Two new species of deep-sea Red Corals (Coralliidae, Genus Hemicorallium Gray, 1867) from the western Indian Ocean

Deep-sea corals are critical to global exploration of deep-sea biodiversity, but research on them in the Indian Ocean is very limited. In this study, we report the first discovery of two new species of red corals at a depth of 1697 m in the western Indian Ocean. The species were identified through detailed morphological analysis, including microscopic examination of colony structure, branches and autozooids (assessing size, abundance and spacing), as well as scanning electron microscopy (SEM) of sclerite morphology and quantity. Two species for the genus Hemicorallium Gray, 1867 were found as new species, designated Hemicorallium indicodensum sp. nov. and Hemicorallium jiaolongensis sp. nov. H. indicodensum sp. nov. is characterised by densely branched structures and numerous globular double-club sclerites, distinguishing it from other species. Meanwhile, H. jiaolongensis sp. nov. features yellowish-white colonies with short rods with sharp, large radial projections in the tentacles. Four mitochondrial regions were used to reveal the phylogenetic relationship in Coralliidae, supporting the taxonomic placement of these two new species. This study provides significant new insights into the biodiversity of deep-sea corals in the Indian Ocean, enriching the known species pool. Additionally, a more comprehensive key to the genus Hemicorallium is provided, further enhancing our understanding of the group’s taxonomy.