A family of fishes in the order Acipenseriformes. The body is bare. The body is bare. Ganoid scales preserved only on the upper paddle of the caudal fin. The snout is elongated, paddle-shaped. There are two antennae on the underside of the snout. Two genera (Polyodon, Psephurus). There is one species in each. The oar-fish (Polyodon spathula) reaches a length of 2 m. Weighs up to 75 kg. It feeds on planktonic organisms in rivers and lakes of North America. It is acclimatised in Russia. It is kept in fish farms in the European part of Russia. The second representative of the paddlefish, the psephurus or Chinese paddlefish (Psephurus gladius), lives in the Yangtze River (China). It can grow up to 7 metres long. It is a predator and feeds on fish.
They are distinguished from other fishes by their elongated rostrum, which is thought to improve electro-reception for detecting prey. Paddlefish are called 'primitive fish' because Acipenseriformes is one of the earliest divergent lineages of ray-finned fish, having diverged from all other living groups more than 300 million years ago. Paddlefish are found almost exclusively in North America and China, both extant and in the fossil record. Eight species are known - six extinct species known only from fossils (five from North America, one from China), one extant species, the American paddlefish (Polyodon spathula) from the Mississippi River basin in the USA, and the Chinese paddlefish (Psephurus gladius), which will be declared extinct in 2022 following a recommendation in 2019. The species was last seen in the Yangtze River basin in China in 2003. Chinese paddlefish are also commonly referred to as 'Chinese swordfish' or 'elephant fish'. The earliest known species is Protopsephurus from early China, which is about 120 million years old. Paddlefish populations have declined dramatically throughout their historic range as a result of overfishing, pollution and human interference, including the building of dams that have blocked their seasonal upstream migration to their ancestral spawning grounds. Other detrimental effects include river modifications that have altered natural flows, resulting in the loss of spawning and nursery grounds.
Paddlefish as a group are among the few organisms that retain a chord after the embryonic stage. They have very few bones and their body is mainly cartilage, with the chordal part acting as a soft backbone. In the early stages of development from embryo to fry, fish do not have a rostrum (snout). It begins to form shortly after hatching. Some common morphological characteristics of paddlefish include a spindle-shaped body without scales with smooth skin, a heterocercal tail and small, poorly developed eyes. Unlike American paddlefish, which fed on filter feeders, Chinese paddlefish were piscivorous and very predatory. Their jaws were more forward facing, suggesting that they fed mainly on small fish in the water column, and sometimes on shrimp, groundfish and crabs. The jaws of American paddlefish are clearly adapted to feeding only through a filter. They feed through rammed suspension filters with a diet consisting mainly of zooplankton and occasionally small insects, insect larvae and small fish. The largest known Chinese paddlefish reached 23 feet (7.0 m) in length and was estimated to weigh several thousand pounds. They typically reach 9.8 ft (3.0 m) and 1,100 lb (500 kg). Although the American paddlefish is one of the largest freshwater fish in North America, its recorded length and weight do not match the larger Chinese paddlefish. American paddlefish typically reach 5 feet (1.5 m) or more in length and can weigh more than 60 pounds (27 kg). Scientists once believed that paddlefish use their rostrums to excavate the bottom substrate, but have since determined through electron microscopy that paddlefish rostrums are covered with electroreceptors called ampullae. These ampullae are tightly packed inside the star-shaped bony projections that protrude from the rostrum. The electroreceptors can detect weak electric fields that not only signal the presence of prey objects in the water column, such as zooplankton, the main food of American paddlefish, but also individual movements of zooplankton appendages as they feed and swim. Paddlefish have poorly developed eyes and rely on their electroreceptors to find food. However, the rostrum is not the only means by which paddlefish detect food. Some reports erroneously claim that a damaged rostrum makes paddlefish less able to obtain food efficiently to maintain good health. Laboratory experiments and field studies suggest otherwise. In addition to the electroreceptors on the rostrum, paddlefish also have sensory pores that cover almost half of the skin surface, extending from the rostrum to the top of the head and down to the tips of the caps (gill flaps). As a result, paddlefish with damaged or shortened rostrums are still able to feed and remain healthy. Paddlefish as a group are among the few organisms that retain a chord after the embryonic stage. Paddlefish have very few bones and their bodies are mainly cartilaginous, with the chordal part acting as a soft spine. During the early stages of development from embryo to juvenile, paddlefish do not have a rostrum (snout). It begins to form shortly after hatching. The snout of the Chinese paddlefish was narrow and sword-like, whereas the snout of the American paddlefish is broad and paddle-like. Some common morphological characteristics of paddlefish include a spindle-shaped, unscaled body with smooth skin, a heterocercal tail and small, poorly developed eyes. Unlike American paddlefish, which fed on filter feeders, Chinese paddlefish were piscivorous and very predatory. Their jaws were more forward pointing, suggesting that they fed mainly on small fish in the water column and sometimes on shrimp, groundfish and crabs. The jaws of American paddlefish are clearly adapted to feeding only through a filter. They feed through rammed suspension filters with a diet consisting mainly of zooplankton and occasionally small insects, insect larvae and small fish.
The paddlefish population has declined over the last half century. Chinese paddlefish are considered to be anadromous with upstream migration, but little is known about their migratory habits and population structure. They were endemic to the Yangtze River basin in China, where they mainly inhabited the broad surface rivers of the main channel and shallow areas along the East China Sea. Studies show that they preferred the middle and lower water columns and sometimes swam into large lakes. The Chinese fish have not been seen since 2003 and were declared extinct in 2019. Previous attempts to artificially breed them for recovery have failed due to difficulties in keeping them alive in captivity. American fish are found in the Mississippi River basin from New York to Montana and as far south as the Gulf of Mexico. They have been found in several drainage channels on the slopes of the Gulf, in medium to large rivers with long, deep pools, and in flooded lakes and canals. In Texas, these fish were historically found in the Angelina River, Bayou Big Cypress, Neches River, Red River tributaries, Sabine River, San Jacinto River, Sulphur River and Trinity River. Its historic range also included habitats in Canada at Lake Guron and Lake Helen, and 26-27 states in the United States. The Ontario Ministry of Natural Resources has listed the paddlefish of Ontario, Canada as endangered under the Endangered Species Act.
Paddlefish are long-lived and reach sexual maturity late in life. Females do not begin spawning until they are six to twelve years old, some as late as sixteen or eighteen. Males begin spawning at four to seven years of age, some as young as nine or ten. Paddlefish spawn in late spring when the right combination of events occur, including water flow, temperature, photoperiod and the availability of gravel substrates suitable for spawning. If these conditions are not met, the fish will not spawn. Studies show that females do not spawn every year, but rather every two or three years, while males spawn more frequently, usually every one or two years. Paddlefish migrate upstream to spawn, preferring non-sedimented gravel banks that would otherwise be exposed to the air or covered by very shallow water were it not for the rising water levels in the river caused by snowmelt and annual spring rains that cause flooding. These are broadcast spawning grounds, also known as mass spawning grounds or synchronised spawning grounds. Gravid females release their eggs into the water on bare rocks or gravel at the same time as the males release their sperm. Fertilisation takes place externally. The eggs are sticky and adhere to the rocky substrate. After hatching, the young are carried downstream and mature in deep freshwater pools.
In this family there is one extant genus, one recently extinct genus and five extinct genera known only from fossil remains.