Morphological and phylogenetic diversity of Waminoa and similar flatworms (Acoelomorpha) in the western Pacific Ocean.

• Acoelomate flatworms specimens are mostly found on host cnidarians. • Acoelomate flatworms specimens include two groups based on external morphology (obcordate, molar-like). • Two morphological groups form well supported phylogenetic clades. • The genus Waminoa is represented by obcordate specimens. • Molar-like specimens constitute potentially an undescribed genus. The genus Waminoa currently contains two described species, which each contains two types of endosymbiotic algae. Waminoa individuals are basically brown in body color, derived from these algal 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 acoelomate flatworms 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 OTUs consisting of one specimen each within the molar-like clade, and one large group of the remaining molar-like specimens. 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. [ABSTRACT FROM AUTHOR]