Cyprinus acutidorsaulis, commonly known as the sea carp, is a unique and rarely seen species within the Cyprinidae family, endemic to the brackish waters of China’s South Sea. Unlike most carp species, it has adapted to thrive in the transitional zones between freshwater rivers and marine environments, making it a fascinating example of euryhaline adaptation. This fish is exclusively found in the river systems of Hainan Island and the lower reaches of the Qinjiang River in Guangxi, highlighting its restricted but vital ecological niche.
Growing up to 60 centimeters in length, the sea carp features a robust, laterally compressed body typical of cyprinids, with a silvery-gray dorsal surface that fades to a white ventral side. Its most distinctive characteristic is the sharply pointed dorsal fin, from which its scientific name "acutidorsaulis" (Latin for "sharp dorsal") is derived. This fin, along with its powerful caudal peduncle, enables efficient swimming in the turbulent currents of estuaries. Small, closely set scales cover its body, and faint dark streaks may run along the lateral line, providing camouflage among the sandy and muddy substrates of its habitat.
As a euryhaline species, Cyprinus acutidorsaulis spends most of its life in brackish estuaries, where it feeds on a diverse diet of aquatic plants, crustaceans, and organic detritus. Its specialized gills and osmoregulatory system allow it to tolerate wide salinity fluctuations, a critical adaptation for surviving in environments influenced by both river runoff and tidal cycles. Spawning typically occurs in freshwater upstream during the rainy season, after which juveniles migrate downstream to grow in the nutrient-rich estuaries.
However, the sea carp faces severe threats due to its limited distribution and specific habitat requirements. Habitat degradation from coastal development, pollution, and the construction of dams—which disrupt migration routes—has led to significant population declines. Overfishing in its narrow range further exacerbates its vulnerability. Listed as "Endangered" on China’s Red List of Threatened Species, conservation efforts such as the establishment of protected areas in Hainan and Guangxi, combined with strict fishing regulations, are urgent to prevent its extinction. As a unique link between marine and freshwater ecosystems, Cyprinus acutidorsaulis serves as an indicator of estuarine health and underscores the importance of preserving China’s rare aquatic biodiversity.
