Melita choshigawaensis, Tomikawa, Ko, Hirashima, Kentaro, Hirai, Atsushi & Uchiyama, Ryu, 2018

Tomikawa, Ko, Hirashima, Kentaro, Hirai, Atsushi & Uchiyama, Ryu, 2018, A new species of Melita from Japan (Crustacea, Amphipoda, Melitidae), ZooKeys 760, pp. 73-88 : 73

Melita choshigawaensis sp. n. Figures 2, 3, 4, 5, 6, 7 New Japanese name: Choshigawamerita-yokoebi

Material examined.

Holotype: male (BL 5.3 mm, NSMT-Cr 25826), Choshi River, Kihoku, Mie Prefecture, Japan (34.108242°N, 136.221998°E), col. Ko Tomikawa, Kentaro Hirashima, Atsushi Hirai, and Ryu Uchiyama, 2 March 2017. Paratypes: male (BL 4.1 mm, NSMT-Cr. 25827), data as for holotype; male (BL 6.8 mm, NSMT-Cr. 25828), female (BL 5.8 mm, NSMT-Cr. 25829), 27 December 2017, locality and collectors as for holotype.

Diagnosis.

Male gnathopod 2 propodus with oblique palmar margin; anterior lobe of female pereopod 6 coxa deep and strongly hooked; male uropod 3 outer ramus uni-articulate, weakly arched, its length 7-8 times its width, lacking long setae.

Description male

(holotype, NSMT-Cr 25826). Head (Fig. 2) slightly shorter than pereonites 1 and 2 combined; rostrum short; eyes ovate; lateral cephalic lobe rounded; antennal sinus quadrate, not incised. Pereonites 1-7 (Fig. 2) dorsally smooth with fine setae. Dorsal margins of pleonites 1-3 (Fig. 3 A–C) with 2, 2, and 7 setae, respectively; epimeral plate 1 (Fig. 3E) ventral submargin with three robust and one slender setae, posterior margin with two setae, posterodistal corner weakly pointed with seta; epimeral plate 2 (Fig. 3F) ventral margin with three robust setae, posterior margin with two setae, posterodistal corner pointed with seta; epimeral plate 3 (Fig. 3G) ventral margin with robust seta, posterior margin bare, posterodistal corner weakly pointed with seta. Dorsal margin of urosomite 2 (Fig. 3D) with four robust setae.

Antenna 1 (Fig. 3H): length 1.1 times that of body; length ratio of peduncular articles 1-3 as 1.0:1.3:0.7; ventral margin of peduncular article 1 with three robust setae, posterodistal corner with robust seta; primary flagellum 28-articulate with a few setae; accessory flagellum (Fig. 3I) 2-articulare, with short terminal article. Antenna 2 (Fig. 3J) half of antenna 1 length; peduncular article 5 length 0.9 times that of article 4; flagellum 7-articulate, article 1 length 1.5 times that of article 2; calceoli absent.

Upper lip (Fig. 3K) ventral margin convex, rounded, with minute setae. Left and right mandibular incisors (Fig. 3 L–N) 4- and 5-dentate, respectively, with left lacinia mobilis quadri-dentate (Fig. 3M) and right (Fig. 3N) multidentate; left and right accessory setal rows (Fig. 3M, N) with five and three bladed setae, respectively; molar process triturative with plumose seta; palp tri-articulate, length ratio of articles 1-3 1.0:2.3:2.0, article 1 bare, article 2 with two setae, article 3 with seven setae. Lower lip (Fig. 3O) outer lobes broad, setulose, mandibular lobes narrow; inner lobes distinct. Maxilla 1 (Fig. 3P, Q) inner plate narrow with six plumose setae; outer plate rectangular with nine serrate robust setae; palp 2-articulate; article 1 rectangular, lacking setae; article 2 expanded, outer margin without setae, apical margin with robust and slender setae. Maxilla 2 (Fig. 3R) inner plate with oblique inner row of seven setae; outer plate slightly longer than inner plate. Maxilliped (Fig. 3S) distal part of inner plate not reaching half of palp article 2; outer plate ovate, exceeding half of palp article 2, apical margin with plumose setae, inner submargin with robust setae; palp quadri-articulate, article 4 with nail.

