A new species of Melita from Japan (Crustacea, Amphipoda, Melitidae)

Abstract A new brackish-water species of melitid amphipod, Melita choshigawaensis, from the Choshigawa River, Mie Prefecture, Japan, is named and described. Melita choshigawaensis sp. n. is distinguished from the most similar M. shimizui (Uéno, 1940) by having an elongate and weakly arched male uropod 3, and a deep and strongly hooked anterior lobe of the coxa on the female’s pereopod 6. Nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I (COI) of M. choshigawaensis and M. shimizui support divergence at the species level. A key to the Japanese species of Melita is provided.

During field surveys of aquatic fish and amphipod faunas in the Choshi River, Mie Prefecture, Japan, a new amphipod species was found. Though DNA nucleotide sequence data have been recently successfully used to differentiate morphologically similar amphipod species (Matsukami et al. 2017;Tomikawa et al. 2016Tomikawa et al. , 2017, previous taxonomic studies on Melita in Japan have focused on morphological characteristics only. Here, both molecular and morphological data are used to differentiate this species from others, which is described and illustrated. A key to species of Melita in Japanese waters using conventional morphological criteria is provided.

Specimens
Specimens were collected using a hand net (mouth 25 cm wide, 17 cm high, mesh size 0.1-0.5 mm) from under stones at the mouth of Choshi River, Kihoku, Mie Prefecture ( Fig. 1), before being fixed in 99% ethanol. The specimens have been deposited in the National Museum of Nature and Science, Tsukuba (NSMT)

Morphological observation
All appendages were dissected 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. Body length (BL, to the nearest 0.1 mm) was measured from the rostrum tip to the telson base, along the dorsal curvature. Type specimens are deposited at the National Museum of Nature and Science, Tsukuba (NSMT).
Gnathopod 1 (Fig. 4A, B) smaller than gnathopod 2; ventral margin and posterior submargin of coxa with setae; basis, anterior and posterior margins with long setae, posterodistal submargin with tiny palmate setae; ischium with tiny palmate  setae; merus with small ventral setae; carpus not lobate, length 1.5 times that of propodus, anterior submargin with small setae, posterior margin with clusters of setae; propodus without anterodistal hood, palmar margin convex with two rows of robust setae, proximal part of palmar margin with distinct protuberance; dactylus short, not exceeding palmar margin. Gnathopod 2 (Fig. 4C) coxa subrectangular, ventral margin and submargin with setae; basis anterior margin bare, posterior margin with long setae, antero-and posterodistal corners with small setae, posterodistal submargin with small palmate setae; carpus not lobate, length 0.5 times that of propodus; propodus large, half as wide as long, palmar margin oblique with nine medial and ten lateral robust setae; dactylus with small posterodistal notch, of similar length to palmar margin.
Uropod 1 (Fig. 5F) extending beyond uropod 2; peduncle with basofacial seta; inner ramus length 0.6 times that of peduncle, with two inner marginal and four distal robust setae, proximal part with slender seta; outer ramus 1.1 times longer than inner ramus, bearing two outer marginal and four distal robust setae. Uropod 2 (Fig. 5G) not extending beyond peduncle of uropod 3; inner ramus 0.9 times as long as peduncle, with two inner robust setae, distal part with five robust setae; outer ramus 0.9 times as long as inner ramus, with one inner and two outer robust setae, distal part with four robust setae. Uropod 3 (Fig. 5H, I) peduncle extending beyond telson; inner ramus length 0.13 times that of outer ramus, with distal robust seta; outer ramus with single article, weakly arched, length 2.9 times that of peduncle and 7.0 times that of outer ramus width, long setae absent. Telson (Fig. 5J) length 1.1 times longer than wide, completely cleft, each lobe with two lateral and three distal robust setae.
Distribution. Known only from the type locality. Etymology. Derived from the name of the type locality. Remarks. Melita choshigawaensis is closely related to M. shimizui (Uéno, 1940), originally described from a freshwater pond on Liaodong Peninsula, China (Uéno 1940), but subsequently recorded from several brackish sites in the Japanese archipelago, such as Honshu, Shikoku, Kyushu, and the main island of Okinawa (Yamato 1988). Recently, Labay (2016) described a new subspecies, M. shimizui sakhalinensis from Sakhalin. The pleonites of both species lack dorsal teeth, urosomite 2 has robust setae on the dorsal margin, the accessory flagellum of antenna 1 is bi-articulate, and the outer ramus of uropod 3 is uni-articulate and lacks long setae. However, M. choshigawaensis can be distinguished from M. shimizui by (features of M. shimizui in parentheses): the outer ramus of male uropod 3 being weakly arched (compared with sublinear) and more than seven times longer than wide (ca. 5), and the anterior lobe of the female pereopod 6 coxa is deep, equal in length to coxal width (shorter than width), and strongly (as opposed to weakly) hooked. These two species also differ genetically in COI (14.9%) greater than distances (3.5-4%) proposed as thresholds for amphipod species discrimination (Witt et al. 2006;Rock et al. 2007;Hou et al. 2009). Thus, we determined M. choshigawaensis represented a novel species.
Melita choshigawaensis is similar to M. laevidorsum Stephensen, 1944 from Korea, andM. myersi Karaman, 1987 from Australia, Fiji, andNew Caledonia in that all three have dorsally smooth pleonites, a urosomite 2 with robust setae on their  (Uéno, 1940). Numbers after localities correspond to locations in Figure 1.  (Uéno, 1940) Lake Hinuma, Ibaraki (2) 14.9 -3 Seno River, Hiroshima (3) 14.9 4.4 -4 Ota River, Hiroshima (4) 14.9 4.4 0.0 -dorsal margins, and an elongate outer ramus of uropod 3 (Stephensen 1944;Karaman 1987). However, M. choshigawaensis differs from M. laevidorsum in having an accessory flagellum of antenna 1 with two articles (compared with four), in lacking an anterodistal hood on the propodus of male gnathopod 1 (compared with having one), and in that the medial surface of the propodus of male gnathopod 2 is sparsely (as opposed to densely) setose. From M. myersi, M. choshigawaensis differs in having a deep antennal sinus (compared with shallow), in lacking an anterodistal hood on the propodus of the male's gnathopod 1 (compared with having one), and in having the meri of pereopods 5 and 6 weakly expanded (as opposed to their not being expanded).

Key to species of Melita in Japan
Since records of three species, M. coroninii Heller, 1867, M. dentata (Krøyer, 1842), and M. palmata (Montagu, 1804) from Japanese waters are dubious (Ishimaru 1994), these species are excluded from the key.