Gammarus somaemulensis, Ahn & Lee & Min, 2022

Ahn, Yong-Uk, Lee, Chi-Woo & Min, Gi-Sik, 2022, A new species of Gammarus (Crustacea, Amphipoda, Gammaridae) from South Korea, ZooKeys 1117, pp. 53-69 : 53

Gammarus somaemulensis sp. nov.

Figs 2 , 3 , 4 , 5 , 6 , 7 New Korean name: so-mae-mul-yeop-sae-u

Material examined.

Holotype: male, dissected on 14 slides (NNIBRIV92290), 9.0 mm, Maejuk-ri (34°37'23"N, 128°32'57.1"E), Somaemuldo Island , Hansan-myeon, Tongyeong-si, Gyeongsangnam-do, South Korea, October 7, 2021, collected by Y. U. Ahn. Paratypes: male, dissected on 10 slides (NNIBRIV92291), 8.4 mm; male, dissected on 10 slides (NNIBRIV92292), 8.8 mm; male, dissected on 10 slides (NNIBRIV92293), 8.2 mm; male, dissected on 11 slides (NNIBRIV92294), 8.4 mm; male, dissected on 11 slides (NNIBRIV92295), 8.9 mm; female, dissected on 10 slides (NNIBRIV92296), 8.2 mm; female, dissected on 10 slides (NNIBRIV92297), 7.8 mm; 11 males and 3 females in ethanol vials (NNIBRIV92298-NNIBRIV92311); all other data same as holotype .

Etymology.

The specific name somaemulensis is derived from the name of the type locality, Somaemuldo Island.

Diagnosis.

Antenna 2 with four clusters of long setae on posterior margin of peduncular article 4, calceoli absent; pereopods 3 and 4 with long straight setae on posterior margins of merus and carpus; pereopods 6 and 7 without anteroproximal setae on basis; inner ramus of uropod 3 reaching approximately 0.8 × as long as outer ramus, outer ramus with plumose setae on both margins, terminal article of outer ramus shorter than adjacent spines.

Description of male.

Head (Fig. 2 ): rostrum short; inferior antennal sinus deep; eyes reniform.

Antenna 1 (Fig. 3A ): peduncular articles 1-3 in length ratio 1.0: 0.7: 0.4, bearing distal setae clusters on each peduncular article; main flagellum 33-articulate, each article with short distal setae; accessory flagellum four-articulate, article 4 very short.

Antenna 2 (Fig. 3B ): peduncular article 1 with three short setae distally; gland cone tapering distally; anterior, posterior and interior margins of peduncular article 4 with four, four and five clusters of setae, respectively, length of longest seta on posterior margin 1.6 × the width of peduncular article 4; peduncular article 5 slightly longer than article 4, anterior, posterior and interior margins with six clusters of setae, respectively, length of longest seta on posterior margin 2.3 × the width of peduncular article 4; flagellum 11-articulate, calceoli absent.

Upper lip (Fig. 3C ): rounded, ventral margin with numerous minute setae.

Lower lip (Fig. 3D ): inner lobes absent, outer lobes broad.

Mandible (Fig. 3E, F ): incisor of left mandible with five teeth; lacinia mobilis of left mandible with four teeth; molar triturative, bearing one plumose seta; palp three-articulate in length ratio 1.0: 3.1: 2.2, article 1 unarmed, article 2 with 19 marginal setae, article 3 bearing eight B-setae on inner surface, six A-setae on outer surface, 28 D-setae on posterior margin and five E-setae apically; right mandible incisor with four teeth; lacinia mobilis of right mandible bifurcate, with small teeth.

Maxilla 1 (Fig. 3G, H ): inner plate with 17 plumose setae; outer plate with 11 serrated spines apically; palp two-articulate and asymmetrical, right palp shorter and stouter than left palp, article 2 of right palp with five stout spines, one slender spine and one seta apically; article 2 of left palp with five slender spines and eight setae apically.

Maxilla 2 (Fig. 3I ): inner plate bearing 17 plumose setae in an oblique row; outer plate broader than inner plate; both plates with numerous long setae apically.

Maxilliped (Fig. 3J ): inner plate bearing three stout spines apically; outer plate with a row of blade-like spines and two plumose setae; palp four-articulate, article 1 unarmed, inner margin of article 2 with numerous setae, article 3 curved, with numerous setae on posterior margin and a row of subapical setae, article 4 hooked, with three setae at hinge of unguis.

