The American paddlefish (Polyodon spathula), also known as a Mississippi paddlefish, spoon-billed cat, or spoonbill, is a species of ray-finned fish. It is the last living species of paddlefish (Polyodontidae). This family is most closely related to the sturgeons; together they make up the order Acipenseriformes, which are one of the most basal living groups of ray-finned fish. Fossil records of other paddlefish species date back 125 million years to the Early Cretaceous, with records of Polyodon extending back 65 million years to the early Paleocene. 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. It has been referred to as a freshwater shark because of its heterocercal tail or caudal fin resembling that of sharks, though it is not closely related. The American paddlefish is a highly derived fish because it has evolved specialised adaptations, such as filter feeding. Its rostrum and cranium are covered with tens of thousands of sensory receptors for locating swarms of zooplankton, its primary food source. The only other species of paddlefish that survived to modern times was the Chinese paddlefish (Psephurus gladius), last sighted in 2003 in the Yangtze River in China and considered to have gone extinct no later than 2010.

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. It commonly inhabited large, free-flowing rivers, braided channels, backwaters, and oxbow lakes throughout the Mississippi River drainage basin, and adjacent Gulf Coast drainages. Its peripheral range extended into the Great Lakes, with occurrences in Lake Huron and Lake Helen in Canada until about 1917. American paddlefish populations have declined dramatically primarily because of overfishing, habitat destruction, and pollution. Poaching has also been a contributing factor to its decline and is liable to continue to be so as long as the demand for caviar remains strong. Naturally occurring American paddlefish populations have been extirpated from most of their peripheral range, as well as from New York, Maryland, Virginia, and Pennsylvania. They have been reintroduced in the Allegheny, Monongahela and Ohio river systems in western Pennsylvania. However, their current range has been reduced to the Mississippi and Missouri River tributaries and Mobile Bay drainage basin. American paddlefish are currently found in twenty-two states in the U.S., and are protected under state, federal and international laws.

Taxonomy, etymology and evolution

thumb|260px|An American paddlefish in a large aquarium tank

In 1797, French naturalist Bernard Germain de Lacépède established the genus Polyodon for paddlefish, which today includes a single extant species, Polyodon spathula. Lacépède disagreed with Pierre Joseph Bonnaterre's description in (1788), which had suggested that paddlefish were a species of shark. When Lacépède established his binomial name Polydon feuille he was unaware the species had already been described in 1792 by taxonomist Johann Julius Walbaum, who had named it as Squalus spathula. Consequently spathula has priority as the specific name (and 'Walbaum, 1792' is the taxonomic authority to be cited). But Walbaum's generic name Squalus was already in use for dogfish, so Lacépède's Polyodon is the valid name for this paddlefish genus. Hence Polyodon spathula (Walbaum, 1792)' is the accepted full scientific name of the American paddlefish.

The American paddlefish is the sole surviving species in the paddlefish family, the Polyodontidae. This is the sister group to the sturgeons (family Acipenseridae); evidence from mitochondrial DNA sequences suggest that their last common ancestor lived roughly 141.4 million years ago. Together these families compose the Acipenseriformes, an order of basal ray-finned fishes. Paddlefish have a long fossil record dating back to the Early Cretaceous 125 million years ago. 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.

The family Polyodontidae comprises six known species: three fossil species from western North America, one fossil species from China, one recently extinct species from China (the Chinese paddlefish, Psephurus gladius; last recorded 2003), and the single extant species, the American paddlefish, native to the Mississippi River Basin in the United States. DNA sequences suggest the Chinese and American paddlefishes diverged about 68 million years ago. An elongated rostrum is a morphological characteristic of Polyodontidae, but only the genus Polyodon has characteristics adapted specifically for filter feeding, including the jaw, gill arches, and cranium. 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". Adult American paddlefish are actually toothless, although numerous small teeth less than were found in a juvenile paddlefish measuring . The name spathula references the elongated, paddle-shaped snout or rostrum. Compared to Chinese paddlefish and fossil genera, American paddlefish (and the fossil relative P. tuberculata) are considered to be highly derived because of their specialised adaptations.

Cladogram after Grande et al. 2002:Unlike the planktivorous American paddlefish, Chinese paddlefish were strong swimmers, grew larger, and were opportunistic piscivores that fed on small fishes and crustaceans. Some distinct morphological differences of Chinese paddlefish include a narrower, sword-like rostrum, and a protrusible mouth. They also had fewer, thicker gill rakers than American paddlefish. They have a shark-like body, average in length, weigh , and can live in excess of thirty years. For most populations the median age is five to eight years and the maximum age is fourteen to eighteen years.

