Chemosynthesis-based ecosystems such as hydrothermal vents and hydrocarbon seeps harbour various endemic species, each uniquely adapted to the extreme conditions. While some species rely on obligatory relationships with bacterial symbionts for nutrient uptake, scavengers and predators also play important roles in food web dynamics in these ecosystems. Acoels, members of the phylum Xenacoelomorpha, are simple, worm-like invertebrates found in marine environments worldwide but are scarcely understood taxa. This study presents a novel genus and species of acoel from a deep-sea hydrocarbon seep off Hatsushima, Japan, Hoftherma hatsushimaensis gen. et sp. nov. Our multi-locus phylogenetic analysis revealed that the acoels are nested within Hofsteniidae, a family previously known exclusively from shallow waters. This finding suggests that at least two independent colonization events occurred in the chemosynthesis-based environments from the phylum Xenoacoelomorpha, represented by hofsteniid acoels and Xenoturbella. Previous reports of hofsteniid species from low-oxygen and sulfide-rich environments, including intertidal habitats with decomposing leaves, in addition to H. hatsushimaensis gen. et sp. nov. from a deep-sea hydrocarbon seep, imply a common ancestral adaptation to sulfide-rich ecosystems within Hofsteniidae. Moreover, the sister relationship between solenofilomorphid acoels predominating in sulfide-rich habitats indicates common ancestral adaptation to sulfide-rich ecosystems between these two families.
We establish a new interstitial polyclad species, Theama japonica sp. nov., based on specimens collected from coarse-sandy habitats in three Japanese main islands (Hokkaido, Honshu, and Shikoku) along the coasts of the Pacific Ocean and the Sea of Japan. Theama japonica is characterized by i) two pairs of cerebral eyespots and four to six precerebral eyespots; ii) eosinophilic secretion glands distributed in the distal half of the inner ventral part of the prostatic vesicle; iii) a conical penis papilla, bent up dorsally, with a sclerotized inner wall; iv) the prostatic sheath with an inner angular fold on the dorso-distal side; and v) the external cilia longer dorsally than ventrally. Partial sequences of the cytochrome c oxidase subunit I (COI) gene from 20 specimens collected at eight localities along Japanese coasts represented 19 haplotypes. The uncorrected p-distances among these COI haplotypes fell within intraspecific variations observed in other polyclads. A network analysis based on these COI haplotypes suggested a geographically non-cohesive genetic structure of the species, possibly indicating the species' high dispersibility. Molecular phylogenetic analyses based on a concatenated dataset of 18S and 28S rDNA sequences showed T. japonica formed a clade with other Theama species. The resulting tree also indicates that our new species is more closely related to Theama sp. from Colombia than species from Panama and Croatia.
A new fish leech, Branchellion brevicaudatae sp. n., is described based on specimens parasitizing the gills of the short-tail stingray, Bathytoshia brevicaudata (Hutton, 1875), collected from Japanese waters. The new species can be distinguished from other congeners by having: i) pulsating vesicles emerging from posterior base of branchiae, one pair per somite; ii) dorsal white spots, not arranged in longitudinal row; and iii) blackish body. A phylogenetic tree based on partial sequences of the mitochondrial cytochrome c oxidase subunit I gene from the new species and other piscicolid worms showed that the new species is sister to Branchellion torpedinis Savigny, 1822. This is the first record of Branchellion Savigny, 1822 from Japanese waters.