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Ecology and Conservation

Research Report

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The species diversity and distribution of cctocorals at Dongsha Atoll were studied by field surveys in 2016-2017. A total of 298 octocoral specimens were collected for taxonomic studies. Based on morphological characters, 119 species of octocorals belonging to 2 orders, 11 families and 31 genera were identified. The octocoral fauna was dominated by soft corals of Subclass Alcyoniina (102 species), while the gorgonians (including subclasses Scleroxonia, Holoxonia, and Calcaxonia) has only 16 species. For the distribution and community structure of octocorals, a total of 30 video-transects (each 50 m long) covering all sectors of Dongsha Atoll were analyzed. Thirty images were randomly selected from each transect and the cover of octocorals as well as other benthos was quantified by CPCe (Coral Point Count with Excel extensions) software. The results showed that living cover of octocoral at Dongsha Atoll was highly heterogeneous and ranged from 0.22% to 73.67%. The highest octocoral cover were found on the outer reef slopes in northern, eastern and southern sectors of the atoll where octocorals occupy more than 30% of the substrate and become dominant in benthic communities, except that on the shallow reef front (<5 m depth) in eastern sector.  Octocoral cover was low on western sector as well as the north and south channels. Very few octocorals were found in the lagoon where the coral communities were dominated by hard corals. The results of cluster analysis and nonmetric multidimensional scaling (nMDS) showed that the reef areas with high octocoral cover were homogeneous with similar composition of alcyonacean genera including Sinularia, Lobophytum, Sarcophyton and Cladiella. While, the reef areas with lower octocoral cover were grouped into several small clusters based on the dominant alcyonacean genera. The composition of octocoral fauna is not uniform, which may be related to the remote island and the species dispersal, and the distribution of octocoral communities is not homogeneous, which may be influenced by internal wave, current and sediments.

Keywords: dongsha atoll, octocoral, soft coral, coral community