A new freshwater species of Gnorimosphaeroma (Crustacea, Isopoda, Sphaeromatidae) from Chichi-jima Island, Ogasawara Islands, Japan

Abstract

This study describes Gnorimosphaeroma rivulare sp. nov. from a stream on Chichi-jima Island, Ogasawara Islands, Japan. This is the second freshwater species of Gnorimosphaeroma and the third Sphaeromatidae from oceanic islands. Gnorimosphaeroma rivulare sp. nov. is morphologically similar to G. boninense Nunomura, 2006, G. naktongense Kwon & Kim, 1987 and G. saijoense Nunomura, 2013. However, G. rivulare sp. nov. differs from these species in various morphological features, such as the shape of pleotelson and pereopod 2, relative length of antennule peduncular articles and pleopod 3 rami, number of setae on maxillula and maxilliped, and setation on pereopod 3. Phylogenetic analyses revealed that G. akanense is sister to G. saijoense, and together they are sister to G. hokurikuense. This three taxa clade is sister to G. rivulare sp. nov. with G. iriei basal to them all. Our analysis concludes that G. boninense from Haha-jima Island, Ogasawara Islands is only distantly related to G. rivulare and may represent an independent colonization event.