Chimaera

Chimaeras[1] are cartilaginous fish in the order Chimaeriformes /kɪˈmɛrɪfɔːrmiːz/, 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.

This article is about the cartilaginous fish order. For the namesake genus, see Chimaera (genus). For the mythological beast, see Chimera (mythology). For other uses, see Chimera.
"Ghost shark" redirects here. For the film, see Ghost Shark (film).

Chimaeras
Temporal range:
Hydrolagus colliei
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: Chimaeriformes
Obruchev, 1953
Families

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.[2] Today, they are largely confined to deep water.[3]

Description and habits
Chimaera egg case
Deep-sea chimaera photographed by the NOAAS Okeanos Explorer. Visible on its snout are tiny pores which lead to electroreceptor cells.

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.[4] They live in all the oceans except for the Arctic and Antarctic oceans.

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.[5]

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. 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.[6] However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead (a type of tentaculum)[7] and in front of the pelvic fins.[5] The females lay eggs in spindle-shaped, leathery egg 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 also have gill covers or opercula like bony fishes.[5] Instead of sharks' many sharp and replaceable teeth, they have just three pairs of large permanent grinding tooth plates. These are unique among vertebrates due to their mineralization. They are made of osteodentin and is covered by an enamel called pleromin formed by mesenchyme-derived cells and made by the mineral whitlockite. Most vertebrates have an enamel of apatite.[8]

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.[9]

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.[10] 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[11] 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.[12] 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 (2.0 in) in length.

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.[2] A preliminary study found 8% of species to be threatened.[13] 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):

Callorhinchus callorynchus
Chimaera monstrosa
Hydrolagus affinis
Harriotta raleighana

See also

References
  1. Froese, Rainer, and Daniel Pauly, eds. (2014). "Chimaeriformes" in FishBase. November 2014 version.
  2. "Ancient And Bizarre Fish Discovered: New Species Of Ghostshark From California And Baja California". ScienceDaily. September 23, 2009. Retrieved 2009-09-23.
  3. Peterson, Roger Tory; Eschmeyer, William N.; Herald, Earl S. (1 September 1999). A Field Guide to Pacific Coast Fishes: North America. Houghton Mifflin Harcourt. p. 13. ISBN 0-618-00212-X. Retrieved 9 August 2015.
  4. Tozer, Helen; Dagit, Dominique D. (2004). "Chapter 33: Husbandry of Spotted Ratfish, Hydrolagus colliei" (PDF). In Smith, Mark; Warmolts, Doug; Thoney, Dennis; Heuter, Robert (eds.). Elasmobranch Husbandry Manual: Captive Care of Sharks, Rays, and their Relatives. Ohio Biological Survey. pp. 487–491. ISBN 0-86727-152-3.
  5. Stevens, John; Last, Peter R. (1998). Paxton, John R.; Eschmeyer, William N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. p. 69. ISBN 0-12-547665-5.
  6. Bullock, T. H.; Hartline, R. H.; Kalmijn, A. J.; Laurent, P.; Murray, R. W.; Scheich, H.; Schwartz, E.; Szabo, T. (6 December 2012). Fessard, A. (ed.). Electroreceptors and Other Specialized Receptors in Lower Vertebrates. Springer Science & Business Media. p. 125. ISBN 978-3-642-65926-3.
  7. 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. ...
  8. A unique mineralization mode of hypermineralized pleromin in the tooth plate of Chimaera phantasma contributes to its microhardness
  9. Inoue, Jun G.; Miya, Masaki; Lam, Kevin; Tay, Boon-Hui; Danks, Janine A.; Bell, Justin; Walker, Terrence I.; Venkatesh, Byrappa (November 2010). "Evolutionary Origin and Phylogeny of the Modern Holocephalans (Chondrichthyes: Chimaeriformes): A Mitogenomic Perspective". Molecular Biology and Evolution. 27 (11): 2576–2586. doi:10.1093/molbev/msq147. PMID 20551041. Retrieved 14 November 2018.
  10. Lebedev, Oleg A.; Popov, Evgeny V.; Bagirov, Sergey V.; Bolshiyanov, Igor P.; Kadyrov, Rail I.; Statsenko, Evgeny O. (2021-10-21). "The earliest chimaeriform fish from the Carboniferous of Central Russia". Journal of Systematic Palaeontology. 19 (12): 821–846. doi:10.1080/14772019.2021.1977732. ISSN 1477-2019. S2CID 239509836.
  11. Gottfried, Michael D.; Fordyce, R. Ewan (2015-05-04). "A Late Triassic chimaeroid egg capsule from New Zealand: early evidence of chimaeroid reproductive mode from the eastern margin of Gondwana". Journal of Systematic Palaeontology. 13 (5): 371–375. doi:10.1080/14772019.2014.880752. ISSN 1477-2019. S2CID 128402250.
  12. Popov, Evgeny V.; Delsate, Dominique; Felten, Roland (2019-07-02). "A New Callorhinchid Genus (Holocephali, Chimaeroidei) from the Early Bajocian of Ottange-Rumelange, on the Luxembourg-French Border". Paleontological Research. 23 (3): 220. doi:10.2517/2018PR021. ISSN 1342-8144. S2CID 198423356.
  13. Finucci, Brittany; Cheok, Jessica; Ebert, David A.; Herman, Katelyn; Kyne, Peter M.; Dulvy, Nicholas K. (2021). "Ghosts of the deep – Biodiversity, fisheries, and extinction risk of ghost sharks". Fish and Fisheries. 22 (2): 391–412. doi:10.1111/faf.12526. ISSN 1467-2979. S2CID 229433827.
  14. Iglésias, S.P., Kemper, J.M. & Naylor, G.J.P. Chimaera compacta, a new species from southern Indian Ocean, and an estimate of phylogenetic relationships within the genus Chimaera (Chondrichthyes: Chimaeridae). Ichthyol Res 69, 31–45 (2022). https://doi.org/10.1007/s10228-021-00810-9
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