Chimaera
Chimaeras Temporal range:
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Hydrolagus colliei (Chimaeridae )
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Chondrichthyes |
Subclass: | Holocephali |
Order: | Chimaeriformes Obruchev, 1953 |
Subgroups | |
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Chimaeras
At one time a "diverse and abundant" group (based on the
Anatomy
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 class
The pectoral fins are large enough to generate lift and a relaxed forward momentum, giving the chimaera the appearance of "flying" through the water. Further back on the body are also a pair of smaller
In many species, the bulbous snout is modified into an elongated sensory organ, capable of electroreception to find prey.[5][6] The cartilaginous skull is holostylic, meaning that the palatoquadrate (upper jaw cartilage) is completely fused to the neurocranium (cranial cartilage). This contrasts with modern sharks, where the palatoquadrate is movable and detachable, a trait known as hyostyly. 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 the incisor teeth of rodents and lagomorphs (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 soft osteodentin, but the active edges are supplemented by a unique hypermineralized tissue called pleromin. Pleromin is an extremely hard enamel-like tissue, arranged into sheets or beaded rods, but it is deposited by mesenchyme-derived cells similar to those that form bone. In addition, pleuromin's hardness is due to the mineral whitlockite, which crystalizes within the teeth as the animal matures. Other vertebrates with hypermineralized teeth rely on enamel, which is derived from ameloblasts and encases round crystals of the mineral apatite.[7]
Chimaeras also differ from sharks in that they have separate
Behavior
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
Diet
The usual diet of chimaeras consist of
Reproduction
Chimaera reproduction resembles that of sharks in some ways: males employ
Unlike sharks, male chimaeras have retractable sexual appendages (known as tentacula) to assist mating.[11][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]
Parasites
As other fish, chimaeras have a number of
Conservation and threats
Despite their secluded habits, some chimaera species may be threatened by overfishing through bycatch or commercial exploitation. No species are listed as Endangered according to the IUCN, but four are listed as Vulnerable, four more as Near Threatened, and many more as Data 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.[12]
Several near-shore species are purposefully caught for their meat, especially callorhinchids, Hydrolagus bemisi (
Another threat is habitat destruction of coastal nurseries (by urban development) or deepwater reefs (by deep sea mining and trawling). Near-shore species such as Callorhinchus milii are vulnerable to the effects of climate change: stronger storms and warmer seawater are predicted to increase egg mortality by disrupting the stable environments necessary to complete incubation.[12]
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.
Suborder Chimaeroidei Patterson 1965
- Family Callorhinchidae Garman, 1901
- Genus Callorhinchus Lacépède, 1798 (3 extant species) Mid-Cretaceous–recent
- Family Chimaeridae Bonaparte, 1831
- Genus Chimaera Linnaeus, 1758 (16 species) Eocene–recent
- Genus Hydrolagus Gill, 1863 (26 species) Miocene–recent
- Family Rhinochimaeridae Garman, 1901
- Genus Harriotta Goode & Bean, 1895 (2 species)
- Genus Neoharriotta Bigelow & Schroeder, 1950 (3 species)
- Genus Rhinochimaera Garman, 1901 (3 species)
Evolution
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.[14]
The group containing chimaeras and their close relatives (Holocephali) is thought to have diverged from Elasmobranchii (the group containing modern sharks and rays) during the Devonian, over 380 million years ago. The oldest known chimaeriform is Protochimaera from the Early Carboniferous (338-332 million years ago) of Russia, which is more closely related to modern chimeras (Chimaeroidei) than any other known extinct groups of Chimaeriformes.[15] The earliest known remains attributable to modern chimaeras are known from the Early Jurassic (Pliensbachian) of Europe, but egg cases from the Late Triassic of Yakutia, Russia and New Zealand[16] 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.[17]
Extinct chimaeriforms include:
- †Genus Echinochimaera Lund, 1977 United States, Lower Carboniferous (Serpukhovian)
- †Genus Protochimaera Lebedev & Popov in Lebedev et al., 2021 Moscow Region, Russia, Lower Carboniferous (Viséan–Serpukhovian)[15]
- †Genus Squaloraja Europe, Early Jurassic (Hettangian–Sinemurian)
- †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)
- †Family Chimaeropsidae
- Suborder Chimaeroidei Patterson 1965
- †Eomanodon Ward and Duffin 1989 United Kingdom, Early Jurassic (Pliensbachian)
- Family Callorhinchidae Garman, 1901
- †Brachymylus A. S. Woodward 1894 Germany, Early Jurassic (Pliensbachian)
- †Bathytheristes Duffin 1995 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
- †Ottangodus Popov, Delsate & Felten, 2019 France, Middle Jurassic (Bajocian)
- †Moskovirhynchus Russia, Upper Jurassic
- †Pachymylus United Kingdom, France, Middle Jurassic
- Family †"Edaphodontidae"
- †Ischyodus (40 species) Worldwide, Middle Jurassic–Miocene (also placed in Callorhinchidae)
- †Elasmodectes Europe, Jurassic–Cretaceous
- †Elasmodus Worldwide, Cretaceous–Paleogene
- †Edaphodon Worldwide, Cretaceous–Neogene
- †Ptyktoptychion Australia, Early Cretaceous
- †Lebediodon Europe, Cretaceous
- Family Chimaeridae Bonaparte, 1831
- †Canadodus Popov, Johns & Suntok, 2020 Sooke Formation, Canada, Oligocene
- Family Rhinochimaeridae Garman, 1901
- †Amylodon Europe, Late Cretaceous–Oligocene
See also
- List of prehistoric cartilaginous fish
- List of chimaeras
- Acanthothoraci
- Ptyctodontida
References
- ^ a b Froese, Rainer, and Daniel Pauly, eds. (2014). "Chimaeriformes" in FishBase. November 2014 version.
- ^ a b "Ancient And Bizarre Fish Discovered: New Species Of Ghostshark From California And Baja California". ScienceDaily. September 23, 2009. Retrieved 2009-09-23.
- ISBN 0-618-00212-X. Retrieved 9 August 2015.
- ^ ISBN 978-1-4398-3924-9
- ^ ISBN 0-12-547665-5.
- ISBN 978-3-642-65926-3.
- PMID 33122684.
- ]
- PMID 34438648.
- PMID 36649436– via CrossRef.
- ^ Madrigal, Alexis (22 September 2009). "Freaky New Ghostshark ID'd Off California Coast". Wired. Retrieved 14 November 2018.
... 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. ...
- ^ S2CID 229433827.
- S2CID 229433827.
- PMID 20551041. Retrieved 14 November 2018.
- ^ S2CID 239509836.
- S2CID 128402250.
- S2CID 198423356.