Gnathopod 1 (Fig. 4A, B ): coxal plate with two setae on both anterodistal and posterodistal corners; basis with long setae on both anterior and posterior margins; length of carpus 1.4 × as long as width, 0.8 × as long as propodus, bearing two clusters of setae on anterior margin; propodus pyriform in shape, palm oblique, with one medial palmar spine and 11 spines on posterior margin; dactylus exceeding near half of propodus, outer margin with one seta.

Gnathopod 2 (Fig. 4C, D ): coxal plate with three setae on anterodistal corner and one seta on posterodistal corner; basis similar to that of gnathopod 1; length of carpus 1.7 × as long as width, 0.8 × the length of propodus, with four clusters of setae on anterior margin; propodus subrectangular in shape, palm concave, with one medial palmar spine and four spines on posterodistal corner; dactylus curved beyond the palmar margin, bearing one seta on outer margin.

Pereopod 3 (Fig. 4E, J ): coxal plate with two setae on anterodistal corner and one seta on posterodistal corner; basis with long setae on both anterior and posterior margins; merus bearing two spines accompanied by setae on anterior margin, eight clusters of long straight setae on posterior margin, the longest seta of them approximately 2.0 × as long as width of merus, anterodistal corner bearing one spine accompanied by setae; carpus with five clusters of long straight setae on posterior margin, one spine accompanied by setae on both anterodistal and posterodistal corners; propodus with three spines accompanied by clusters of setae on posterior margin, one spine on posterodistal corner; dactylus bearing one plumose seta on anterior margin, two setae at hinge of unguis.

Pereopod 4 (Fig. 4F ): coxal plate with posterior excavation, bearing two setae on anterodistal corner and four setae on posterior margin; basis similar to that of pereopod 3; merus with one spine accompanied by setae on anterior margin, four clusters of long straight setae on posterior margin, the longest seta of them approximately 1.4 × as long as width of merus, anterodistal corner bearing one spine accompanied by setae; carpus with three clusters of long straight setae on posterior margin, one spine accompanied by setae on both anterodistal and posterodistal corners; propodus with three spines accompanied by clusters of setae on posterior margin; dactylus similar to that of pereopod 3.

Pereopod 5 (Fig. 4G-I ): coxal plate bilobed, posterior lobe with three setae on posterior margin; basis with two anteroproximal setae and six small spines on anterior margin, anterodistal corner bearing two spines accompanied by setae, posterior margin with 11 short setae, posterodistal lobe developed; merus with five clusters of setae on anterior margin, one spine on posterior margin, one and two spines accompanied by setae on anterodistal and posterodistal corners, respectively; carpus with three clusters of setae and two spines on anterior margin, two spines accompanied by setae on posterior margin; propodus with four groups of spines accompanied by setae on anterior margin; dactylus bearing one plumose on posterior margin, two setae at hinge of unguis.

Pereopod 6 (Fig. 5A, B ): coxal plate bilobed, posterior lobe with three setae on posterior margin; basis with five small spines on anterior margin and without anteroproximal setae, posterior margin with 14 short setae, posterodistal lobe not developed; merus with six clusters of setae and two spines on anterior margin, two spines on posterior margin, one and two spines accompanied by setae on anterodistal and posterodistal corners, respectively; carpus with three groups of spines accompanied by setae on anterior margin, two groups of spines accompanied by setae on posterior margin; propodus with four groups of spines accompanied by setae on anterior margin; dactylus similar to that of pereopod 5.

Pereopod 7 (Fig. 5C-E ): coxal plate shallowly concave ventrally, four setae on posterior margin; anterior margin of basis with five small spines and without anteroproximal setae, posterior margin with 15 short setae, inner surface near posterodistal corner with four short setae, posterodistal lobe not developed; merus with five clusters of setae and two spine on anterior margin, one spine on posterior margin, two spines accompanied by setae on both anterodistal and posterodistal corners; carpus with three groups of spines accompanied by setae on anterior margin, one spine and one cluster of setae on posterior margin; propodus with four groups of spines accompanied by setae on anterior margin; dactylus similar to those of pereopods 5 and 6.

Coxal gills present on gnathopod 2 and pereopods 3-7.

Pleonites 1-3 (Fig. 5K-M ): posterodorsal margins of pleonites 1-3 with four, four and five setae, respectively.

Epimeral plates 1-3 (Fig. 5H-J ): plate 1 with three long setae on anteroventral margin and four short setae on posterior margin; plate 2 with two spines on ventral margin and six short setae on posterior margin; plate 3 with three spines on ventral margin and four short setae on posterior margin.

