Frilled shark

The zoologist Ludwig Döderlein first identified, described, and classified the frilled shark as a discrete species of shark. After three years (1879–1881) of marine research in Japan, Döderlein took two specimen sharks to Vienna, but lost the taxonomic manuscript of the research. Three years later, in the Bulletin of the Essex Institute (vol. XVI, 1884) the zoologist Samuel Garman published the first taxonomy of the frilled shark, based upon his observations, measurements, and descriptions of a –long female shark from Sagami Bay, Japan. In the article "An Extraordinary Shark" Garman classified the new species of shark within its own genus and family, and named it Chlamydoselachus anguineus (eel-like shark with frills). Initially, marine scientists considered the frilled shark a living, evolutionary representative of extinct groups of elasmobranchs (rays, sharks, skates, sawfish), because the shark's body featured primitive anatomic traits, such as long jaws with trident-shaped, multi-cusp teeth; amphistyly, the direct articulation of the jaws to the cranium, at a point behind the eyes; and a quasi-cartilaginous notochord (a proto-spinal-column) composed of indistinct vertebrae. Nonetheless, the systematic biologist Shigeru Shirai proposed the Chlamydoselachiformes taxonomic order exclusively for the C. anguinesis and the C. africana species of frilled sharks. In 2009, marine biologists identified, described, and classified the Chlamydoselachus africana (southern African frilled shark) of the Atlantic waters of southern Angola and of southern Namibia as a species of frilled shark different from the Chlamydoselachus anguineus identified in 1884. ==Habitat and distribution==

habitat, in the western Atlantic Ocean (2004). The habitats of the frilled shark include the waters of the outer continental shelf and the upper-to-middle continental slope, favoring upwellings and other biologically productive areas. Usually, the shiver lives close to the ocean floor, yet its diet of cephalopods, smaller sharks, and bony fish, indicates that the frilled shark practices diel vertical migration, and swims up to feed at night at the surface of the ocean. In their Atlantic- and Pacific-ocean habitats, frilled sharks practice spatial segregation determined by the individual size, the sex, and the reproductive condition of each shark in the shiver. In Suruga Bay, on the Pacific coast of Honshu, Japan, the frilled shark is most common at the depth of , except in the August-to-November period, when the temperature at the water-layer exceeds , and then the sharks swim into deeper, cooler water. In the eastern Atlantic Ocean, the frilled shark occurs off northern Norway, northern Scotland, and western Ireland, ranging from France to Morocco, the archipelago of Madeira, and the coast of Mauritania, in northwest Africa. In the central Atlantic Ocean, the frilled shark has been caught along the region of the Mid-Atlantic Ridge, ranging from north of the Azores islands to the Rio Grande Rise, off southern Brazil, and the Vavilov Ridge, off West Africa. Frilled sharks tend to be very solitary organisms; interacting with multiple individuals of their kind is rare. However, in the late 2000s a large capture was made over an underwater seamount of the Mid-Atlantic Ridge, hauling in over 30 frilled sharks. The mass capture of a wide variety of male and female specimens emphasized these seamounts as a location for the mating of the species. In the western Atlantic, the frilled shark occurs in the waters of New England and Georgia, in the US, and in the waters of Suriname, in the northeastern coast of South America. In the western Pacific Ocean, the frilled shark ranges from southeastern Honshu, Japan, north to Taiwan, off the coast of China, to the coast of New South Wales, Australia, and the islands of Tasmania and New Zealand. In the central and eastern Pacific Ocean, the frilled shark occurs in the regional waters of Hawaii and the coast of California, in the US, and the northern coast of Chile, in western South America. Although it has been caught at the depth of , the frilled shark usually does not occur deeper than . ==Description==

