Thursday, November 30, 2023

[Crustacea • 2023] Gnorimosphaeroma rivulare • A New Freshwater Species of Gnorimosphaeroma (Isopoda: Sphaeromatidae) from Chichi-jima Island, Ogasawara Islands, Japan

Gnorimosphaeroma rivulare Tomikawa, Yoshii & Nunomura, 

in Tomikawa, Yoshii, Noda, Lee, Sasaki, Kimura & Nunomura, 2023

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.

Key Words: freshwater, inland water, isopod, molecular phylogeny, oceanic island, stream, taxonomy

Family Sphaeromatidae Latreille, 1825

Genus Gnorimosphaeroma Menzies, 1954

Gnorimosphaeroma rivulare Tomikawa, Yoshii & Nunomura, sp. nov.
  New Japanese name: Chichijima-kotsubumushi
 Diagnosis: Pleonites incompletely fused; anterior suture line longer than posterior one. Pleotelson posterior margin rounded. Maxillula medial lobe with 4 plumose setae and a short single seta; lateral lobe with 10 robust setae. Maxilla medial lobe with 17 setae; middle lobe with 12 setae; lateral lobe with 13 setae. Pereopod 1 basis with a single seta on posterodistal corner; merus with 4 setae on anterodistal corner. Pereopod 2 propodus subrectangular, not swollen. Pereopod 3 merus, carpus, and propodus sparsely setulose. Uropod exopod length 0.7 times as long as endopod.

Etymology: The specific name rivulare is derived from a Latin adjective rivularis, which means brook living, referring to the habitat of the new species.

Distribution and habitat: This species is known only from the type locality. The specimens were collected from beneath the cobbles in an upper stream of Nagatani River.

 Ko Tomikawa, Junpei Yoshii, Akari Noda, Chi-Woo Lee, Tetsuro Sasaki, Naoya Kimura and Noboru Nunomura. 2023. A New Freshwater Species of Gnorimosphaeroma (Crustacea, Isopoda, Sphaeromatidae) from Chichi-jima Island, Ogasawara Islands, Japan. Zoosystematics and Evolution. 99(2): 545-556. DOI: 10.3897/zse.99.108032