Tissue loss in corals infested by acoelomorph flatworms (Waminoa sp.) Little is known about species and host specificity of epizoic acoelomorphs that live in association with corals (Barneah et al. 2012; Hoeksema et al. 2012). Most records concern zooxanthellate acoels of the genus Waminoa Winsor, 1990, which reproduce both sexually and asexually and may harm their hosts by shading and by eating their protective mucus layer, but no coral injuries have been reported so far (Haapkylä et al. 2009; Naumann et al. 2010). During a biodiversity survey around Lembeh Island (North Sulawesi, Indonesia) in January-February 2012, various damaged corals were observed that were partly or completely covered by acoels. A specimen of the mushroom coral Danafungia scruposa (Klunzinger, 1879) had 100 % Waminoa cover on its upper surface, except for a part with tissue loss (Fig. 1). Its mouth appeared clogged by sediment (Fig. 1a). The damaged area was not similar to the contact zone with neighbouring corals, although interaction with these corals may have weakened the mushroom coral's defence capacity. Apparently, acoels can harm corals by smothering them, which may hinder their respiration, feeding and sediment shedding capacities. The infestation may be contagious, since a Sandalolitha robusta (Quelch, 1886) mushroom coral, with more than 90 % acoel cover was observed to be in close contact with one of another species, Pleuractis granulosa (Klunzinger, 1879), that was only infested at the nearest side, implying that acoels crawled or swam from one coral to another. All presently reported mushroom coral species represent new host records for Waminoa (see Hoeksema et al. 2012).

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.