A new species of Leipsuropus Stebbing, 1899 (Amphipoda: Podoceridae) from Japan

Abstract A new podocerid amphipod, Leipsuropus seisuiae sp. nov., from 338-340 m depth in the Kumano Sea, Japan, Northwestern Pacific, is described. This is the deepest record of any Leipsuropus species. Leipsuropus seisuiae sp. nov. is distinguished from four other congeneric species in having: i) pereonites 2-5 each with five narrow spiniform projections on tergal plate, ii) one dorsal tubercle on the telson, and iii) one small, proximal, denticulate projection on gnathopod-2 palmar margin. A key to males of the species of Leipsuropus is provided.


INTRODUCTION
The family Podoceridae Leach, 1814 is one of five families of the superfamily Caprelloidea Leach, 1814 (Lowry and Myers, 2017).Among which, Podoceridae closely resembles Dulichiidae Dana, 1849 but differs in having a rectangular head and pereopod 7 attached ventrally (not posteroventrally) to pereonite 7 (cf.Myers and Lowry, 2003).The family currently contains eight genera and 89 species (Horton et al., 2022).From Japanese waters, four podocerid species have been reported, and all of them were collected from localities shallower than 100 m depth (Ariyama, 2012;Tomikawa et al., 2019).
Leipsuropus Stebbing, 1899 is one of eight podocerid genera characterized by i) pereonite 1 shorter than head, ii) segmented urosomites 1-3, and iii) two paired uropods (uropod 2 absent).As with other podocerids, members of this genus are efficient climbers on algae and invertebrates such as sponges,
In this paper, we describe a new Leipsuropus species based on a single male individual collected from 338-340 m depth in the Kumano Sea, Japan, Northwestern Pacific.This extends the vertical limit of the known distribution of this genus from 108 m to 338 m.In addition, we present a key to males of the currently known species of Leipsuropus.

MATERIAL AND METHODS
The single individual was obtained in the Kumano Sea (Fig. 1) with a biological dredge on 7 November 2017 during a research cruise of TR/V "Seisui-maru" (Mie University, Japan).Collected bottom sediment was sieved by a 0.45-mm mesh; the podocerid was picked from the residue and then fixed and preserved in 70% ethanol.The methods used for dissection, preparation of slides, light microscopy, and drawing were as described by Kakui and Angsupanich (2012).The studied specimen was deposited in the Invertebrate Collection of the Hokkaido University Museum (ICHUM), Sapporo, Japan.
Morphological terminology follows LeCroy (2000) and Ariyama (2012).Body length was measured from the base of the antenna 1 along the lateral margin to the posterior end of the urosomite 3. Pereopod lengths were measured from the proximal end of basis along the lateral margin to the distal end of dactylus.Pleopod lengths were measured from the proximal end of peduncle to the distal end of the inner ramus.All measurements were done with ImageJ (https:// imagej.nih.gov/ij/).Diagnosis (male).Pereonites 2-5 each with five narrow spiniform projections on tergal plate.Pereonites 6 and 7 segmented.Telson with dorsal tubercle.Gnathopod-2 palm with small, proximal, denticulate projection.
Etymology.The specific name is a noun in the genitive case and derived from TR/V "Seisui-maru", the vessel from which the type specimen was collected.
Distribution.So far known only from the type locality.
Remarks.Leipsuropus seisuiae sp.nov. is the fifth named species in Leipsuropus.Pereonites 2-7 of male L. seisuiae sp.nov. each has five projections on the tergal plate: one dorsal, a pair of dorsolateral, and a pair of lateral projections.This condition was also found in male L. hongi, but L. seisuiae sp.nov. is distinguished from L. hongi by: i) dorsal projection on the pereonites is narrow and spiniform (broad, keellike in L. hongi), ii) gnathopod-2 palmar margin has one small, proximal, denticulate projection (one wide and two large triangular projections in L. hongi), and iii) the telson has a dorsal tubercle (no tubercle in L. hongi) (Ariyama, 2012).Differences among congeners including L. seisuiae sp.nov. in a combination of five character states are presented in Tab. 1.A key to world Leipsuropus males is given below.2012) Barnard and Drummond (1981) Ren (2012)

Figure 1 .
Figure 1.Map showing the sampling site.A, B, map showing the sampling site in Japan and the Kumano Sea.Maps were generated by using GMT6 (A, Wessel et al., 2019) or based on GSI Maps (B, https://maps.gsi.go.jp).
, J1) with inner plate bearing 6 distal setulose setae and 3 distal short robust setae; outer plate with 4 distal setulose setae and 5 medial robust setae and several medial simple setae.Palp with 4 articles; article 1 naked; article 2 with distal and medial setulose setae; articles 3-4 with setulose setae on almost all surfaces.

Table 1 .
Comparison of selected characters between males in Leipsuropus.