First record of Australatyaobscura Han & Klotz, 2015 (Decapoda, Atyidae) from the Ryukyu Islands, Japan

Abstract Background The freshwater shrimp, Australatyaobscura Han & Klotz, has been known only from Taiwan and Panay Island, Philippines. New information An adult A.obscura was collected from a river on Ishigaki Island, Ryukyu Islands, Japan. This is the first record of the species in Japan and the northernmost specimen-supported record to date. The species is suggested as having been transported northwards by the Kuroshio Current.


Introduction
The Ryukyu Islands are located southwest of the Japanese mainland and are the northernmost region with a tropical rainforest climate. The region is known for its high diversity of atyid shrimps which currently includes 22 species belonging to seven genera, namely Antecaridina, Atyoida, Atyopsis, Caridina, Halocaridinides, Neocaridina and Paratya (Cai and Shokita 2006, Chace 1983, Toyota and Seki 2014, Weese et al. 2012. In addition, an undescribed species, considered to belong to a new genus, was reported as Atyidae gen. sp. by Fujita et al. (2017). However, shrimps of the genus Australatya Chace, 1983 have not been previously recorded.
Australatya obscura Han & Klotz, 2015 is a rare freshwater filter-feeding atyid shrimp. Although the small and numerous eggs indicate a possibly wide distribution range in South-East Asia, the species has been reported only from Taiwan and Panay Island, Philippines (Han and Klotz 2015). This study reports the first Japanese record of A. obscura, based on an adult specimen.

Materials and methods
The shrimp was caught by hand net from under a stone in a fast-flowing stream of the Fukido River, Ishigaki Island, Ryukyu Islands, Japan on 11 March 2018. Its distinctive colouration clearly separated it from other filter-feeding shrimps (Atyopsis spinipes) collected at the time. After being photographed, the specimen was preserved in 70% ethanol and later examined under a microscope and measured using a digital caliper. The carapace length was measured from the postorbital margin to the posterior margin of the carapace. The specimen was deposited in the Kanagawa Prefectural Museum of Natural History (registration number: KPM-NH 3213).

Diagnosis
The specimen was a non-ovigerous adult female with carapace length 6.5 mm and rostral formula 0/4. The third maxilliped was slender, lacking terminal spiniform seta. The merus of the third pereiopod had a row of plumose setae. The live specimen had black and white vertical bands on the abdominal segments (Fig. 1). The morphological features and live colouration of the specimen agreed well with the original description of Australatya obscura (Han and Klotz 2015).

Discussion
The present report is the first record of the species in Japan and the northernmost record based on a voucher specimen. However, a photograph of an immature A. obscura collected on Okinawa Island, Ryukyu Islands was taken by Tsuhako (2016). Known localities of A. obscura are shown in Fig. 3. It is believed that individuals of the species found in the Ryukyu Islands were transported from southern habitats by the Kuroshio Current during their larval stage. Considering that the current specimen was an adult and must have overwintered, it seems likely that the species could gain a foothold in the Ryukyu islands.  The new standard Japanese name "Hime-oni-numa-ebi" is proposed for A. obscura. The body size of A. obscura is relatively small compared with other Atya-like species in Japan, the new Japanese name reflecting its small size.