Paddlefish

of paddlefish Paddlefish are among the few extant organisms known to 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. 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 in length and in weight. The largest specimen on record was speared in 1916 in Okoboji Lake, Iowa. This fish measured long with a girth of . Rostrum and electroreception organs (ampullae of Lorenzini) The shape of the rostrum (elongated snout or "paddle") varies dramatically between the species; the Chinese paddlefish possessed a narrow, sword-like rostrum, while the American paddlefish has a broad, paddle-shaped one. Scientists initially believed paddlefish used their rostrums to dig through bottom sediments, but electron microscopy revealed a different purpose entirely; the rostrum's surface is covered with electroreceptors called ampullae. These ampullae concentrate densely within star-shaped bony projections that branch from the rostrum's bony core. 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. In contrast, the American paddlefish jaw is specialized for filter feeding; as ram suspension filter feeders, they consume primarily zooplankton, occasionally taking small aquatic insects, insect larvae, and small fish. Fossil non-Polyodon paddlefish are thought to have had a similar ecology to the Chinese paddlefish as primarily piscivorous predators. ==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. They inhabit several Gulf Slope drainages, favoring medium to large rivers with long, deep pools that move slowly, along with backwater lakes and bayous. 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. 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). 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. Research suggests they favored the middle and lower layers of the water column and occasionally entered large lakes. ==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. Paddlefish are broadcast spawners, also known as mass or synchronous spawners, with fertilization occurring 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 by the river's flow, growing to adulthood in deep freshwater pools. ==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. 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. 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. In 1988, fertilized paddlefish eggs and larvae from Missouri hatcheries were first introduced into China. 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. ==Classification and evolution==

Protopsephurus '' There is one currently extant genus in this family, one recently extinct genus, and five extinct genera known exclusively from fossils. Classification following , The oldest unambiguous members of Acipenseriformes are known from the Early Jurassic 201–175 million years ago. The ancestors of sturgeons and paddlefish are suggested to have diverged from each other by at least the Late Jurassic, around 155 million years ago. Protopsephurus from the Early Cretaceous of China, around 120 million years ago, represents the oldest known paddlefish. Fragmentary remains of paddlefish have also been reported from the late Early Cretaceous and early Late Cretaceous (Bissekty Formation) of Uzbekistan, assigned to the species Hesperopsephurus kyatensis and Psephuroides kazakhorum respectively. The earliest record of paddlefish in North America dates to the Santonian stage of the Late Cretaceous, around 85.7-83.6 million years ago. Relationships of the genera are from . ==Notes==