| Paddlefishes | |
|---|---|
 | |
| American paddlefish,Polyodon spathula | |
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| Chinese paddlefish,†Psephurus gladius | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Acipenseriformes |
| Suborder: | Acipenseroidei |
| Family: | Polyodontidae Bonaparte, 1838 |
| Genera | |
Recent genera Fossil genera | |
Paddlefish are members of theray-finned fish familyPolyodontidae, which belong to the orderAcipenseriformes, one of two living groups within this order alongsidesturgeons (Acipenseridae).[3][4] Their most distinctive feature is an elongatedrostrum that enhanceselectroreception, allowing them to detect prey in murky water. Often called "primitive fish," paddlefish represent one of the earliest diverging lineages of ray-finned fish, having separated from all other living groups over 300 million years ago. Both living and fossil paddlefish occur almost exclusively in North America and China.[5]
Eight species are known, six of which survive only as fossils—five from North America and one from China.[2] Of the twoextant species, theAmerican paddlefish (Polyodon spathula) inhabits theMississippi River basin in the United States, while theChinese paddlefish (Psephurus gladius) was declared extinct in 2022 following a 2019 recommendation.[6][7][8] The last confirmed sighting of a Chinese paddlefish occurred in theYangtze River Basin in 2003.[9][10] The Chinese paddlefish is also known as the "Chinese swordfish" or "elephant fish."[11] The earliest known paddlefish fossil,Protopsephurus, dates to approximately 120 million years ago during theearly Cretaceous (Aptian) period in China.
Throughout their historic range, paddlefish populations have declined dramatically due to overfishing, pollution, and human development. Dam construction has proven particularly destructive, blocking seasonal migrations to ancestral spawning grounds. River alterations have changed natural flow patterns, destroying spawning habitat and nursery areas critical to population survival.[12][13]
Paddlefish are among the few extant organisms known to retain anotochord beyond theembryonic stage. Their bodies contain very fewbones, consisting primarily ofcartilage with the notochord serving as a flexiblespine.[14] Both species share several morphological features: aspindle-shaped body with smooth,scaleless skin, aheterocercal tail, and small, poorly developedeyes.[11]
Chinese paddlefish are the largest known paddlefish.[15] It is often stated that the largest Chinese paddlefish measured 23 ft (7.0 m) in length and was estimated to weigh several thousand pounds.[10] However, this considerable length, reported byJohn Treadwell Nichols in 1943 as originating from a Chinese language publication byC. Ping in 1931, may be the result of a translation error.[15] The last known individual of the species, a female caught in 2003, was one of the largest recorded individuals, measured to be 3.63 metres (11.9 ft) long, with a body mass of approximately 200 kilograms (440 lb).[16]
Though the American paddlefish ranks among the largestfreshwater fishes in North America, it falls short of its Chinese cousin's impressive dimensions. American paddlefish commonly exceed 5 ft (1.5 m) in length and 60 lb (27 kg) in weight. The largest specimen on record was speared in 1916 in Okoboji Lake, Iowa.[a][17] This fish measured 7 ft 1 in (2.16 m) long with a girth of 45.5 in (1.16 m).[17] According to a 1969 report by J.R. Harlan and E.B. Speaker inIowa Fish and Fishing, it weighed over 198 lb (90 kg).[18]
The world record (American) paddlefish caught on rod and reel weighed 144 lb (65 kg) and measured 54.25 in (1.378 m) long. Clinton Boldridge caught this fish in a 5-acre pond inAtchison County, Kansas on May 5, 2004.[19][20] This record was broken twice in 2020. On June 28, an Oklahoma man caught a 146 lb (66 kilograms) paddlefish inKeystone Lake, west ofTulsa. Less than a month later on July 23, another Oklahoma angler caught a 151 lb (68 kilograms), nearly 6 ft (1.8 metres) long paddlefish in the same lake.[21]
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The shape of therostrum (elongated snout or "paddle") varies dramatically between the species; theChinese paddlefish possessed a narrow, sword-like rostrum, while the American paddlefish has a broad, paddle-shaped one.[14] During early development from embryo tofry, paddlefish lack their rostrum, which begins forming shortly after hatching.[14]
Scientists initially believed paddlefish used their rostrums to dig throughbottom sediments,[14][22] butelectron microscopy revealed a different purpose entirely; the rostrum's surface is covered withelectroreceptors calledampullae.[23] These ampullae concentrate densely within star-shaped bony projections that branch from the rostrum's bony core.[15] These electroreceptors detect weak electrical fields that signal prey presence in thewater column. Remarkably, they can sense not justzooplankton—the primary food source for American paddlefish—but the individual feeding and swimming movements of zooplanktonappendages.[5][14]
Some sources incorrectly suggest that rostrum damage would severely impair a paddlefish's ability to feed and maintain health, but laboratory experiments and field research demonstrate otherwise; even paddlefish with damaged or severed rostrums can forage effectively. Given their poorly developed eyes, paddlefish rely heavily on electroreception for foraging, though the rostrum is not their only sensory tool: sensory pores cover nearly half the skin surface, extending from the rostrum across the top of the head down to the tips of theoperculum (gill flaps).[5][14]
The two species employed vastly different feeding strategies. Unlike its filter-feeding American relative, the Chinese paddlefish was apiscivore—a highlypredatory fish. Its forward-pointing jaws suggest it hunted primarily for small fish in the water column, supplementing its diet with shrimp,benthic fishes, and crabs.[11][24] In contrast, the American paddlefish jaw is specialized for filter feeding; asram suspension filter feeders, they consume primarilyzooplankton, occasionally taking smallaquatic insects, insect larvae, and small fish.[5]
American paddlefish are native to the Mississippi River basin, ranging from New York to Montana and south to the Gulf of Mexico.[25] They inhabit several Gulf Slope drainages, favoring medium to large rivers with long, deep pools that move slowly, along with backwater lakes and bayous.[26] In Texas, paddlefish historically occurred in theAngelina River,Big Cypress Bayou,Neches River, tributaries of theRed River,Sabine River,San Jacinto River,Sulphur River, andTrinity River.[25]
Their historical range once extended into Canada—specificallyLake Huron andLake Helen—and across 26 to 27 U.S. states. American paddlefish have been extirpated from much of their northern peripheral range, including theGreat Lakes and populations inNew York,Maryland,Pennsylvania, and Canada. TheOntario Ministry of Natural Resources now lists paddlefish as extirpated fromOntario, Canada under their Endangered Species Act.[27] TheIUCN Red List designates Canadian populations as extirpated, noting the last Canadian records date to the early 1900s and the species' presence there was marginal.
