Sinocyclocheilus is a genus of freshwater cyprinid fish endemic to the karst landscapes of southwestern China, predominantly in the provinces of Guizhou, Yunnan, and the Guangxi Zhuang Autonomous Region. Almost all of its species live in or around caves and most of these have adaptions typical of cavefish such as a lack of scales, lack of pigmentation and reduced eyes (some are completely blind). Several species have an unusual hunchbacked appearance and some of the cave-dwellers have a "horn" on the back (above the forehead), the function of which is unclear. In contrast, the non-cave or surface-associated lineages within the genus lack many of these highly specialized features. Adult sizes vary across species, but many reach up to ~23 cm (9.1 in) in total length. Many species are highly restricted in range (often to single caves or cave systems) and have small populations. Conservation status of evaluated species is often of concern, while many species populations in the genus have yet to be evaluated by the IUCN. The type species is S. tingi. The name is derived from the Latin word sino, meaning "from China", and the Greek word kyklos, meaning "circle", and the Greek word cheilos, meaning "lip".
Evolution Sinocyclocheilus is a genus of cyprinid fish in the family Cyprinidae, they are known for their adaptations to cave environments. All fish in this genus are endemic to the karst regions of the Yunnan-Guizhou Plateau and adjacent areas in southwest China. They represent one of the most diverse groups of cavefish in the world.
Phylogenetic position Molecular phylogenetic studies based on mitochondrial genomes strongly supports Sinocyclocheilus as a monophyletic group within the "Barbinae". The conception of a subfamily "Barbinae" that includes Sinocyclocheilus, although molecularlly consisient on its own (it does form a clade including Barbus) and very popular among Chinese researchers of this genus, is incompatible with the common view of Barbinae. In mainstream views, Sinocyclocheilus belongs to subfamily Cyprininae; a further classification into a tribe may be used if more granularity is desired.
Evolutionary history Molecular evidences estimate the origin of the genus Sinocyclocheilus to the late Miocene (approximately 6.49 million years ago). This diversification period is strongly associated with the intensive uplift of the Tibetan Plateau, a major geological event that dramatically altered the topography of the region. It is hypothesized that the ancestors of Sinocyclocheilus were widespread in the region and repeatedly colonized the karst cave systems as the geological changes created new ecologically isolated populations. The species S. jii is consistently identified as the sister species to all other species within the genus.
Multiple independent origins of cave adaptation When life habits and other traits of Sinocyclocheilus are overlaid onto their molecular phylogeny, it appears that the cave-dwelling species do not form a single evolutionary lineage. This pattern indicates that their adaptations to the cave environment, a process known as troglomorphy (e.g., eye degeneration, loss of pigmentation), evolved multiple times independently within the genus.
Polyploidy Many species within the genus, including S. microphthalmus, are allotetraploids, possessing four sets of chromosomes derived from ancestral hybridization events. Genomic analyses indicate that gene duplication resulting from polyploidy may have contributed to evolutionary diversification within the genus and has been associated with adaptations to cave environments, including eye degeneration and modification of sensory systems. Based on subgenome comparison, S. anshuiensis (and presumably other species of the genus) shares the same allo-tetraploidization event with the common carp Cyprinus carpio. The two species are estimated to have diverged around 10 million years ago, so the event predates that time. This event is also shared with the goldfish, which subsequently reverted to diploid.
Ecology The ecology of the genus Sinocyclocheilus is mainly shaped by its habitat. Surface species exhibit normal ecology, deep cavefishes have evolved behavioral, sensorial, and life-history adaptations to thrive in the darkness. Although facing many challenges, they are able to survive in the karst cave systems with their unique physiologies.
Habitat Most cave-dwelling Sinocyclocheilus species inhabit freshwater environments of limestone caves. Where their habitat is characterized by a complete absence of light, stable low water temperatures, and often oligotrophic (nutrient-poor) conditions. Their degree of troglomorphism (cave adaptation) is often correlated with the extent of isolation within the subterranean systems, with highly specialized eyeless species being mostly restricted to the deepest isolated cave chambers.
Behavioral ecology With the absence of light, Sinocyclocheilus cavefish cannot rely on their vision for foraging. Instead, they have evolved non-visual sensory systems to locate food. Genomic studies revealed a significant expansion of taste receptor genes, such as the duplication of Tas1r1 and Tas2r200-2 genome in S. anshuiensis, S. grahami, and S. rhinocerous, suggesting improvements to the sense of taste in the cave-restricted areas, which is likely used to detect prey and organic matter in the dark. A key behavioral adaptation for foraging is wall-following (WF), a form of thigmotaxis where the fish swims closely along the boundaries of its environment. This behavior is common in the genus but is significantly enhanced in eyeless species. Comparative studies showed that eyeless Sinocyclocheilus have a longer swimming distance and spend more time moving at higher speeds along walls compared to their normal-eyed, surface-dwelling relatives. This enhanced wall-following is an adaptive strategy for efficient spatial exploration in the dark, allowing the fish to navigate through their habitat to locate resources. Activity patterns also reflect energy conservation strategies. While eyeless forms are mostly constantly active in wall-following behavior, normal-eyed species exhibit significantly more resting behavior. This suggests that eyed species may conserve energy by relying more on visual cues when available, while blind species must invest continuous energy in active, non-visual exploration.
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