Unexpected diversity of Isidoides (Anthozoa: Octocorallia: Isidoidae) revealed by morphology and phylogenomic analysis with descriptions of three 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 is 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 new species and develop reliable features for species identification. The 23 specimens could be classified into four well-supported clades 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 three new species, viz., I. elegans sp. nov., I. gracilis 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, sclerite forms with their surface sculpturing are more diagnostic features for species identification. Our phylogenetic and species delimitation analyses indicate that UCEs have higher resolution than the nuclear 28S rDNA and the mitochondrial genes mtMutS and cox1 for species discrimination within Isidoidae. ZooBank: urn:lsid:zoobank.org:pub:392485F5-502E-4383-B153-45B167571190.