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The American paddlefish (''Polyodon spathula'') is a species of basal ray-finned fish closely related to sturgeons in the order Acipenseriformes. Fossil records of paddlefish date back over 300million years, nearly 50million years before dinosaurs first appeared. American paddlefish are smooth-skinned freshwater fish commonly called paddlefish, but are also referred to as Mississippi paddlefish, spoon-billed cats, or spoonbills. They are one of only two extant species in the paddlefish family, Polyodontidae. The other is the critically endangered Chinese paddlefish (''Psephurus gladius'') endemic to the Yangtze River basin in China. American paddlefish are often referred to as primitive fish, or relict species because they retain some morphological characteristics of their early ancestors, including a skeleton that is almost entirely cartilaginous, a paddle-shaped rostrum (snout) that extends nearly one-third their body length, and a heterocercal tail or caudal fin, much like that of sharks. American paddlefish are a highly derived fish because they have evolved with adaptations such as filter feeding. Their rostrum and cranium are covered with tens of thousands of sensory receptors for locating swarms of zooplankton, which is their primary food source. American paddlefish are native to the Mississippi River basin and once moved freely under the relatively natural, unaltered conditions that existed prior to the early 1900s. They commonly inhabited large, free-flowing rivers, braided channels, backwaters, and oxbow lakes throughout the Mississippi River drainage basin, and adjacent Gulf drainages. Their peripheral range extended into the Great Lakes, with occurrences in Lake Huron and Lake Helen in Canada until about 90 years ago. American paddlefish populations have declined dramatically primarily because of overfishing, habitat destruction, and pollution. Poaching has also been a contributing factor to their decline and will continue to be as long as the demand for caviar remains strong. Naturally occurring American paddlefish populations have been extirpated from most of their peripheral range, as well as from New York, Maryland, Virginia, and Pennsylvania. The current range of American paddlefish has been reduced to the Mississippi and Missouri River tributaries and Mobile Bay drainage basin. They are currently found in twenty-two states in the U.S., and those populations are protected under state, federal and international laws. ==Taxonomy, etymology and evolution== American paddlefish are closely related to sturgeons in the order Acipenseriformes, an order of basal ray-finned fishes that includes sturgeon and paddlefish, several species of which are now extinct. Paddlefish are among the oldest of fishes as evidenced in the fossil record which dates their first appearance approximately 300 to 400million years ago, almost 50million years before the dinosaurs. Fossils of a second extinct species, ''P. tuberculata'', which date back approximately 60million years ago, were found in the Lower Paleocene Tullock Formation in Montana. In 1797, French naturalist Bernard Germain de Lacépède established the genus ''Polyodon'' for paddlefish, which today includes a single extant species, ''Polyodon spathula''. Lacépède disagreed with Pierre Joseph Bonnaterre's description in ''Tableau encyclopédique et méthodique'' (1788), which suggested paddlefish were a species of shark. Lacépède noted, "The country and habits of this fish are still unknown." When Lacépède established the binomial, ''Polydon feuille'', he was unaware the species had already been named in 1772 by taxonomist, Johann Julius Walbaum, who described paddlefish as ''Squalus spathula''. As a result of Lacépède's inadvertent double naming, ''Polyodon spathula'' became the preferred scientific name of American paddlefish, and ''Squalus spathula'' became the synonym as one of two names applied to the group. Walbuam, 1792, is recognized and cited as the authority. The family Polyodontidae comprises five known taxa; three extinct taxa from western North America, and two extant taxa including the American paddlefish (''Polyodon spathula'') native to the Mississippi River Basin in the United States, and the critically endangered Chinese paddlefish (''Psephurus glades'') endemic to the Yangtze River Basin in China.〔 American paddlefish are the only living species in the genus ''Polyodon''. They are often referred to as primitive fish, or relict species, because of morphological characteristics they retain from some of their early ancestors as evidenced in the fossil record which dates them back to the Late Cretaceous, 70 to 75million years ago. Some of their primitive characteristics include a skeleton composed primarily of cartilage, and a deeply forked heterocercal caudal fin similar to that of sharks, although they are not closely related.〔 Fossil paddlefishes with recognizable rostrums date from the Upper Cretaceous and Paleocene periods 65million years ago.〔 An elongated rostrum is a morphological characteristic of Polyodontidae, but only the genus ''Polyodon'' (''P. spathula'' and the extinct ''P. tuberculata'') have characteristics adapted specifically for filter feeding, including the jaw, gill arches, and cranium. The gill rakers of American paddlefish are composed of extensive comb-like filaments believed to have inspired the etymology of the genus name, ''Polyodon'', a Greek compound word meaning "many toothed". Adult American paddlefish are actually toothless, although numerous small teeth less than were found in a juvenile paddlefish measuring . ''Spathula'' references the elongated, paddle shaped snout or rostrum. Compared to Chinese paddlefish and fossil genera, American paddlefish (and by extension, a fossil relative, ''P. tuberculata'') are considered to be a highly derived species because of their novel adaptations. Chinese paddlefish are the closest extant relative of American paddlefish. They are currently listed as critically endangered on the IUCN Red List. Several reports have suggested that Chinese paddlefish may now be extinct.〔 The primary reasons for their decline are similar to those of American paddlefish and include overfishing, the construction of dams, and destruction of habitat. Unlike the planktivorous American paddlefish, Chinese paddlefish are strong swimmers, grow larger, and are opportunistic piscivores that feed on small fishes and crustaceans. Some distinct morphological differences of Chinese paddlefish include a narrower, sword-like rostrum, and a protrusible mouth. They also have fewer, thicker gill rakers than American paddlefish.〔〔 The last confirmed sighting of a live Chinese paddlefish was made on the Yangtzee River on January 24, 2003. From 2006 to 2008, scientists conducted surveys in an effort to locate the fish. They used several boats, deployed 4762 setlines, 111 anchored setlines and 950 drift nets covering on the upper Yangtze River, most of which lies within the protected area of the Upper Yangtze National Nature Reserve. They did not catch a single fish. They also used hydroacoustic equipment to monitor active sound in water (sonar), but were unable to confirm the presence of paddlefish.〔 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「American paddlefish」の詳細全文を読む スポンサード リンク
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