The freshwater heteronemertean Apatronemertes albimaculosa Wilfert and Gibson, 1974 has previously been reported from Germany, Austria, and the USA. All these records were from aquarium tanks with commercially distributed tropical and/or subtropical water plants; by hiding among their roots, the worms are thought to have been introduced from an unidentified place of origin. We report the occurrence of A. albimaculosa for the first time from Japan, based on specimens found in private home aquaria for guppy breeding, with a total of 18 species of water plants grown for different periods and lengths of time but not all at the same time. Histological examination confirmed the presence of diagnostic features of this species, including a complex precerebral vascular system and hermaphroditism. The partial sequence of the mitochondrial cytochrome c oxidase subunit I gene (607 bp) of the Japanese material is identical with that of a heteronemertean specimen from a freshwater tank in Madrid, Spain, which had been deposited in the public databases but left unidentified as to its species, genus, or family. Apatronemertes albimaculosa appears unlikely to be able to survive below 10°C for more than about a week, which suggests that its native locality is in the tropics or subtropics, not the temperate zone. The barcoding sequence herein determined will serve in future studies to help locate the natural place of origin.
We are grateful to Patrick Beckers and Jörn von Döhren (University of Bonn) for their help and effort in providing the information about the type specimen of A. albimaculosa, and also to Ms Tomoko Ikari (now T. Miyamoto: formerly Lake Biwa Museum) for taking digital photographs of ICHUM 5112 in life.