Although the cichlid fishes from Lake Tanganyika are treated as a textbook example of adaptive radiation, many taxonomic problems remain unresolved. Cyathopharynx furcifer, which belongs to the currently monospecific genus Cyathopharynx, contains two colour morphs at the southern end of the lake: one has a yellow anal fin, and the other has a black anal fin. Some books for hobbyists of ornamental fish treat these morphs as different species, but taxonomic studies have neither mentioned the existence nor addressed the status of these colour morphs. In the present paper, we analysed these two colour morphs using mitochondrial, microsatellite, morphometric, and meristic data sets. Both molecular and morphological data allowed clear discrimination between these morphs, suggesting the existence of two distinct sympatric species. Three taxonomic species have been described in this genus, and only C. furcifer is currently considered valid. Observations of type specimens of these three nominal species will be needed to determine the scientific names of these colour morphs. 1. Introduction Lake Tanganyika is one of the ancient lakes of the East African Rift Valley. This lake harbours about 250 cichlid species, and 98% of these species are endemic to the lake [1]. These fish exhibit high morphological, behavioural, ecological, and genetical diversification, and are treated as a textbook example of adaptive radiation (e.g., [2–7]). Cyathopharynx Regan is one of the genera belonging to the endemic tribe Ectodini from Lake Tanganyika [8, 9]. This genus is morphologically well defined, namely, fish of this genus have small scales on the sides of the body (48–64 scales in longitudinal line), a lower pharyngeal bone with a rounded posterior margin, and in males, long pelvic fins. These morphological features are also found in some other genera of Ectodini [8], but only Cyathopharynx has all of these features combined. A phylogenetic study based on mitochondrial DNA does not contradict the monophyly of Cyathopharynx and shows that this genus nest within a monophyletic group including Ophthalmotilapia Pellegrin and Cardiopharynx Poll [10]. Three species have been described in Cyathopharynx: C. furcifer (Boulenger) (originally described as Paratilapia furcifer in 1898 [11]), C. foae (Vaillant) (originally described as Ectodus foae in 1899 [12]), and C. grandoculis (Boulenger) (originally described as Tilapia grandoculis in 1899 [13]). The latter two nominal names are currently considered as junior synonyms of C. furcifer, and only C. furcifer is considered valid in this
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