Chimaera

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-Recent[1]
Hydrolagus colliei
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Holocephali
Order: Chimaeriformes
Obruchev, 1953
Families

Callorhinchidae
Chimaeridae
Rhinochimaeridae

Chimaeras[1] are cartilaginous fishes in the order Chimaeriformes, known informally as ghost sharks, rat fish (not to be confused with the rattails), spookfish (not to be confused with the true spookfish of the family Opisthoproctidae), or rabbit fish (not to be confused with the true rabbitfishes of the family Siganidae).

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

Description and habits

Chimaera egg
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 largely covered by placoid 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. They also use electroreception to locate 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 three families are described; an additional three genera and two families are only known from fossils):

Callorhinchus callorynchus
Chimaera monstrosa
Hydrolagus affinis
Harriotta raleighana

Phylogenetics

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.)

See also

References

  1. 1 2 3 Froese, Rainer, and Daniel Pauly, eds. (2014). "Chimaeriformes" in FishBase. November 2014 version.
  2. 1 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, H. & Dagit, D.D. (2004): Husbandry of Spotted Ratfish, Hydrolagus colliei., Chapter 33 in: Smith, M., D. Warmolts, D. Thoney, & R. Hueter (editors). Elasmobranch Husbandry Manual: Captive Care of Sharks, Rays, and their Relatives. Ohio Biological Survey, Inc.
  5. 1 2 3 Stevens, J.; Last, P.R. (1998). Paxton, J.R.; Eschmeyer, W.N., eds. Encyclopedia of Fishes. San Diego: Academic Press. p. 69. ISBN 0-12-547665-5.
  6. ↑ T.H. Bullock; R.H. Hartline; A.J. Kalmijn; P. Laurent; R.W. Murray; H. Scheich; E. Schwartz; T. Szabo (6 December 2012). Electroreceptors and Other Specialized Receptors in Lower Vertrebrates. Springer Science & Business Media. p. 125. ISBN 978-3-642-65926-3.
  7. ↑ Freaky New Ghostshark ID’d Off California Coast, a September 22, 2009 blog post from Wired Science
  8. ↑ Inoue, J.G., Miya, M., Lam, K., Tay, B.H., Danks, J.A., Bell, J., Walker, T.I. Venkatesh, B. (2010): Evolutionary origin and phylogeny of the modern holocephalans (Chondrichthyes: Chimaeriformes): A mitogenomic perspective. Molecular Biology and Evolution, 27 (11): 2576-2586.
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