Chimaeras[ are Chondrichthyes in the order Chimaeriformes (), known informally as ghost sharks, rat fish (not to be confused with ), spookfish, or rabbit fish; the last two names are also applied, respectively, to the Actinopterygii groups of Barreleye and Rabbitfish.
]
At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are and rays, though their last common ancestor with them lived nearly 400 million years ago.
Anatomy
Chimaeras are soft-bodied, shark-like fish with bulky heads and long, tapered tails; measured from the tail, they can grow up to in length. Like other members of the class Chondrichthyes, chimaera skeletons are entirely cartilaginous, or composed of cartilage. Males use forehead denticles to grasp a female by a fin during copulation.
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 smaller Pelvic fin, and some genera bear an anal fin in front of the tail. In Chimaeridae and Rhinochimaeridae, the tail is leptocercal, meaning that it is thin and whip-like, edged from above and below by fins of similar size. In Callorhinchus, the tail is instead heterocercal, 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 a spine on the front edge of the dorsal fin.
In many species, the bulbous snout is modified into an elongated sensory organ, capable of electroreception to find prey.
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 Rodent and Lagomorpha (hence the name "rabbit fish").
Chimaeras also differ from sharks in that they have separate Anus and urogenital openings.
Behavior
Chimaeras live in temperate ocean floors, with some species inhabiting depths exceeding ,
Diet
The usual diet of chimaeras consists of Crustacean, brittle stars, and molluscs.
Reproduction
Chimaera reproduction resembles that of sharks in some ways: males employ Clasper for internal fertilization of females and females lay eggs within spindle-shaped, leathery egg cases.
Unlike sharks, male chimaeras have retractable sexual appendages (known as tentacula) to assist mating.
Parasites
As other fish, chimaeras have a number of fish parasites. Chimaericola leptogaster (Chimaericolidae) is a parasite of the of Chimaera monstrosa; the species can attain in length.
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.
Several near-shore species are purposefully caught for their meat, especially callorhinchids, Hydrolagus bemisi (pale ghost shark), and Hydrolagus novaezealandiae (Dark ghostshark). Modern quotas have helped to moderate collection of these species to a sustainable level, though Callorhinchus milii (the Australian ghostshark) experienced severe overfishing in the 20th century before protections were enacted. Neoharriotta pinnata (sicklefin chimaera) is targeted along the coast of India 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), and Hydrolagus 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 the Shark finning, which threatens many true sharks.
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.
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 Osteichthyes.
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
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.
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.
It has commonly been assumed that due to being an evolutionarily basal group that is largely found in the deep ocean, modern chimaeras likely colonized the deep ocean during the Mesozoic and used it as a refugium 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 the Cretaceous-Paleogene extinction event. The Callorhinchus are the only group to still inhabit shallower waters, in the manner of ancestral chimaera groups.
Taxonomy
Extinct chimaeriforms include:
-
†Suborder Echinochimaeroidei Lund, 1977
-
†Family Echinochimaeridae Lund, 1977
-
†Suborder Squalorajoidei Patterson, 1965 (Lower Carboniferous–Early Jurassic)
-
?†Genus Sulcacanthus Itano & Duffin, 2023 United States, Lower Carboniferous (Viséan)
-
Family †Squaloraja Woodward, 1886
-
†Suborder Myriacanthoidei Patterson 1965 (Late Triassic–Late Jurassic; possible Carboniferous records)
-
†Family Chimaeropsidae
-
† Chimaeropsis Zittel 1887 Belgium, Early Jurassic (Sinemurian), Germany, Late Jurassic
-
†Family Myriacanthidae Woodward 1889
-
† Acanthorhina Fraas 1910 Posidonia Shale, 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)
-
†Suborder Protochimaeroidei Lebedev & Popov in Lebedev et al., 2021
-
Suborder Chimaeroidei Patterson 1965
See also