The eel-like bodies of C. anguineus and C. africana differ anatomically; C. anguineus has a longer head and shorter gill slits, a spinal column with more vertebrae (160–171 vs. 147), and a lower-intestine spiral valve with more turns (35–49 vs. 26–28) than does C. africana. The recorded, maximum body-length of a male frilled shark is , and the recorded, maximum body-length of a female frilled shark is . The head of the frilled shark is broad and flat, with a short, rounded snout. The nostrils are vertical slits, separated by a flap of skin that forms the incurrent opening and the excurrent opening. The moderately large eyes are horizontal ellipsoids, which have no nictitating membrane, which is a protective, third eyelid. Ligaments articulate the long jaws to the cranium, and the corners of the mouth have neither furrows nor folds. The jaws contain 300 trident-shaped teeth, each needle-tooth has a cusp and two cusplets; the rows of teeth are widely spaced, with 19–28 tooth rows in the upper jaw, and 21–29 tooth rows in the lower jaw. Frilled sharks are able to open their jaws and devour food sources that are considerably larger than their own body size, a physical trait that is also present in gulper eels and viperfish. At the throat, there are six pairs of long gill slits; the first pair of gill slits form a collar, while the extended tips of the gill filaments create a fleshy frill, hence, the frilled shark name of this fish. The pectoral fins are short and rounded; the single, small dorsal fin has a rounded margin, and is positioned at the far end of the body, approximately opposite the anal fin. The pelvic and the anal fins are large, broad and rounded, and are positioned to the tail-end of the frilled shark's body. The very long caudal fin is a triangular tail that has neither a lower lobe nor a ventral notch in the upper lobe, and has a margin equipped with sharp, chisel-shaped dermal denticles, which the shark can enlarge. The underside of the shark's eel-like body features a pair of long, thick folds of skin, separated by a groove, which run the length of the belly; the function of the ventral skin-folds is unknown. In the female frilled shark, the mid-section of the body is longer, with the pelvic fins located closer to the anal fin. ==Biology and ecology==

A cartilaginous skeleton and a large liver (filled with low-density lipids) are the mechanical means with which the frilled shark controls and maintains its buoyancy in the deep waters of the ocean. which suggested that the sharks lived inland, in shallow bodies of water far from the ocean. The shallow-water frilled shark had larger, stronger teeth, suitable for eating mollusks; scarcity and plenty of food are indicated in the tooth's morphology of sharper points (cusps) oriented into the mouth. From the Late Paleocene epoch (66–56 mya) until the contemporary era, other species of sharks out-matched the Chlamydoselachus sharks in competition for feeding grounds and living space, which restricted their geographic distribution to the deep-water ocean. Regarding the frilled shark's survival of the mass-extinction event which occurred at the Cretaceous–Paleogene time-boundary, one hypothesis proposes that the sharks survived in bodies of shallow water, both inland and on the continental shelf; afterwards, the frilled shark migrated to deep-water habitats. Diet The frilled shark eats a diet of cephalopods, Nudibranchs, smaller sharks, and bony fish; In hunting and eating prey that are tired or exhausted or dying (after spawn), When the embryo is long, the mother shark expels the egg capsule, at which developmental stage the frilled shark's external gills are developed. Throughout embryonic development, the size of the yolk sac remains constant, until the shark embryo is long, whereupon the sac shrinks until disappearing when the embryo has grown to in length. In the course of pregnancy, the embryo's average rate-of-growth is per month until birth, when the shark pups are long, therefore, the frilled shark's gestation period can be as long as 3.5 years; at birth, a frilled shark's litter comprises 2–15 pups, with an average litter comprises 6.0 pups. ==Human interactions==

In pursuit of food, the frilled shark usually is a bycatch of commercial fishing, accidentally caught in the nets used for trawl-, gillnet-, and longline-fishing. In 2016, consequent to the depletion of food sources caused by commercial overfishing of the feeding areas of the shark's deep-water habitat, and because of the shark's slow rate of reproduction, the International Union for the Conservation of Nature (IUCN) classified the frilled shark as a fish species under near-threat of extinction, and then reclassified it as a species of Least Concern of extinction. ==See also==