Unexpected diversity of Isidoides (Anthozoa: Octocorallia: Isidoidae) revealed by morphology and phylogenomics analysis with descriptions of four new species

Isidoidae Heestand Saucier, France & Watling, 2021 is a rare octocoral family currently represented by a single genus and species, Isidoides armata Nutting, 1910 recorded in the western Pacific Ocean. The taxonomic status and diversity of Isidoides remain unclear, due to the lack of diagnostic taxonomic features and limited taxon sampling. Based on 23 Isidoides specimens obtained from the northwestern to southwestern Pacific, we carried out morphological and phylogenetic analyses to reveal the taxonomic status of species and develop reliable features for species identification. The 23 specimens could be classified into five well-separated groups by the phylogenomic analysis of ultraconserved elements (UCEs), four groups by 28S rDNA , and two groups by mtMutS-cox1 . Integrating morphology and molecular data, we uncovered unexpected diversity of Isidoides composed of the known species Isidoides armata and four new species, viz., I. elegans sp. nov., I. gracilis sp. nov., I. incrassata sp. nov. and I. pseudarmata sp. nov. The morphological analysis showed high intraspecific morphological variation in colony color and the size, shape and arrangement of polyps. By contrast, the sclerite forms with their surface sculpturing are more reliable features for species identification. Our phylogenetic analyses indicate that UCE phylogenomics has higher resolution than the nuclear 28S rDNA and the mitochondrial genes mtMutS and cox1 for species delimitation within Isidoidae.