Paddlefish

Paddlefish (family Polyodontidae) are a family of ray-finned fish belonging to the order Acipenseriformes, one of two living groups within this order alongside sturgeons (Acipenseridae).<ref name=Crow2012>Template:Cite journal</ref><ref name=TexasPWD-sp-dsc>Template:Cite web</ref> Their most distinctive feature is an elongated rostrum that enhances electroreception, 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.<ref name=Wilkens-Hofmann-2007>Template:Cite journal</ref>

Eight species are known, six of which survive only as fossils—five from North America and one from China.<ref name=Hilton2023> Template:Cite journal </ref> Of the two extant species, the American paddlefish (Polyodon spathula) inhabits the Mississippi River basin in the United States, while the Chinese paddlefish (Psephurus gladius) was declared extinct in 2022 following a 2019 recommendation.<ref name=Reuters2022>Template:Cite news</ref><ref name=SD2019>Template:Cite journal</ref><ref name=Oceanographic-2020>Template:Cite web</ref> The last confirmed sighting of a Chinese paddlefish occurred in the Yangtze River Basin in 2003.<ref name=SCMPost-2020>Template:Cite web</ref><ref name=IUCN-Chinese>Template:Cite iucn</ref> The Chinese paddlefish is also known as the "Chinese swordfish" or "elephant fish."<ref name=FAO>Template:Cite web</ref> The earliest known paddlefish fossil, Protopsephurus, dates to approximately 120 million years ago during the early 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.<ref>Template:Cite news</ref><ref name=NatGeo>Template:Cite web</ref>

Morphology

General morphology of paddlefish

Paddlefish are among the few organisms that retain a notochord beyond the embryonic stage. Their bodies contain very few bones, consisting primarily of cartilage with the notochord serving as a flexible spine. During early development from embryo to fry, paddlefish lack a rostrum (snout), which begins forming shortly after hatching.<ref name=LSU>Template:Cite report</ref>

The rostrum shape varies dramatically between species. The Chinese paddlefish possessed a narrow, sword-like rostrum, while the American paddlefish displays a broad, paddle-shaped one. Both species share several morphological features: a spindle-shaped body with smooth, scaleless skin, a heterocercal tail, and small, poorly developed eyes.<ref name=FAO/><ref name=LSU/>

Feeding strategies

The two species employed vastly different feeding strategies. Unlike its filter-feeding American relative, the Chinese paddlefish was a piscivore—a highly predatory 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.<ref name=FAO/><ref name=Miller>Template:Cite book</ref> In contrast, the American paddlefish jaw is specialized exclusively for filter feeding. As ram suspension filter feeders, they consume primarily zooplankton, occasionally taking small insects, insect larvae, and small fish.<ref name=Wilkens-Hofmann-2007/>

Size

Closeup of the head, showing the presence of electrorecepting organs (ampullae of Lorenzini)

The largest recorded Chinese paddlefish measured Template:Convert in length and was estimated to weigh several thousand pounds.<ref name=IUCN-Chinese/> More commonly, these fish reached Template:Convert and Template:Convert.<ref name=IUCN-Chinese/><ref name=FAO/><ref name=BBC>Template:Cite news</ref>

Though the American paddlefish ranks among the largest freshwater fishes in North America, it falls short of its Chinese cousin's impressive dimensions. American paddlefish commonly exceed Template:Convert in length and Template:Convert in weight. The largest specimen on record was speared in 1916 in Okoboji Lake, Iowa.Template:Efn<ref name=Nichols>Template:Cite journal</ref> This fish measured Template:Convert long with a girth of Template:Convert.<ref name=Nichols/> According to a 1969 report by J.R. Harlan and E.B. Speaker in Iowa Fish and Fishing, it weighed over Template:Convert.<ref name=AFS-7>Template:Cite book</ref>