Overall, the American paddlefish is classified as vulnerable (VU) on the IUCN Red List. International trade has been restricted since June 1992 under Appendix II of theConvention of International Trade in Endangered Species of Wild Flora and Fauna (CITES).[28] Concerns about remaining populations continue to grow across other states.
The Chinese paddlefish was consideredanadromous, migrating upstream to spawn, though little is known about their migration patterns or population structure. They were endemic to theYangtze River Basin in China, living primarily in the broad main stem rivers and shoal zones along theEast China Sea.[10][29] Research suggests they favored the middle and lower layers of the water column and occasionally entered large lakes.[10]
With no confirmed sightings since 2003, the species was declared extinct in 2019.[7] Past attempts atartificial propagation for restoration failed because captive specimens could not be kept alive.[30]
Paddlefish are long-lived and reachsexual maturity late in life; females do not beginspawning until they reach six to twelve years of age, some not until sixteen to eighteen years old. Males begin spawning around four to seven years of age, with some waiting until nine or ten years.[14][31][4]
Spawning occurs in late spring only when specific environmental conditions align: appropriate water flow, temperature,photoperiod, and the presence of suitable gravel substrates. If any condition is absent, paddlefish will not spawn that year. Females spawn every second or third year, while males spawn more frequently—typically annually or every other year.[14]
To spawn, paddlefishmigrate upstream, seekingsilt-freegravel bars. During normal conditions, these bars would be exposed to air or covered by very shallow water, but springsnowmelt and rainfall raise river levels sufficiently to submerge them.[32] Paddlefish arebroadcast spawners, also known as mass or synchronous spawners, withfertilization occuring externally:gravid females release theireggs over bare rocks or gravel while males simultaneously releasesperm. The adhesive eggs stick to the rocky substrate. After hatching, young paddlefish are swept downstream by the river's flow, growing to adulthood in deep freshwater pools.[33]
Advances in biotechnology have significantly improved paddlefish propagation and captive rearing. Success rates for reproduction, adaptation, and survival of cultured paddlefish have increased substantially, benefiting both broodstock development and stock rehabilitation programs. These improvements have enabled successful reservoir ranching and pond rearing, generating growing interest in the global market for paddlefish polyculture.[34][35]
In the early 1970s, a cooperative scientific effort between theU.S. Fish & Wildlife Service and itsUSSR counterpart brought American paddlefish to the former Soviet Union foraquaculture. The program began with five thousand hatched larvae fromMissouri hatcheries. These fish were introduced into several European and Asian rivers, establishing the first brood stock that successfully reproduced in 1984–1986 in Russia.[36] Paddlefish are now raised inGermany,Austria, theCzech Republic, and thePlovdiv andVidin regions inBulgaria. Reproduction was successful in 1988 and 1989, resulting in the exportation of juvenile paddlefish toRomania andHungary. In May 2006, specimens of various sizes and weights were caught by professional fishermen nearPrahovo in theSerbian part of theDanube River.[36]
In 1988, fertilized paddlefish eggs and larvae from Missouri hatcheries were first introduced into China.[36] Since that time, China imports approximately 4.5 million fertilized eggs and larvae annually from hatcheries in Russia and the United States. Some paddlefish are polycultured in carp ponds and sold to restaurants, while others are cultured for brood stock and caviar production. China has also exported paddlefish to Cuba, where they are farmed for caviar production.[35]
There is one currentlyextant genus in this family, one recentlyextinct genus, and five extinct genera known exclusively fromfossils.
Classification followingGrande & Bemis (1991),[15] withParapsephurus andPugiopsephurus added inHilton et al. (2023):[2]
Relationships of the genera are fromGrande et al. (2002).[37]
| Polyodontidae | |
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