The American paddlefish is a smooth-skinned freshwater fish with an almost entirely cartilaginous skeleton and a paddle-shaped rostrum (snout), which extends nearly one-third its body length.

Its rostrum and cranium are covered with tens of thousands of sensory receptors for locating swarms of zooplankton, its primary food source.

The American paddlefish is native to the Mississippi River basin and once moved freely under the relatively unaltered conditions that existed prior to the early 1900s.

Lacépède disagreed with Pierre Joseph Bonnaterre's description in Tableau encyclopédique et méthodique (1788), which had suggested that paddlefish were a species of shark.

This is the sister group to the sturgeons (family Acipenseridae); evidence from DNA sequences suggest that their last common ancestor lived roughly 140 million years ago.

[16] American paddlefish are often referred to as primitive fish, or relict species, because of morphological characteristics that they retain from some of their early fossil ancestors.

[17] These characteristics include a skeleton composed primarily of cartilage, and a deeply forked heterocercal (spine extending into the upper lobe) caudal fin similar to that of sharks, although they are not closely related.

[14] The oldest fossils of paddlefish belonging to Polyodon are those of P. tuberculata from the Lower Paleocene Tullock Member of the Fort Union Formation in Montana, dating to around 65 million years ago.

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".

[25][26] Other distinguishing characteristics include a deeply forked heterocercal caudal fin and dull coloration, often with mottling, ranging from bluish gray to black dorsally grading to a whitish underbelly.

[29] However, laboratory experiments in 1993 that utilized advanced technology in the field of electron microscopy have established conclusively that the rostrum of American paddlefish is covered with tens of thousands of sensory receptors.

These receptors are morphologically similar to the ampullae of Lorenzini of sharks and rays, and are indeed passive ampullary-type electroreceptors used by American paddlefish to detect plankton.

[29] When a swarm of zooplankton is detected, the paddlefish swims forward continuously with its mouth wide open, forcing water over the gill rakers to filter out prey.

Some reports suggest a damaged rostrum would render American paddlefish less capable of foraging efficiently to maintain good health, but laboratory experiments and field research indicate otherwise.

[33] They spawn on silt-free gravel bars that would otherwise be exposed to air or covered by very shallow water were it not for the rises in the river from snow melt and annual spring rains that cause flooding.

They occur most frequently in deeper, low current areas such as side channels, oxbows, backwater lakes, bayous, and tailwaters below dams.

[37][35] In 1991, Pennsylvania implemented a reintroduction program utilizing hatchery-reared American paddlefish in an effort to establish self-sustaining populations in the upper Ohio and lower Allegheny rivers.

[29] The artificial propagation of American paddlefish began with the efforts of the Missouri Department of Conservation during the early 1960s, and focused primarily on maintenance of the sport fishery.

[40] However, it was the growing importance of American paddlefish for their meat and roe that became the catalyst for further development of culture techniques for aquaculture in the United States.

[40] Artificial propagation requires broodstock which, because of the late sexual maturation of American paddlefish, are initially obtained from the wild and brought into a hatchery environment.

Unlike most teleosts, the oviduct branches of American paddlefish and sturgeons are not directly attached to the ovaries; rather, they open dorsally into the body cavity.

Introduction began when five thousand hatched larvae from Missouri hatcheries in the United States were exported to the former Soviet Union for aquacultural utilization.

[47] The largest American paddlefish on record was captured in West Okoboji Lake, Iowa, in 1916 by a spear fisherman; it measured 85 in (2.2 m) and weighed an estimated 198 lb (90 kg).

The assessment concluded that "an overall population size reduction of at least 30% may occur within the next 10 years or three generations due to actual or potential levels of exploitation and the effects of introduced taxa, pollutants, competitors or parasites.

"[1] American paddlefish are filter-feeding pelagic fish that require large, free-flowing rivers with braided channels, backwater areas, oxbow lakes that are rich in zooplankton, and gravel bars for spawning.

[34] Channelization and groynes or wing dykes have caused the narrowing of rivers and altered flow, destroying crucial spawning and nursery habitat.

During this initial stage of development, which usually lasts a few weeks, veligers are able to swim freely in the water column with other microscopic animals comprising zooplankton.

[51] Poaching has been a contributing factor to declining populations of American paddlefish in the states where they are commercially exploited, particularly while the demand for caviar remains strong.

[29] State and federal regulations restricting the harvest of American paddlefish populations in the wild, and the illegal trafficking of their roe, are strictly enforced.

Related violations such as the illegal transport of American paddlefish roe have resulted in convictions with substantial fines and prison sentences.