Pleopods (Fig. 5F, G ): peduncle with two retinacula accompanied by one seta; inner ramus slightly longer than outer ramus, both rami fringed with plumose setae.

Urosomites 1-3 (Fig. 6F-H ): dorsally flat; urosomites 1 and 2 with one-one-one-one spines accompanied by setae on dorsal margins from left to right, respectively; urosomite 3 with two spines accompanied by setae on left and right sides each, and three setae on dorsal margin.

Uropod 1 (Fig. 6A ): peduncle bearing two basofacial spines, two and three spines on inner and outer margins, respectively, with one spine on both inner and outer distal corners; inner ramus approximately 0.7 × the length of peduncle and almost the same length as outer ramus, with two and one spines on inner and outer margins, respectively; outer ramus with two and three spines on inner and outer margins, respectively; both rami with five distal spines.

Uropod 2 (Fig. 6B ): peduncle with one spine on inner margin and two spines on outer margin, one spine on both inner and outer distal corners; inner ramus approximately 0.9 × the length of peduncle and 1.3 × as long as outer ramus, with two and one spines on inner and outer margins, respectively; outer ramus with two spines on outer margin; both rami with five distal spines.

Uropod 3 (Fig. 6C, D ): peduncle with several spines and setae on distal margin; inner ramus approximately 2.0 × as long as peduncle, reaching 0.8 × the length of outer ramus, bearing one distal spine, both inner and outer margins with plumose and simple setae; outer ramus two-articulate, proximal article with three spines on outer margin, bearing three distal spines, both inner and outer margins with plumose and simple setae, terminal article shorter than adjacent spines.

Telson (Fig. 6E ): cleft nearly to base, width 0.9 × as long as length, each lobe with one cluster of setae and two single setae on surface, bearing one distal spine accompanied by five setae.

Descrption of female. General appearance similar to male. Observed sexual dimorphism as follows:

Antenna 2 (Fig. 7A ): setae of peduncular articles 4 and 5 longer than those of male, the longest seta on article 4 posterior margin 1.9 × as long as width of article 4, the longest seta of article 5 posterior margin 2.9 × as long as width of article 5.

Gnathopod 1 (Fig. 7B, C ): palm not as oblique as that of male, with six spines posterior margin, medial palmar spine absent; dactylus not exceeding half of propodus.

Gnathopod 2 (Fig. 7D, E ): carpus more elongate than that of male, length 1.2 × as long as propodus; palm with two spines on posterodistal corner, medial palmar spine absent.

Oostegites: present on gnathopod 2 (Fig. 6D ) and pereopods 3-5, with numerous marginal setae.

Uropod 3 (Fig. 7F ): both rami shorter than those of male, inner ramus 1.3 × as long as peduncle length, and 0.7 × the length of outer ramus.

Habitat.

The specimens were collected from a small brook flowing along a cliff on Somaemuldo Island.

Molecular analysis.

The COI sequences of Gammarus somaemulensis sp. nov. (GenBank accession numbers: ON980527-ON980532) were obtained from six individuals. Additionally, the sequences of G. wangbangensis (GenBank accession number: ON980560) and G. soyoensis (GenBank accession number: ON980559) were determined in this study. The intraspecific variation of the COI sequence of the new species ranged between 0.0-0.2%. The interspecific variation between new species and the related species ranged between 21.5-26.3% (Table 2 ).

Remarks.

Gammarus somaemulensis sp. nov. is most similar to G. wangbangensis Lee & Seo, 1990 in the following features: 1) antenna 2 peduncular article 4 with few clusters of long setae, calceoli absent, 2) pereopods 3 and 4 with long straight setae on posterior margins of merus and carpus, 3) pereopods 5-7 with short setae on posterior margins of basis, and 4) outer ramus of uropod 3 with plumose setae on both margins, setae length of outer margin longer than width of proximal article. However, the new species differs from G. wangbangensis in the following features (features of G. wangbangensis in parentheses): 1) article 3 of mandibular palp with one group of B-setae (two groups of B-setae), 2) bases of pereopods 6 and 7 without anteroproximal setae (with long anteroproximal setae), 3) terminal article of outer ramus in uropod 3 shorter than adjacent spines (longer than adjacent spines), and 4) anteroventral margin of epimeral plate 1 with three or four setae (six or more setae).

