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The genus Waminoa currently contains two described species, which contain two types of endosymbiotic algae. Waminoa individuals are basically brown in body color, derived from these symbionts, and their body shape has been described as "discoid to obcordate". They have been found as associates of various anthozoans (Cnidaria) in the Indo-Pacific Ocean and the Red Sea. In order to reveal the diversity of the genus Waminoa and their hosts, we conducted phylogenetic and morphological analyses on Waminoa specimens collected from Japan, Palau and Indonesia. At least 18 Waminoa morphotypes were found on at least 20 anthozoan host species, and two specimens were found on species of two sea stars. Overall, there were two main body shapes of specimens; obcordate -as seen in W. litus and W. brickneri, and the other molar-like with an elongated body. These two body shapes each represented a separate clade in 18S rDNA and mitochondrial cytochrome c oxidase subunit 1 (COI) phylogenetic trees, with W. brickneri included in the obcordate subclade. Automatic Barcode Gap Discovery (ABGD) analyses on COI sequences of our specimens revealed the presence of at least five operational taxonomic units (OTUs). These five OTUs consisted of one large group of all obcordate animals, three singletons within the molar-like clade, and one large group of the other molar-like individuals. Both clades contain numerous morphotypes and were associated with a variety of hosts. Finally, based on genetic distances, the molar-like specimens are considered as an unnamed genus-group separate from Waminoa, which needs to be clarified in future studies that will include histology and additional molecular data.

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Associations between different taxa constitute critical data to recognize ecological symbioses. Acoela flatworms of the genus Waminoa are an overlooked but often present group in coral reef ecosystems. More than 97% of the reports on Waminoa individuals until now have found them on scleractinian coral species, and the relationships between Waminoa species and other potential hosts such as octocorals, which are often large and ecologically important components in coral reefs, are not known well. Here we report the first observation of Waminoa sp. from a species of Dendronephthya, at Zanpa, Okinawa, Japan. We observed three morphotypes on a colony of D. aff. rigida (Studer, 1888), with almost all Waminoa individuals in shaded locations.