Jesogammarus (Jesogammarus) ikiensis, Tomikawa, Ko, 2015

Tomikawa, Ko, 2015, A new species of Jesogammarus from the Iki Island, Japan (Crustacea, Amphipoda, Anisogammaridae), ZooKeys 530, pp. 15-36 : 19-26

Taxon classification Animalia Amphipoda Anisogammaridae

Jesogammarus (Jesogammarus) ikiensis sp. n. Figures 3, 4, 5, 6, 7, 8, 9

Material examined.

Holotype: NSMT-Cr 24107, male (13.1 mm, 8 slides), river at Ishida (33°45'1.7"N, 129°44'33.7"E), Iki, Nagasaki Prefecture, Japan, collected by K. Tomikawa and S. Tashiro on 9 March 2012. Paratypes: NSMT-Cr 24108, ovigerous female (10.4 mm, 6 slides), NSMT-Cr 24109, 1 male and 1 ovigerous female in ethanol vial, data same as for holotype; NSMT-Cr 24110, 2 males and 2 ovigerous females in ethanol vial, river at Ishida (33°45'1.7"N, 129°44'33.7"E), Iki, Nagasaki Prefecture, Japan, collected by K. Tomikawa and S. Tashiro on 2 April 2015; NSMT-Cr 24111, male (12.0 mm, 6 slides), NSMT-Cr 24112, ovigerous female (9.4 mm, 5 slides), river at Katsumoto (33°49'30.1"N, 129°42'51.5"E), Iki, Nagasaki Prefecture, Japan, collected by K. Tomikawa and S. Tashiro on 8 March 2012; NSMT-Cr 24113, male (11.9 mm, 5 slides), NSMT-Cr 24114, ovigerous female (10.0 mm, 5 slides), river at Ashibe (33°47'3.1"N, 129°45'3.8"E), Iki, Nagasaki Prefecture, Japan, collected by K. Tomikawa and S. Tashiro on 9 March 2012; NSMT-Cr 24115, male (9.2 mm, 5 slides), NSMT-Cr 24116, female with offsprings (7.4 mm, 5 slides), irrigation ditch at Gonoura (33°43'26"N, 129°41'52"E), Iki, Nagasaki Prefecture, Japan, collected by K. Tomikawa and S. Tashiro on 9 March 2012.

Description of male

(holotype, NSMT-Cr 24107). Head (Fig. 4) with short rostrum; ventral margin of lateral cephalic lobe weakly concave; antennal sinus rounded; eyes reniform, major axis 0.4 × height of head. Dorsal surfaces of pereonites smooth (Fig. 4). Dorsal margins of pleonites 1-3 (Fig. 9 C–E) with three, two, and two setae, respectively. Posterior margin of epimeral plate 1 rounded with seta, anteroventral cor ner with many setae (Fig. 9I); posterior margin of plate 2 with one seta, posteroventral corner quadrate, anteroventral corner with three setae, ventral submargin with four robust setae (Fig. 9J); posterior margin of plate 3 with two setae, posteroventral corner quadrate, anteroventral to ventral margin with six setae (Fig. 9K). Urosomites 1-3 (Fig. 9 F–H) with seven, four, and two robust setae associated with slender setae.

Antenna 1 (Fig. 5A): length 0.7 × body length; peduncular articles 1-3 in length ratio of 1.0: 0.9: 0.5; posterodistal corner of peduncular article 1 with one robust seta, posterior margin of peduncular article 2 with one cluster and three pairs of setae, posterior margin of peduncular article 3 with one cluster and one pair of setae; accessory flagellum seven-articulate; primary flagellum 29-articulate, each article with one aesthetasc.

Antenna 2 (Fig. 5B): length 0.7 × antenna 1; posterior margin of peduncular article 4 with three clusters of setae, posterior margin of peduncular article 5 with three clusters of setae and one single seta; flagellum 18-articulate, calceoli present (Fig. 5C).

