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Chimaera

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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: Early Devonian-Present[1]
Hydrolagus colliei
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: Chimaeriformes
Obruchev, 1953
Families

Callorhinchidae
Chimaeridae
Rhinochimaeridae

Chimaeras[1] 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.[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 relatively shallow depths. Consequently, these are also among the few species from the chimaera order kept in public aquaria.[4] 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 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.

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 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.[5]

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] They 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

Phylogenetics

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.[8]

The order appears to have originated about 420 million years ago during the Silurian. The 39 extant species fall into three families—the Callorhinchids, Rhinochimaerids and Chimaerids with the callorhinchids being the most basal clade. The families appear to have diverged during the late Jurassic to early Cretaceous (170–120 mya).

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.

See also

References

  1. ^ a b c Froese, Rainer; Pauly, Daniel (eds.). "Order Chimaeriformes". FishBase. November 2014 version.
  2. ^ a b "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. ^ a b c 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. ^ 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.
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