aquaria.[9] They live in all the oceans except for the Arctic and Antarctic oceans.
The usual diet of chimaeras consist ofcrustaceans, and more specifically, they include ophiurans and molluscs.[10] Modern species aredemersaldurophages, but they used to be more diverse. The Carboniferous period had forms that lived as specialised suction feeders in the water column.[11]
Chimaera reproduction resembles that of sharks in some ways: males employclaspers for internal fertilization of females and females layeggs withinspindle-shaped, leatheryegg cases.[1]
Unlike sharks, male chimaeras have retractable sexual appendages (known as tentacula) to assist mating.[12][5] The frontal tentaculum, a bulbous rod which extends out of the forehead, is used to clutch the females' pectoral fins during mating. The prepelvic tentacula are serrated hooked plates normally hidden in pouches in front of the pelvic fins, and they anchor the male to the female. Lastly, the pelvic claspers (sexual organs shared by sharks) are fused together by a cartilaginous sheathe before splitting into a pair of flattened lobes at their tip.[4]
As other fish, chimaeras have a number ofparasites.Chimaericola leptogaster (Chimaericolidae) is amonogenean parasite of thegills ofChimaera monstrosa; the species can attain 50 mm (2.0 in) in length.
Despite their secluded habits, some chimaera species may be threatened byoverfishing throughbycatch or commercial exploitation. No species are listed asEndangered according to theIUCN, but four are listed asVulnerable, four more asNear Threatened, and many more asData Deficient (too rare to evaluate). Many species have restricted ranges and practically none have had their movement patterns studied. In addition, bycatch reports are usually insufficiently precise to the species or even genus level, so it is difficult to keep track of bycatch on a species-by-species basis. This lack of data renders chimaera species especially susceptible to overlooked population declines.[13]
Several near-shore species are purposefully caught for their meat, especially callorhinchids,Hydrolagus bemisi (pale ghost shark), andHydrolagus novaezealandiae (dark ghost shark). Modern quotas have helped to moderate collection of these species to a sustainable level, thoughCallorhinchus milii (theAustralian ghostshark) experienced severe overfishing in the 20th century before protections were enacted.Neoharriotta pinnata (sicklefin chimaera) is targeted along the coast ofIndia for its liver oil, and a recent decline of catch rates may indicate a population crash. Even species without commercial exploitation can fall victim to bycatch:Callorhinchus callorynchus (American elephantfish),Neoharriotta carri (dwarf sicklefin chimaera),Chimaera monstrosa (rabbit fish),Chimaera ogilbyi (Ogilby's ghostshark),Hydrolagus colliei (spotted ratfish), andHydrolagus melanophasma (eastern Pacific black ghostshark) all have bycatch rates exceeding 10% in certain parts of their range, and some are experiencing steep declines. Chimaeras have mostly avoided harvesting for thefin trade, which threatens many true sharks.[13]
Another threat is habitat destruction of coastal nurseries (by urban development) or deepwater reefs (bydeep sea mining andtrawling). Near-shore species such asCallorhinchus milii are vulnerable to the effects ofclimate change: stronger storms and warmer seawater are predicted to increase egg mortality by disrupting the stable environments necessary to complete incubation.[13]
In some classifications, the chimaeras are included (as subclassHolocephali) in the classChondrichthyes of cartilaginous fishes; in other systems, this distinction may be raised to the level of class. Chimaeras also have some characteristics ofbony 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.[2] A preliminary study found 8% of species to be threatened.[14] There areover 50 extant species in six genera and three families, with other genera known from fossils. The extant species fall into three families—the Callorhinchidae, Rhinochimaeridae andChimaeridae with the callorhinchids being the most basalclade.
SuborderChimaeroideiPatterson 1965
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.[15]
The group containing chimaeras and their close relatives (Holocephali) is thought to have diverged fromElasmobranchii (the group containing modern sharks and rays) during theDevonian, over 380 million years ago. The oldest known chimaeriform isProtochimaera from the EarlyCarboniferous (338–332 million years ago) of Russia, which is more closely related to modern chimeras (Chimaeroidei) than any other known extinct groups of Chimaeriformes.[16] The earliest known remains attributable to modern chimaeras are known from theEarly Jurassic (Pliensbachian) of Europe, but egg cases from theLate Triassic of Yakutia, Russia and New Zealand[17] that resemble those of rhinochimaerids and callorhinchids respectively indicates that they had a global distribution prior to the end of theTriassic. Unlike modern chimaeras,Mesozoic representatives are often found in shallow water settings.[18] Most modern chimaera groups appear to have originated during theMesozoic Marine Revolution.[19] Modern chimaeras reached their highest ecological diversity during the mid-Cretaceous (Albian toCenomanian), when they acquired a variety of different dentition types.[20][21]
It has commonly been assumed that due to being an evolutionarilybasal group that is largely found in the deep ocean, modern chimaeras likely colonized the deep ocean during the Mesozoic and used it as arefugium to survive mass extinction events. However, more recent studies indicate that chimaeras were likely a shallow-water group for most of their existence, and only colonized the deep ocean in the aftermath of theCretaceous-Paleogene extinction event. Theplough-nosed chimaeras are the only group to still inhabit shallower waters, in the manner of ancestral chimaera groups.[19]
Extinct chimaeriforms include:
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| Forerunners | 
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| Jawless fish |
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| Jawed fish |
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Authority control databases: National |
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