Mouthparts. Upper lip (= labrum) (Fig. 5D) with rounded distal margin, bearing fine setae. Lower lip (= labium) (Fig. 5E) with broad outer lobes, inner lobes indistinct. Mandibles (Fig. 5 F–H) with left and right incisors six- and four-dentate, respectively, left lacinia mobilis five-dentate, right one bifid, bearing many teeth; molar process triturative, with plumose seta; accessory setal rows of left and right mandibles each with seven blade-like setae; left palp three-articulate with length ratio of 1.0: 3.8: 3.8, palp article 1 bare, article 2 with 28 setae, article 3 with two clusters and one pair of A-setae, one pair of B-setae, and many C-, D-, and E-setae, article 3 of right palp with three clusters of A-seta and one B-seta. Maxilla 1 (Fig. 5I) with inner and outer plates and palp; medial margin and apical submargin of inner plate with 31 plumose setae; outer plate subrectangular, with 11 serrate teeth apically (Fig. 5J); right palp two-articulate, much longer than outer plate, article 1 lacking marginal setae, article 2 with seven robust and six slender setae on its apical margin, outer margin with three setae, left palp lacking setae on outer margin of article 2. Maxilla 2 (Fig. 5K) with oblique inner row of 23 plumose setae on inner plate; outer plate slightly longer than inner plate. Maxilliped (Fig. 6A) with inner and outer plates and palp; inner plate (Fig. 6C) with six robust setae along apical and inner margins; outer plate (Fig. 6B) with plumose setae on apical margin and robust setae on inner margin; palp four-articulate, article 2 with inner marginal and submarginal rows of setae, article 3 with facial setae, article 4 slightly curved inward, with slender nail.

Gnathopod 1 (= pereopod 1) (Fig. 6D): coxa (= article 1) with six setae on ventral margin; anterior and posterior margins of basis (= article 2) with long setae; carpus (= article 5) length 1.4 × width, anterior margin with seta; propodus (= article 6) length 1.2 × length of carpus and 1.3 ×width of propodus, anterior margin with one pair and two clusters of setae, palmar margin (Fig. 6E) oblique, weakly convex, with 16 peg-spines (= robust setae); dactylus (= article 7) (Fig. 6E) as long as palmar margin, with posterior accessory blade longer than nail, blade basally elevated.

Gnathopod 2 (= pereopod 2) (Fig. 6F): coxa with seven marginal and one submarginal setae on ventral part, posteroproximal part with two setae; anterior and posterior margins of basis with long setae; carpus length 1.7 × width, anterior margin with cluster of setae and single seta; propodus almost as long as carpus and 1.5 × width of propodus, anterior margin with two clusters of setae, palmar margin (Fig. 6G) oblique, weakly convex, with 12 peg-spines (= robust setae) and one serrate seta; dactylus (Fig. 6G) as long as palmar margin, with posterior accessory blade longer than nail.

Pereopod 3 (Fig. 7A, B): coxa with seven marginal setae on ventral part, posterio-proximal part with two setae; anterior and posterior margins of basis with long setae, anterio-distal corner of basis with robust seta.

Pereopod 4 (Fig. 7C, D): coxa expanded with posterior concavity, bearing one seta on anterodistal corner and five setae on posterodistal margin; anterior and posterior margins of basis with long setae, anterodistal corner with robust seta.

Pereopod 5 (Fig. 7F, G): coxa bilobed, anterior lobe with apical seta, ventral margin of posterior lobe with three setae; posterior margin of basis weakly expanded, with ten setae; anterior and posterior margins of merus to propodus with robust and slender setae.

Pereopod 6 (Fig. 8A, B): coxa bilobed, anterior lobe with apical seta and anterio-proximal setae, ventral margin of posterior lobe with three setae; posterior margin of basis weakly expanded, with 18 setae; anterior and posterior margins of merus to propodus with robust and slender setae.

Pereopod 7 (Fig. 7D, E): ventral margin of coxa weakly concave, bearing slender setae on anterior part and three setae on posteroventral part; posterior margin of basis weakly expanded, with 20 setae; anterior and posterior margins of merus to propodus with robust and slender setae.

Coxal gills on gnathopod 2 and pereopods 3-5 (Figs 6H, 7A, C, E) with two accessory lobes, gills on pereopods 6 and 7 (Fig. 8C, F) each with one accessory lobe.

Pleopods 1-3 (Fig. 8G) each with paired retinacula (Fig. 8H) on inner margin of peduncle, and bifid plumose setae (= clothes-pin setae) (Fig. 8I) on inner basal margin of inner ramus.

Uropods. Uropod 1 (Fig. 8J): peduncle with robust seta on basofacial part, inner and outer margins each with three robust setae, inner proximal part with three short setae; inner ramus length 0.8 × peduncle, inner margin with three robust setae and outer margin with robust seta and minute seta; outer ramus length 0.9 × inner ramus, inner and outer margins each with two and three robust setae. Uropod 2 (Fig. 8K): peduncle with three robust setae on inner and outer margins, respectively; inner ramus length 0.9 × peduncle, its inner and outer margins with two robust setae, respectively; outer ramus length 0.8 × inner ramus, its outer margin with robust seta. Uropod 3 (Fig. 9A): peduncle length 0.3 × outer ramus; inner ramus length 0.25 × outer ramus (both proximal and terminal articles), with two robust setae on inner margin; outer ramus two-articulate, inner margin of proximal article with five plumose setae, and several robust setae and simple setae, outer margin with robust setae and simple setae, terminal article length 0.2 × proximal article, with short setae apically.

