Sinocyclocheilus is a genus of freshwater fish in the family Cyprinidae endemic to China. First described in 1936, it includes 76 valid species, of which 71 are grouped into five species groups.
Most members of this genus live in or around caves, and most have adaptations typical of cave fish, such as lack of scales, pigmentation, and impaired vision. Some species will have “horns” on the back of the head, but the function of these horns is still unclear.
In contrast, terrestrial species and a few found underground did not show a straightforward adaptation of cavefish.
There are many theories that Sinocyclocheilus adapted the cave habitat to cope with the aridity of China during the Late Miocene and Pliocene epochs.
“Sinocyclocheilus species almost all have differently developed horn-like eyes and structures on the back of the head,” the scientists said.
“Eye morphology includes normal, small-eyed, and ophthalmic conditions.”
“The species with normal eyes and small eyes are distributed from eastern Guangxi through southern Guizhou to eastern Yunnan, while species without eyes are mainly distributed in the Red River basin in the north. Guangxi and the South Panjiang River basin in eastern Yunnan”.
“The horn-like structure is present mainly in species of Sinocyclocheilus angles and Sinocyclocheilus microphthalmus.”
“These horned species are distributed in the basins of the Nam Banjiang, Bac Panjiang, and Hongshui rivers of the Pearl River upstream.”
Adaptive traits, such as eye degeneration and horn evolution, are found in clade-related cave systems in Guangxi and Guizhou provinces. Studies incorporating various methods, such as morphology, phylogenetics, rate analysis, molecular dating, and distribution, have revealed distinct patterns and outliers in the population—the evolution of Sinocyclocheilus making it a valuable model for exploring evolutionary novelty.
The newly described species, Sinocyclocheilus longicornus, has been discovered in only one place, a vertical cave at an altitude of 2,276 m from the town of Hongguo in China’s Guizhou province.
The researchers said: “There is no light inside the cave. Individuals of the species Sinocyclocheilus longicornus are all lying in a small puddle about 25 meters from the mouth of the cave.”
“This puddle is about 1.8 meters wide and 80 centimeters deep, the water temperature at the time of sampling was 16 degrees Celsius, and the pH of the water was 7.4.”
Sinocyclocheilus longicornus has an albino fish-like body, without scales, without pigment, and with small, degenerated eyes. It also has a single, relatively long, unbranched horn-like structure at the back of its head.
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