American paddlefish are smooth-skinned and almost entirely cartilaginous. Their eyes are small and directed laterally. They have a large, tapering operculum flap, a large mouth, and a flat, paddle-shaped rostrum that measures approximately one-third of their body length. During the initial stages of development from embryo to hatchling, American paddlefish have no rostrum. It begins to form shortly after hatching. The rostrum is an extension of the cranium, not of the upper and lower jaws or olfactory system as with the long snouts of other fish. They had once believed it was used to excavate bottom substrate or functioned as a balancing mechanism and navigational aid. Their electroreceptors can detect weak electrical fields that signal not only the presence of zooplankton, but also the individual feeding and swimming movements of zooplankton appendages.

American paddlefish begin their upstream spawning migration sometime during early spring; some begin in late fall. Although availability of preferred spawning habitat is essential, there are three precise environmental events that must occur before American paddlefish will spawn. and by late July the fingerlings are around long. They are currently found in 22 states in the US, and are protected under state and federal laws. There are 13 states that allow commercial or sport fishing for American paddlefish. 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. The fish are injected with LH-RH hormone to stimulate spawning. The number of eggs a female may produce depends on the size of the fish and can range anywhere from 70,000 to 300,000 eggs. 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. To determine the status of progression toward maturation, ova staging is performed. The process begins with a minor procedure that involves a small abdominal incision from which to extract a few sample oocytes. The oocytes are boiled in water for a few minutes until the yolk is hardened, and then they are cut in half to expose the nucleus. The exposed nucleus is examined under a microscope to determine stage of maturity.

A spermiating male indicates successful production of mature spermatozoa which results in the release of large volumes of milt over the course of three to four days. Milt is collected by inserting a short plastic tube with syringe attached into the urogenital opening of the male and applying light suction with the syringe to draw the milt. The collected milt is diluted in water just prior to adding it to the eggs and the combination is gently stirred for about a minute to achieve fertilization. Fertilized eggs are adhesive and demersal, therefore if incubation is to take place in a flow-through hatching jar, the eggs must be treated to prevent clumping. Incubation usually takes anywhere from five to twelve days. Their last common ancestor is estimated to have lived 141.4 million years ago based on mitochondrial DNA sequences, while the Bayesian molecular clock analysis estimated an earlier divergence time of 184.4 million years ago. Reproduction was successful in 1988 and 1989, and resulted in the exportation of juveniles to Romania and Hungary. American paddlefish are now being raised in Ukraine, Germany, Austria, the Czech Republic, and the Plovdiv and Vidin regions in Bulgaria. In May 2006, specimens of different sizes and weights were caught by professional fisherman near Prahovo in the Serbian part of the Danube River.

Sport fishing

American paddlefish are a popular sport fish where their populations are sufficient to allow such activity. Areas where there are no self-sustaining populations rely on state and federal restocking programs to maintain a viable fishery. A 2009 report includes the following states as allowing American paddlefish sport fishing per their respective state and federal regulations: Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Mississippi, Missouri, Montana, Nebraska, North Dakota, Oklahoma, South Dakota and Tennessee. This fish measured long with a girth of . The official state record in Kansas is an American paddlefish snagged in 2004 that weighed . In Montana, an American paddlefish was snagged in 1973 weighing . In North Dakota, one snagged in 2024 weighed .

The world record (American) paddlefish caught on rod and reel weighed and measured long. Clinton Boldridge caught this fish in a 5-acre pond in Atchison County, Kansas on May 5, 2004. This record was broken twice in 2020. On June 28, an Oklahoma man caught a paddlefish in Keystone Lake, west of Tulsa. Less than a month later on July 23, another Oklahoma angler caught a , nearly long paddlefish in the same lake. According to Missouri Conservation, a new world record (American) set by Chad Williams of Olathe Kansas on Lake of The Ozarks, Missouri March 17, 2025 weighing and in length.

Population declines

thumbnail|260px|Angler landing a large paddlefish

Overfishing and habitat destruction

American paddlefish populations have declined dramatically, primarily as a result of overfishing and habitat destruction. In 2004 they were listed as Vulnerable (VU A3de ver 3.1) on the IUCN Red List of Threatened Species. In 2022 the status category was changed to VU A2cd throughout their range as the result of a U.S. Fish & Wildlife Service assessment. 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." A few days after the fertilization of zebra mussel eggs, a microscopic larva emerges called a veliger. 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. Veligers are poor swimmers, making them susceptible to predation by any animal that feeds on zooplankton. However, natural predation of zebra mussels at any stage of development has not made a significant contribution to the long-term reduction of zebra mussel populations.

Poaching and overexploitation

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.

The roe of American paddlefish can be processed into caviar similar in taste, color, size and texture to sevruga sturgeon caviar from the Caspian Sea. American paddlefish are also protected under Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning international trade in the species (including parts and derivatives) is regulated.

Notes

References

  • The Paddlefish: An American Treasure – documentary chronicling the biology and life history of paddlefish
  • Study: 96 percent of vertebrates descended from common ancestor with 'sixth sense'