|
The frilled shark (''Chlamydoselachus anguineus'') is one of two extant species of shark in the family Chlamydoselachidae, with a wide but patchy distribution in the Atlantic and Pacific Oceans. This species is found over the outer continental shelf and upper continental slope, generally near the bottom, though there is evidence of substantial upward movements. It has been caught as deep as , although it is uncommon below .〔Dianne J. Bray, 2011, Frill Shark, Chlamydoselachus anguineus, in Fishes of Australia, accessed 07 Oct 2014, http://www.fishesofaustralia.net.au/home/species/3419〕 In Suruga Bay, Japan, it is most common at depths of . Exhibiting several "primitive" features, the frilled shark has often been termed a "living fossil". It reaches a length of and has a dark brown, eel-like body with the dorsal, pelvic, and anal fins placed far back. Its common name comes from the frilly or fringed appearance of its six pairs of gill slits, with the first pair meeting across the throat. Seldom observed, the frilled shark may capture prey by bending its body and lunging forward like a snake. The long, extremely flexible jaws enable it to swallow prey whole, while its many rows of small, needle-like teeth make it difficult for the prey to escape. It feeds mainly on cephalopods, leavened by bony fishes and other sharks. This species is aplacental viviparous: the embryos emerge from their egg capsules inside the mother's uterus, where they survive primarily on yolk. The gestation period may be as long as three and a half years, the longest of any vertebrate. Litter sizes vary from two to fifteen, and there is no distinct breeding season. Frilled sharks are occasional bycatch in commercial fisheries, but have little economic value. The International Union for Conservation of Nature has assessed it as Near Threatened, since even incidental catches may deplete its population given its low reproductive rate. This shark, or a supposed giant relative, is a suggested source for reports of sea serpents. ==Taxonomy and phylogeny== The frilled shark was first scientifically recognized by German ichthyologist Ludwig Döderlein, who visited Japan between 1879 and 1881 and brought two specimens to Vienna. However, his manuscript describing the species was lost, so the first description of the frilled shark was authored by American zoologist Samuel Garman, working from a 1.5-m-long female caught from Sagami Bay in Japan. His account, entitled "An Extraordinary Shark", was published in an 1884 volume of ''Proceedings of the Essex Institute''.〔〔 Garman placed the new species in its own genus and family, and gave it the name ''Chlamydoselachus anguineus'' from the Greek ''chlamy'' ("frill") and ''selachus'' ("shark"), and the Latin ''anguineus'' for "eel-like".〔 Other common names for this species include frill shark, lizard shark, scaffold shark, and silk shark.〔〔 Several early authors believed the frilled shark to be a living representative of otherwise long-extinct groups of elasmobranchs (sharks, rays, and their ancestors), based on its multiple-pointed teeth, the articulation of its upper jaw directly to the cranium behind the eyes (called "amphistyly"), and its "notochord-like" spinal column with indistinct vertebrae.〔 Garman proposed that it was allied with the "cladodonts", a now-obsolete taxonomic grouping containing forms that thrived during the Palaeozoic era, such as ''Cladoselache'' from the Devonian period (416–359 Mya). His contemporaries Theodore Gill and Edward Drinker Cope suggested it was instead related to the hybodonts, which were the dominant sharks during the Mesozoic era. Cope went as far as to assign this species to the fossil genus ''Didymodus''.〔〔 More recent investigations have found the similarities between the frilled shark and extinct groups may have been overstated or misinterpreted, and this shark exhibits a number of skeletal and muscular traits that firmly place it with the neoselachians (modern sharks and rays), and more specifically with the cow sharks in the order Hexanchiformes (though systematist Shigeru Shirai has proposed that it be placed in its own order, Chlamydoselachiformes).〔〔 Nevertheless, the frilled shark belongs to one of the oldest still-extant shark lineages, dating back to at least the Late Cretaceous (about 95 Mya) and possibly to the Late Jurassic (150 Mya).〔 Because of their ancient ancestry and "primitive" characteristics, the frilled shark and other members of this lineage have been described as a "living fossil".〔 However, the frilled shark itself is a relatively recent species, with the earliest known fossil teeth belonging to this species dating to the early Pleistocene epoch.〔Marsili, S. (2007). Analisi sistematica, paleoecologica e paleobiogeografica della selaciofauna plio-pleistocenica del Mediterraneo.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「frilled shark」の詳細全文を読む スポンサード リンク
|