﻿A new species of the genus Podocerus from the Seto Inland Sea, Japan (Crustacea, Amphipoda, Podoceridae)

﻿Abstract A new podocerid amphipod, Podocerussetouchiensissp. nov., is described from the Etajima Island, the Seto Inland Sea, Japan. This new species differs from its congeners by the dorsal carination of pereonites and pleonites, and form of the antenna 1, gnathopods 1 and 2, uropods 1 and 2, and telson. Nucleotide sequence data of the mitochondrial cytochrome c subunit I (COI) from a paratype of Podocerussetouchiensissp. nov. is provided for future molecular systematic studies.


Introduction
Podocerus Leach, 1814, is an amphipod crustacean genus belonging to the family Podoceridae Leach, 1814 and is cosmopolitan in world seas (Barnard and Karaman 1991). So far, 63 species of Podocerus have been described worldwide (Horton et al. 2022). In Japan, Nagata (1965b) recorded P. inconspicuus (Stebbing, 1888) from the sandy mud bottom of the Seto Inland Sea. Hughes (2012) noted morphological differences between Nagata's (1965b) description of P. inconspicuus recorded from Japan and the original description of this species, and considered them to be distinct species. Yamato (1992) described P. umigame Yamato, 1992 from green algae growing on the shell of the loggerhead sea turtle Caretta caretta. However, this podocerid species is now considered a synonym of P. chelonophilus (Chevreux & Guerne, 1888) (Baldinger 2001;Kilgallen 2009;Hughes 2016). Recently, Tomikawa et al. (2019) described P. jinbe Tomikawa, Yanagisawa & Vader, 2019, a unique species that lives in the mouth of a whale shark (Tomikawa et al. 2019).
The Seto Inland Sea is the largest inland sea in Japan, surrounded by the three of four largest islands in Japan (excluding Okinawa), Honshu, Shikoku, and Kyushu, with more than 700 islands and a rich marine ecosystem. More than 90 species of amphipods have been reported from the Seto Inland Sea (Nagata 1965a;Hirayama 1987;Tomikawa et al. 2016). During our field survey of shallow-water amphipods in the Seto Inland Sea, an undescribed species of Podocerus was collected from coasts of Hiroshima and Okayama Prefectures. In this study, this undescribed species is described and illustrated. In addition, DNA sequence data will be provided for future taxonomic studies based on molecular data.

Sampling and morphological observation
Specimens were collected using a hand net and fixed in 99% ethanol on-site (Fig. 1). Some specimens were frozen, then fixed and preserved with polyvinyl alcohol.
All appendages were dissected using insect pins in 80% ethanol and mounted in gum-chloral medium on glass slides using a stereomicroscope (Olympus SZX7). Slides were examined using a light microscope (Nikon Eclipse Ni), with appendages illustrated using a camera lucida. Bodies were dehydrated through a graded ethanol series, and dried using hexamethyldisilazane (HMDS) (Nation 1983). They were then sputter coated with gold and observed using scanning electron microscopy (SEM, JSM-6510LV). Body length was measured from the rostrum tip to the telson base, along the dorsal curvature to the nearest 0.1 mm. The specimens have been deposited in the National Museum of Nature and Science, Tsukuba (NSMT).
Etymology. The specific name is derived from the Seto Inland Sea, where this new species is distributed. DNA Sequence. A sequence of COI (GenBank accession number LC719250; 658 bp) was determined from the paratype female (NSMT-Cr 30868).