Record of Euilyoida takewakii (Kato, 1935) (Platyhelminthes, Polycladida, Acotylea) from Fukuoka, Japan, with notes on its phylogenetic position

Abstract

While most polyclad flatworms are free-living, a handful of species exhibit intriguing associations with other invertebrates. One such case is Euilyoida takewakii (Kato, 1935), which inhabits the genital bursa of the ophiuroid species Ophioplocus japonicus H.L. Clark, 1911, found in Japan. This study presents the second report of E. takewakii from the Japan Sea coast, based on specimens collected in Fukuoka, northern Kyushu, Japan. We additionally identified a 4.1% infestation rate of the ophiuroid O. japanicus by E. takewakii. We further elucidated the phylogenetic position of E. takewakii within acotyleans using partial 18S and 28S rDNA sequences for molecular phylogenetic analysis. Despite previous confusion regarding its genus assignment between Discoplana Bock, 1913 and Euilyoida Faubel, 1983, our results support the classification as Euilyoida. This conclusion is based on the finding that E. takewakii did not form a clade with Discoplana gigas (Schmarda, 1859), the only species whose sequences are available in public databases currently among Discoplana.