Occurrence of 12 Acoela Species in the Seto Inland Sea

The Acoela is a notable taxon in terms of the early evolution of bilaterians and the photosynthetic symbiosis between animals and microalgae. There are approximately 416 described species of Acoela worldwide, which are classified into 16 families. In total, 21 species have been reported in Japan, of which five have been reported in the Seto Inland Sea. We surveyed acoels in the intertidal zone of beaches along the Seto Inland Sea coast of Hiroshima Prefecture and collected specimens. A comparison of mitochondrial cytochrome oxidase subunit I (COI) sequences and molecular phylogenetic analysis suggested that they could be divided into 12 species. Molecular phylogenetic analysis using 18S rRNA sequences suggested that these species belonged to five families: Convolutidae, Otocelididae, Dakuidae, Actinoposthiidae, and Isodiametridae. There have been no previous reports of Dakuidae or Actinoposthiidae in Japan and no reports of Isodiametridae in the Seto Inland Sea. These results suggested that the diversity of Acoela in Japan and the Seto Inland Sea is far richer than is currently known.