The world record paddlefish caught on rod and reel weighed Template:Convert and measured Template:Convert long. Clinton Boldridge caught this fish in a 5-acre pond in Atchison County, Kansas on May 5, 2004.<ref name=Kansas>Template:Cite web</ref><ref name=KWPT>Template:Cite web</ref> This record was broken twice in 2020. On June 28, an Oklahoma man caught a 146-pound paddlefish in Keystone Lake, west of Tulsa. Less than a month later on July 23, another Oklahoma angler caught a 151-pound, nearly 6-foot-long paddlefish in the same lake.<ref>Template:Cite news</ref>

Rostrum and electroreception

Scientists initially believed paddlefish used their rostrums to dig through bottom sediments,<ref name=LSU/><ref name=Nachtrieb>Template:Cite journal</ref> but electron microscopy revealed a different purpose entirely. The rostrum surface is covered with electroreceptors called ampullae.<ref name=Jorgensen>Template:Cite journal</ref> These ampullae concentrate densely within star-shaped bone projections that branch from the rostrum.<ref name=Grande1991>Template:Cite journal</ref>

These electroreceptors detect weak electrical fields that signal prey presence in the water column. Remarkably, they can sense not just zooplankton—the primary food source for American paddlefish—but the individual feeding and swimming movements of zooplankton appendages.<ref name=Wilkens-Hofmann-2007/><ref name=LSU/>

Given their poorly developed eyes, paddlefish rely heavily on electroreception for foraging. However, the rostrum is not their only sensory tool. Some sources incorrectly suggest that rostrum damage would severely impair a paddlefish's ability to feed and maintain health. Laboratory experiments and field research demonstrate otherwise. Sensory pores cover nearly half the skin surface, extending from the rostrum across the top of the head down to the tips of the operculum (gill flaps). Even paddlefish with damaged or shortened rostrums can forage effectively.<ref name=Wilkens-Hofmann-2007/><ref name=LSU/>

Distribution and habitat

American paddlefish

American paddlefish are native to the Mississippi River basin, ranging from New York to Montana and south to the Gulf of Mexico.<ref name=TPW>Template:Cite web</ref> They inhabit several Gulf Slope drainages, favoring medium to large rivers with long, deep pools that move slowly, along with backwater lakes and bayous.<ref name=INHS>Template:Cite report</ref> In Texas, paddlefish historically occurred in the Angelina River, Big Cypress Bayou, Neches River, tributaries of the Red River, Sabine River, San Jacinto River, Sulphur River, and Trinity River.<ref name=TPW/>

Their historical range once extended into Canada—specifically Lake Huron and Lake Helen—and across 26 to 27 U.S. states. American paddlefish have been extirpated from much of their northern peripheral range, including the Great Lakes and populations in New York, Maryland, Pennsylvania, and Canada. The Ontario Ministry of Natural Resources now lists paddlefish as extirpated from Ontario, Canada under their Endangered Species Act.<ref name=SAR>Template:Cite web</ref> The IUCN 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 the Convention of International Trade in Endangered Species of Wild Flora and Fauna (CITES).<ref name=Redlist>Template:Cite iucn</ref> Concerns about remaining populations continue to grow across other states.

Chinese paddlefish

The Chinese paddlefish was considered anadromous, migrating upstream to spawn, though little is known about their migration patterns or population structure. They were endemic to the Yangtze River Basin in China, living primarily in the broad main stem rivers and shoal zones along the East China Sea.<ref name=IUCN-Chinese/><ref name=UN-HFAO>Template:Cite web</ref> Research suggests they favored the middle and lower layers of the water column and occasionally entered large lakes.<ref name=IUCN-Chinese/>

With no confirmed sightings since 2003, the species was declared extinct in 2019.<ref name=SD2019/> Past attempts at artificial propagation for restoration failed because captive specimens could not be kept alive.<ref name=Conservation>Template:Cite book</ref>

Life cycle

Paddlefish are long-lived and reach sexual maturity late in life. Females do not begin spawning 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.<ref name=LSU/><ref name=ND>Template:Cite web</ref><ref name=TexasPWD-sp-dsc/>