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.

Pereopod 3 (Fig. 4D) coxa subrectangular, ventral margin and submargin with setae; basis arched, anterior and posterior margins with long and short setae; length ratio of merus, carpus, propodus and dactylus 1.0:0.9:0.8:0.3. Pereopod 4 (Fig. 4E): coxa expanded with posterior concavity, bearing ventral and surface setae; basis anterior and posterior margins with long and short setae; length ratio of merus, carpus, propodus and dactylus 1.0:0.9:0.8:0.3. Pereopod 5 (Fig. 5A) coxa bilobate, anterior lobe large with small seta on distal margin, posterior lobe with small setae on ventral margin and posterodistal corner; basis with posterodistal lobe; length ratio of merus, carpus, propodus and dactylus 1.0:0.8:0.9:0.2; merus weakly expanded, half as wide as long. Pereopod 6 (Fig. 5B) coxa bilobate, shallower than that of pereopod 5, posterior lobe with small seta on posterodistal corner; basis posterior margin weakly serrate, posterodistal corner lobate; length ratio of merus, carpus, propodus and dactylus 1.0:0.9:1.3:0.3; merus weakly expanded, half as wide as long. Pereopod 7 (Fig. 5C) coxa semicircular, with seta on posterior margin; basis subovate, posterior margin weakly serrate, bearing posterodistal lobe; length ratio of merus, carpus, propodus and dactylus 1.0: 0.8:1.2:0.3; merus 0.4 times as wide as long.

Coxal gills (Fig. 2) present on gnathopod 2, and pereopods 3-6.

Pleopod 1-3 (Fig. 5D) peduncles with paired retinacula (Fig. 5E) on inner distal margin, and bifid plumose setae (clothes-pin setae) on inner ramus inner basal 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.

Description female

(paratype, NSMT-Cr 25829). Antenna 1 (Fig. 6A) 0.6 times body length; length ratio of peduncle articles 1-3 1.0:1.2:0.7; ventral margin of peduncular article 1 without robust setae; primary flagellum 17-articulate. Antenna 2 (Fig. 6B) length half that of antenna 1.

Gnathopod 1 (Fig. 6C, D) coxa elongate, anterior margin weakly concave; carpus length 1.6 times that of propodus; proximal part of palmar margin without protuberance. Gnathopod 2 (Fig. 6E, F): coxa elongate; anterior margin of basis with seta; carpus length 0.8 times that of propodus; propodus 0.6 times as wide as long, palmar margin with six medial and six lateral robust setae.

Pereopods 5-7 (Fig. 6G, H, J). Depth of pereopod 6 (Fig. 6I) anterior lobe equal to coxal width, strongly hooked; merus width 0.4 times that of length.

Uropod 3 (Fig. 6K) inner ramus length 0.15 times that of outer ramus; outer ramus sublinear, length 2.5 times that of peduncle and 6.6 times outer ramus width.

16 eggs.

Variation.

Uropod 3 outer ramus length 2.8 times that of peduncle and 8.2 times outer ramus width (male 6.8 mm, NSMT-Cr 25828).

Sequences and COI genetic distances.

In total, 658 bp of six nucleotide sequences were determined: paratypes of M. choshigawaensis sp. n. (NSMT-Cr 25827-25829), three sequences (LC371923-371925); and M. shimizui from three localities, one from Lake Hinuma (LC371926), one from Seno River (LC371927), and one from Ota River (LC371928). Uncorrected p-distances between M. choshigawaensis and M. shimizui were 14.9% (Table 1). Intraspecific distances of M. choshigawaensis and M. shimizui were up to 0.2% and 4.4%, respectively (Table 1).

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, and M. myersi Karaman, 1987 from Australia, Fiji, and New Caledonia in that all three have dorsally smooth pleonites, a urosomite 2 with robust setae on their 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).