Latitudinal shift of the associated hosts in Sagamiscintilla thalassemicola (Galeommatoidea: Galeommatidae), a rare ectosymbiotic bivalve that lives on the proboscis of echiuran worms

Sagamiscintilla thalassemicola (Bivalvia: Galeommatoidea: Galeommatidae) is a rare ectocommensal bivalve that lives on the proboscis of echiuran worms, Anelassorhynchus spp. (Annelida: Thalassematidae: Thalassematinae: Thalassematini), and has been known only from the temperate zones of Japan. In this study, we found S. thalassemicola on the proboscis of the large echiuran Ochetostoma sp. (Thalassematidae: Thalassematinae: Thalassematini) on intertidal flats of three islands of the Ryukyu Archipelago, southern Japan. These are the first records of S. thalassemicola on non-Anelassorhynchus hosts and also from the subtropical regions. Additionally, we also collected S. thalassemicola from an intertidal flat of Kushimoto, Wakayama, Kii Peninsula, Japan, which is an update of the easternmost record of this species. The genetic differences in the mitochondrial cytochrome c oxidase subunit I and nuclear internal transcribed spacer 2 genes among S. thalassemicola, including those with Ochetostoma sp. from the subtropical region and with Anelassorhynchus spp. from the temperate region, can be considered within the intraspecific variation. These suggest that S. thalassemicola uses different echiuran hosts in the temperate and subtropical regions, respectively.