Surgeonfishes (Teleostei: Acanthuridae) of the Socotra Archipelago: Diversity and Distributional Biogeography, with Two New Records

An updated account of the exceptional diversity of surgeonfishes from the Socotra Archipelago is provided. Thirty species of four genera (Acanthurus, Ctenochaetus, Naso and Zebrasoma) are reported, including 25 species positively recorded based on underwater observations and photographs, and partly by samples. Acanthurus bariene and A. xanthopterus are recorded from the Archipelago for the first time. Additional five species are reported based on visual observations, pending further documentation. With likely 30 species the Archipelago hosts about the entire surgeonfish diversity of wider Arabia (31 species). The 25 documented species still exceed by far the richness of any other Arabian ecoregion. A species account, accompanied by photographs, provides distinctive characters and distribution details, including records from the Arabian region. The distributional biogeography of the family pertinent to the Socotra Archipelago and the Arabian region is analysed in the context of the Western Indian Ocean. Two main Arabian units are identified: a ‘Red Sea unit’, and a ‘Gulfs-eastern Arabia unit’. Both units do form a discrete “pan-Arabian” cluster opposite to a wider northern and western Indian Ocean cluster. The Socotra Archipelago is excluded from pan-Arabia, yet represents also an outgroup within the other cluster, underscoring its transitional biogeographic position. Accordingly, the Archipelago hosts eight species whose ranges are restricted to either main cluster, Arabia (3) or the adjacent northern and western Indian Ocean (5). The Acanthuridae thus strongly contrast overall distributional patterns of coastal and reef fishes in Arabia, and do not conform to major biogeographic schemes.