Entospheninae

Entospheninae is a subfamily of freshwater fish belonging to the order Salmoniformes, commonly known as the "paddlefishes." This group consists of two extant genera, *Polyodon* and *Psephurus*, both of which are characterized by a unique, elongated snout resembling a paddle, which they use to filter feed on plankton and small organisms from the water column. The subfamily is notable for its ancient lineage, with fossils dating back over 100 million years, indicating a long evolutionary history.

The most well-known member of Entospheninae is the North American paddlefish (*Polyodon spathula*), which inhabits large rivers and lakes across the central and eastern United States. This species can grow to impressive lengths of up to 2.4 meters (8 feet) and weigh over 60 kilograms (130 pounds). The Chinese paddlefish (*Psephurus gladius*), once widespread in the Yangtze River basin, was declared functionally extinct in 2020 due to overfishing, habitat destruction, and dam construction. Both species are considered living fossils, representing a primitive branch of the salmoniform lineage.

Entospheninae exhibits several distinctive anatomical features, including a reduced number of vertebrae, a lack of scales, and a highly specialized sensory system for detecting water currents. Their diet primarily consists of zooplankton, making them ecologically important as filter feeders in their respective ecosystems. Conservation efforts for the North American paddlefish focus on habitat protection, sustainable fishing practices, and restocking programs, as populations have declined due to human activities.

The subfamily is also of significant scientific interest due to its unique genetic and developmental traits. Research on paddlefish has contributed to understanding early vertebrate evolution and the genetic mechanisms behind specialized feeding adaptations. Despite their ecological and evolutionary importance, Entospheninae remains one of the least studied groups of salmoniform fishes.