Telson (Fig. 9B) length 1.1 × width, cleft for 59% of length in V-shape; each lobe with one lateral and one apical robust seta.

Description of ovigerous female

(paratype, NSMT-Cr 24108). Antenna 1 (Fig. 9L): length 0.7 × body length; peduncular articles 1-3 in length ratio of 1.0: 0.8: 0.5; accessory flagellum seven-articulate; primary flagellum 36-articulate.

Antenna 2 (Fig. 9M): length 0.5 × antenna 1; flagellum 12-articulate, calceoli absent.

Gnathopod 1 (Fig. 10A): carpus length 1.7 × width, with cluster of setae and single seta on anterior margin; propodus almost as long as carpus and 1.5 × width of propodus, bearing two clusters and one pair of setae on anterior margin; palmar margin (Fig. 10B) with seven robust setae and two pectinate setae.

Gnathopod 2 (Fig. 10C): carpus length 2.2 × width, with one cluster, one pair, and one single seta on anterior margin; propodus length 0.9 and 2.0 × carpus and width of propodus, respectively, bearing one cluster and one pair of setae on anterior margin; palmar margin (Fig. 10D) with two robust and 10 pectinate setae.

Posterior margin of bases of pereopods 5-7 more expanded than in male (Fig. 10 F–H).

Brood plates (= oostegites) (Fig. 10E): broad, with numerous marginal setae.

Uropod 3 (Fig. 10I): peduncle length 0.3 × outer ramus; inner ramus length 0.3 × outer ramus (both proximal and terminal articles), with robust seta on inner margin; inner margin of proximal article of outer ramus with plumose seta, terminal article length 0.2 × proximal article.

Egg number: 175.

Variations.

The number of setae and/or setal bundles on posterior margin of peduncular articles of antennae is variable: antenna 1, two or three on article 1, three or four on article 2, one or two on article 3; antenna 2, two to four on article 4, three to five on article 5. Most specimens have a pair of setae on dorsal margins of pleonites 1-3 but several specimens have three setae. The length ratio of inner ramus of uropod 3 to outer ramus ranged from 0.2 to 0.3 in both sexes. The number of plumose setae on inner margin of outer ramus of uropod 3 varied from two to eight in males and one to three in females. Ovigerous females have 58 to 175 eggs.

Remarks.

Jesogammarus ikiensis sp. n. is assigned to the subgenus Jesogammarus in having well developed posterior accessory lobe of coxal gills on gnathopod 2 and pereopods 3-5, and pectinate setae on palmar margin of female gnathopod 2. The new species is distinguished from Jesogammarus fontanus Hou & Li, 2004, Jesogammarus hebeiensis Hou & Li, 2004, Jesogammarus hinumensis Morino, 1993, and Jesogammarus spinopalpus Morino, 1985 by absence (vs. presence) of setae on article 1 of mandibular palp. Jesogammarus ikiensis is distinguished from Jesogammarus mikadoi Tomikawa, Morino & Mawatari, 2003 by absence (vs. presence) of setae on dorsal margin of pereonites 5-7 and two or three (vs. more than seven) setae on dorsal margins of pleonites 1-3. Jesogammarus ikiensis is distinguished from Jesogammarus paucisetulosus Morino, 1984 by medium eye, major axis of eyes 0.4 × height of head (vs. small, less than 0.3), posterodistal corner of peduncular article 1 of antenna 1 with a robust (vs. slender) seta, posterior margin of peduncular article 2 of antenna 1 with three or four (vs. more than five) setae and/or setal bundles, and posterio-marginal setae on peduncular article 4 of antenna 2 shorter (vs. longer) than width of article 4 in male;. Jesogammarus ikiensis differs from the Jesogammarus jesoensis complex including Jesogammarus fujinoi Tomikawa & Morino, 2003, Jesogammarus hokurikuensis Morino, 1985, Jesogammarus jesoensis (Schellenberg, 1937), Jesogammarus shonaiensis Tomikawa & Morino, 2003, by two or three (vs. more than seven) setae on dorsal margins of pleonites 1-3 and three or four (vs. two) setae and/or setal bundles on posterior margin of peduncular article 2 of antenna 1. Jesogammarus ikiensis differs from Jesogammarus ilhoii Lee & Seo, 1992 by absence (vs. presence) of pectinate setae on palmar margin of propodus of male gnathopod 2 and two or three (vs. more than ten) setae on dorsal margins of pleonites 1-3.

Etymology.

The specific name is from the Latinized Japanese ikiensis (of Iki), referring to the type locality of the new species.

Distribution.

Known only from Iki Island.

Habitat.

River and irrigation ditch.