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.<ref name=LSU/>

To spawn, paddlefish migrate upstream, seeking silt-free gravel bars. During normal conditions, these bars would be exposed to air or covered by very shallow water, but spring snowmelt and rainfall raise river levels sufficiently to flood them.<ref name=MDC>Template:Cite web</ref> Paddlefish are broadcast spawners, also known as mass or synchronous spawners. Fertilization occurs externally: Gravid females release their eggs over bare rocks or gravel while males simultaneously release sperm. The adhesive eggs stick to the rocky substrate. After hatching, young paddlefish are swept downstream, growing to adulthood in deep freshwater pools.<ref name=EoF>Template:Cite book</ref>

Aquaculture

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.<ref name=WAS>Template:Cite conference</ref><ref name=Mims>Template:Cite report</ref>

In the early 1970s, a cooperative scientific effort between the U.S. Fish & Wildlife Service and its USSR counterpart brought American paddlefish to the former Soviet Union for aquaculture. The program began with five thousand hatched larvae from Missouri hatcheries. These fish were introduced into several European and Asian rivers, establishing the first brood stock that successfully reproduced in 1984–1986 in Russia.<ref name=Lenhardt-Hegediš-etal-2006>Template:Cite journal</ref> Paddlefish are now raised in Germany, Austria, the Czech Republic, and the Plovdiv and Vidin regions in Bulgaria. Reproduction was successful in 1988 and 1989, resulting in the exportation of juvenile paddlefish to Romania and Hungary. In May 2006, specimens of various sizes and weights were caught by professional fishermen near Prahovo in the Serbian part of the Danube River.<ref name=Lenhardt-Hegediš-etal-2006/>

In 1988, fertilized paddlefish eggs and larvae from Missouri hatcheries were first introduced into China.<ref name=Lenhardt-Hegediš-etal-2006/> 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.<ref name=Mims/>

Classification

Restoration of the Cretaceous Template:ExtinctProtopsephurus

Restoration of Template:ExtinctCrossopholis

There are two recent taxa, and five extinct genera known exclusively from fossils.

Classification following Template:Harvp,<ref name=Grande1991/> with Parapsephurus and Pugiopsephurus added in Template:Harvp:<ref name=Hilton2023/>

• genus Template:Extinct Protopsephurus Lu, 1994 (Early Cretaceous, China)
  • species Template:Extinct Protopsephurus liui Lu, 1994
• genus Template:Extinct Pugiopsephurus Hilton et al., 2023<ref name=Hilton2023/> (Late Cretaceous, North America) (Incertae sedis)
  • species Template:Extinct Pugiopsephurus inundatus Hilton et al., 2023<ref name=Hilton2023/>
• clade Polyodonti
  • genus Template:Extinct Paleopsephurus MacAlpin, 1947 (Late Cretaceous, North America)
    • species Template:Extinct Paleopsephurus wilsoni MacAlpin, 1947
  • genus Template:ExtinctParapsephurus Hilton et al., 2023<ref name=Hilton2023/> (Late Cretaceous, North America)
    • species Template:Extinct Parapsephurus willybemisi Hilton et al., 2023<ref name=Hilton2023/>
  • subfamily Polyodontinae
    • genus Template:Extinct Psephurus Günther, 1873
      • Template:ExtinctPsephurus gladius E. von Martens, 1862 Chinese paddlefish (extinct c. 2003)
    • tribe Polyodontini
      • genus Template:Extinct Crossopholis Cope, 1883 (Paleogene, North America)
        • species Template:Extinct Crossopholis magnicaudatus Cope, 1883
      • genus Polyodon Lacépède, 1797 (Paleocene-Recent, North America)
        • Polyodon spathula Walbaum, 1792 American paddlefish
        • Template:Extinct Template:Ill Grande & Bemis, 1991

Relationships of the genera are from Template:Harvp.<ref>Template:Cite journal</ref>Template:Clade

External links

• One hour PBS documentary
• The Chinese Paddlefish Website – containing many photographs of Psepherus.
• images and movies of the paddlefish (Polyodon spathula) ARKive
• FishBase entry for Polyodontidae
• USGS UMESC Paddlefish Study
• Fisheries.org
• Paddlefish Fisheries Management
• Stochastic synchronization of electroreceptors in the paddlefish