Gammarus somaemulensis sp. nov. is also similar to G. sobaegensis Uéno, 1966 in the following features: 1) antenna 2 with long setae on peduncular articles, calceoli absent, 2) pereopods 3 and 4 with long straight setae on posterior margins of merus and carpus, and 3) inner ramus of uropod 3 reaching 0.8 × the length of outer ramus. However, the new species can be distinguished from G. sobaegensis by the following features (features of G. sobaegensis in parentheses): 1) posterior margin of peduncular article 4 in antenna 2 with four clusters of long setae (six or more clusters of long setae), 2) bases of pereopods 6 and 7 without anteroproximal setae (with anteroproximal setae), and 3) outer ramus of uropod 3 with plumose setae on both margins (outer margin without plumose setae).

Gammarus soyoensis Lee & Kim, 1980 also share the following features with the new species: 1) antenna 2 calceoli absent and 2) uropod 3 outer margin of outer ramus with plumose setae, terminal article shorter than adjacent spines. However, the new species can be distinguished from G. soyoensis by following features (features of G. soyoensis in parentheses): 1) male gnathopods 1 and 2 with medial palmar spine, each (without medial palmar spine), 2) setae on posterior margin of merus in pereopod 4 longer than width of merus (shorter than width of merus), and 3) setae on outer margin of outer ramus in uropod 3 longer than width of proximal article (shorter than width of proximal article).

The interspecific variation within the COI sequence ranged from 21.5-26.3% for G. somaemulensis sp. nov. and related species (Table 2 ). Previous studies have reported similar or lower levels of COI sequence divergences among Gammarus species. Hou et al. (2009) suggested that the mean inter-specific divergence of the COI sequence among Chinese Gammarus species was 21.9%. Copilaș-Ciocianu et al. (2019) reported a 13.3% between G. hamaticornis and G. kischineffensis . Similarly, Zhang et al. (2022) reported 16.6% difference between G. zhouqiongi and G. takesensis . Therefore, COI sequence divergence, which is 21.5-26.3% among related species, supports G. somaemulensis sp. nov. as a new species.

Figures

Figure 2. Gammarus somaemulensis sp. nov., male, paratype (NNIBRIV 92298), habitus. Scale bar: 1.0 mm.

Figure 3. Gammarus somaemulensis sp. nov., male, holotype (NNIBRIV 92290) A antenna 1, omitted from main flagellar article 7 B antenna 2 C upper lip D lower lip E left mandible F incisor and lacinia mobilis of right mandible G right maxilla 1 H palp of left maxilla 1 I maxilla 2 J maxilliped. Scale bars: 0.5 mm (A-D); 0.2 mm (E, G-J); 0.1 mm (F).

Figure 4. Gammarus somaemulensis sp. nov., male, holotype (NNIBRIV 92290) A gnathopod 1 B palm of propodus and dactylus in gnathopod 1, setae omitted C gnathopod 2 D palm of propodus and dactylus in gnathopod 2, setae omitted E pereopod 3 F pereopod 4 G coxal plate of pereopod 5 H basis to dactylus of pereopod 5 I dactylus of pereopod 5 J dactylus of pereopod 3. Scale bars: 0.5 mm (A, C, E-H); 0.2 mm (B, D); 0.1 mm (I, J).

Figure 5. Gammarus somaemulensis sp. nov., male, holotype (NNIBRIV 92290) A coxal pate of pereopod 6 B basis to dactylus of pereopod 6 C coxal pate of pereopod 7 D basis to dactylus of pereopod 7 E inner surface near posterodistal corner of basis in pereopod 7 F pleopod 1 G inner distal corner of peduncle in pleopod 1 H-J epimeral plates 1 - 3, respectively K-M pleonites 1 - 3, respectively. Scale bars: 0.5 mm (A-D, F, H-M); 0.2 mm (E); 0.05 mm (G).

Figure 6. Gammarus somaemulensis sp. nov., male, holotype (NNIBRIV 92290) A uropod 1 B uropod 2 C uropod 3 D terminal article of outer ramus in uropod 3, distal setae omitted E telson F-H urosomites 1 - 3, respectively. Scale bars: 0.5 mm (A-C); 0.05 mm (D); 0.2 mm (E-H).

Figure 7. Gammarus somaemulensis sp. nov., female, paratype (NNIBRIV 92296) A antenna 2, omitted from flagellar article 3 B gnathopod 1 C palm of propodus and dactylus in gnathopod 1, setae omitted D gnathopod 2 E palm of propodus and dactylus in gnathopod 2, setae omitted F uropod 3. Scale bars: 0.5 mm (A, B, D, F); 0.2 mm (C, E).