American paddlefish

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...

Scientists began to debate the function of the American paddlefish's rostrum when the species was described in the late 1700s. They had once believed it was used to excavate bottom substrate or functioned as a balancing mechanism and navigational aid. 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. Clusters of electroreceptors also cover the head and operculum flaps. The diet of the American paddlefish consists primarily of zooplankton. Examples of animals in the P. spathula diet are copepods, cladocerans such as Daphnia pulex, and ephemeropteran nymphs. 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. 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. Such feeding behavior is considered ram suspension-feeding. Further research has indicated that the electroreceptor of the paddlefish may serve as a navigational aid for obstacle avoidance. American paddlefish have small undeveloped eyes that are directed laterally. Unlike most fishes, American paddlefish hardly respond to overhead shadows or changes in illumination. Electroreception appears to have largely replaced vision as a primary sensory modality, which indicates a reliance on electroreceptors for detecting prey. However, the rostrum is not their only means of food detection. 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. As well as electroreceptors on the rostrum, American paddlefish have sensory pores covering nearly half of the skin surface extending from the rostrum to the top of the head down to the tips of the operculum flaps. Studies have indicated that American paddlefish with damaged or abbreviated rostrums are still able to forage and maintain good health.

American paddlefish are highly mobile and well adapted to living in rivers. They inhabit many types of riverine habitats throughout much of the Mississippi Valley and adjacent Gulf slope drainages. They occur most frequently in deeper, low current areas such as side channels, oxbows, backwater lakes, bayous, and tailwaters below dams. They have been observed to move more than 2,000 mi (3,200 km) in a river system. American paddlefish are endemic to the Mississippi River Basin, historically occurring from the Missouri and Yellowstone rivers in the northwest to the Ohio and Allegheny rivers of the northeast; the headwaters of the Mississippi River south to its mouth, from the San Jacinto River in the southwest to the Tombigbee and Alabama rivers of the southeast. They were extirpated from New York, Maryland and Pennsylvania, as well as from much of their peripheral range in the Great Lakes region, including Lake Huron and Lake Helen in Canada. 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. In 1998, New York initiated a stocking program upstream in the Allegheny Reservoir above Kinzua Dam, and a second stocking in 2006 in Conewango Creek, a relatively unaltered section of their historic range. Reports of free ranging adults captured by gill nets have since been documented in Pennsylvania and New York, but there is no evidence of natural reproduction. 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.

Scientists began to debate the function of the American paddlefish's rostrum when the species was described in the late 1700s. They had once believed it was used to excavate bottom substrate or functioned as a balancing mechanism and navigational aid. 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. Clusters of electroreceptors also cover the head and operculum flaps. The diet of the American paddlefish consists primarily of zooplankton. Examples of animals in the P. spathula diet are copepods, cladocerans such as Daphnia pulex, and ephemeropteran nymphs. 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. 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. Such feeding behavior is considered ram suspension-feeding. Further research has indicated that the electroreceptor of the paddlefish may serve as a navigational aid for obstacle avoidance. American paddlefish have small undeveloped eyes that are directed laterally. Unlike most fishes, American paddlefish hardly respond to overhead shadows or changes in illumination. Electroreception appears to have largely replaced vision as a primary sensory modality, which indicates a reliance on electroreceptors for detecting prey. However, the rostrum is not their only means of food detection. 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. As well as electroreceptors on the rostrum, American paddlefish have sensory pores covering nearly half of the skin surface extending from the rostrum to the top of the head down to the tips of the operculum flaps. Studies have indicated that American paddlefish with damaged or abbreviated rostrums are still able to forage and maintain good health.

American paddlefish are long-lived, sexually late maturing pelagic fish. Females do not begin spawning until they are seven to ten years old, some as late as sixteen to eighteen years old. Females do not spawn every year; rather they spawn every second or third year. Males spawn more frequently, usually every year or every other year beginning around age seven, some as late as nine or ten years of age. American paddlefish begin their upstream spawning migration sometime during early spring; some begin in late fall. 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. Although availability of preferred spawning habitat is essential, there are three precise environmental events that must occur before American paddlefish will spawn. The water temperature must be from 55 to 60 °F (13 to 16 °C); the lengthened photoperiod which occurs in spring triggers biological and behavioral processes that are dependent on increasing day length; and there must be a proper rise and flow in the river before a successful spawn can occur. Historically, American paddlefish did not spawn every year because the precise environmental events occurred just once every 4 or 5 years. American paddlefish are broadcast spawners, also referred to as mass spawners or synchronous spawners. Gravid females release their eggs into the water over bare rocks or gravel at the same time males release their sperm. Fertilization occurs externally. The eggs become sticky after they are released into the water and will attach to the bottom substrate. Incubation varies depending on water temperature, but in 60 °F (16 °C) water the eggs will hatch into larval fish in about seven days. After hatching, the larval fish drift downstream into areas of low flow velocity where they forage on zooplankton. Young American paddlefish are poor swimmers which makes them susceptible to predation. Therefore, rapid first-year growth is important to their survival. Fry can grow about 1 in (2.5 cm) per week, and by late July the fingerlings are around 5–6 in (13–15 cm) long. Their rate of growth is variable and highly dependent on food abundance. Higher growth rates occur in areas where food is not limited. The feeding behavior of fingerlings is quite different from that of older juveniles and adults. They capture individual plankton one by one, which requires detection and location of individual Daphnia on approach, followed by an intercept maneuver to capture the selected prey. By late September fingerlings have developed into juveniles, and are around 10–12 in (25–30 cm) long. After the 1st year their growth rate slows and is highly variable. Studies indicate that by age 5 their growth rate averages around 2 in (5.1 cm) per year depending on the abundance of food and other environmental influences.

American paddlefish are among the largest and longest-lived freshwater fishes in North America. They have a shark-like body, average 1.5 m (4.9 ft) in length, weigh 27 kg (60 lb), 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. The age of American paddlefish is best determined by dentary studies, a process which usually occurs on fish harvested during snagging season, a popular sport fishing activity in certain parts of the U.S. The dentary is removed from the lower jawbone, cleaned of any remaining soft tissue, and cross-sectioned to expose the annual rings. The dentary rings are counted in much the same way a tree is aged. Dentary studies suggest that some individuals can live 60 years or longer, and that females typically live longer and grow larger than males. 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. 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.

A 2020 paper reported that eggs from three Russian sturgeons were crossbred with American paddlefish using sperm from four male paddlefish, resulting in hybrids called sturddlefish, a blend of the two names. The offspring had a survival rate of 62–74% and on average reached 1 kg (2.2 lb) after a year of growth. This was the first time such fish from different families were successfully crossbred. 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.

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 Tableau encyclopédique et méthodique (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. 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. 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. 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. 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 1 mm (0.039 in) were found in a juvenile paddlefish measuring 630 mm (25 in). 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.