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Paddlefish (family Polyodontidae) are basal Chondrostean ray-finned fish. They have been referred to as "primitive fish" because they have evolved with few morphological changes since the earliest fossil records of the Late Cretaceous, seventy to seventy-five million years ago.〔(【引用サイトリンク】 title=INHS padfish )〕 Polyodontids are exclusively North American with the exception of the genus ''Psephurus'', which includes solely the Chinese paddlefish (''Psephurus gladius''). There are five known taxa: three extinct taxa from western North America, and two extant taxa, including the American paddlefish (''Polyodon spathula'') which is endemic to the Mississippi River basin in the U.S. and the critically endangered Chinese paddlefish (''Psephurus glades'') which is endemic to the Yangtze River Basin in China. Chinese paddlefish are also commonly referred to as "Chinese swordfish", or "elephant fish".〔(【引用サイトリンク】 title=Chinese Paddlefish )〕 Paddlefish populations have declined dramatically throughout their historic range as a result of overfishing, pollution, and the encroachment of human development, including the construction of dams that have blocked their seasonal upward migration to ancestral spawning grounds. Other detrimental effects include alterations of rivers which have changed natural flows resulting in the loss of spawning habitat and nursery areas. Chinese paddlefish have not been seen in the wild since 2003, and may now be extinct for many of the same reasons that have plagued the American species.〔(【引用サイトリンク】title=Chinese Paddlefish )〕 == Morphology == During the initial stages of development from embryo to fry, paddlefish have no rostrum (snout). It begins to form shortly after hatching. The rostrum of a Chinese paddlefish is narrow and sword-like while the rostrum of the American paddlefish is broad and paddle-like. Some common morphological characteristics of paddlefish include a spindle-shaped, smooth skinned scaleless body, heterocercal tail, and small poorly developed eyes.〔〔 Unlike the filter feeding American paddlefish, Chinese paddlefish are piscivores, and highly predaceous. Their jaws are more forward pointing which suggest they forage primarily on small fishes in the water column, and occasionally on shrimp, benthic fishes, and crabs.〔 The jaws of the American paddlefish are distinctly adapted for filter feeding only.〔 They are ram suspension filter feeders with a diet that consists primarily of zooplankton, and occasionally small insects, insect larvae, and small fish.〔 The largest Chinese paddlefish on record measured in length, and weighed over several thousand pounds. They commonly reach and .〔〔 Although the American paddlefish is one of the largest freshwater fishes in North America, their recorded lengths and weights fall short in comparison to the larger Chinese paddlefish. American paddlefish commonly reach or more in length and can weigh more than . The largest American paddlefish on record weighed , and was caught by Clinton Boldridge in the Atchison Watershed in Kansas. Scientists once believed paddlefish used their rostrums to excavate bottom substrate,〔 but have since determined with the aid of electron microscopy that paddlefish have electroreceptors on their rostrum's ampulla (hair cells) which are similar in structure to other Lorenzini. The electroreceptors can detect weak electrical fields which not only signal the presence of prey items in the water column, such as zooplankton which is the primary diet of the American paddlefish, but they can also detect the individual feeding and swimming movements of zooplankton's appendages.〔〔 Paddlefish have poorly developed eyes, and rely on their electroreceptors for foraging. However, the rostrum is not the paddlefish's sole means of food detection. Some reports incorrectly suggest that a damaged rostrum would render paddlefish less capable of foraging efficiently to maintain good health. Laboratory experiments, and field research indicate otherwise. In addition to electroreceptors on the rostrum, paddlefish also have sensory pores covering nearly half of the skin surface extending from the rostrum to the top of the head down to the tips of the operculum (gill flaps). Therefore, paddlefish with damaged or abbreviated rostrums are still able to forage and maintain good health.〔〔 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「paddlefish」の詳細全文を読む スポンサード リンク
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