Paddlefish
Paddlefish (family Polyodontidae) are basal Chondrostean ray-finned fish.[2] They have been referred to as "primitive fish" because they have evolved with few morphological changes since the earliest fossil records of the Early Cretaceous, 120 to 125 million years ago.[3] Polyodontids are almost exclusively North American and Chinese, both extant and in the fossil record.[4]
There are six known species: four extinct species known only from fossil remains (three from western North America, one from China), one extant species, the American paddlefish (Polyodon spathula) which is native to the Mississippi River basin in the U.S., and the Chinese paddlefish (Psephurus gladius), reported as likely extinct in 2019.[5] The species was last sighted in 2003 in the Yangtze River Basin in China.[6][7] Chinese paddlefish are also commonly referred to as "Chinese swordfish", or "elephant fish".[8] The earliest known species is Protopsephurus from the Early Cretaceous (Barremian) of China.
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.[9] Other detrimental effects include alterations of rivers which have changed natural flows resulting in the loss of spawning habitat and nursery areas.[10]
Paddlefish as a group are one of the few organisms that retain a notochord past the embryonic stage. Paddlefish have very few bones and their bodies mostly consist of cartilage with the notochord functioning as a soft spine. During the initial stages of development from embryo to fry, paddlefish have no rostrum (snout). It begins to form shortly after hatching.[11] The rostrum of the Chinese paddlefish was narrow and sword-like whereas 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.[8][11] Unlike the filter-feeding American paddlefish, Chinese paddlefish were piscivores, and highly predatory. Their jaws were more forward pointing which suggested they foraged primarily on small fishes in the water column, and occasionally on shrimp, benthic fishes, and crabs.[8][12] The jaws of the American paddlefish are distinctly adapted for filter feeding only.[4] They are ram suspension filter feeders with a diet that consists primarily of zooplankton, and occasionally small insects, insect larvae, and small fish.[4]
