Melita aestuarina Choi & Kim, 2024, sp. nov.

Melita aestuarina sp. nov. (Figs. 2–5) (Korean name: Gi-su-mel-li-ta-yeop-sae-u, new) Type material. Holotype, adult male, 5.2 mm, cat no. NIBRIV0000919532, Hoecheon Stream, Hoeryeong-ri, Hoecheon-myeon, Boseong-gun, Jeollanam-do, Korea, 35°03′48″N, 128°24′58″E, 20 April 2018, collected by S.H. Im. Paratypes, adult female, 3.6 mm, cat no. DKUAMP202403, data same as holotype. The remaining paratypes (15 ♂, 20 ♀, DKUAMP202404) in the collection of the corresponding author. Additional material examined. See figure 1 and table 1 for station details. Etymology. The species name is derived from its habitat in various estuarine areas of Korea. Description. Holotype adult male, NIBRIV0000919532. Body (Figs. 2A, 3A) dorsally smooth. Head, lateral cephalic lobe subrounded, with notch; eye round, small. Epimeral plates 1–3, posterior margin convex, produced ventrodistally. Urosomite 2 with 2 pairs of robust setae dorsally. Antenna 1 (Fig. 3B) elongate; peduncular article 1, with 1 small robust seta on ventral margin; peduncular article 2 with 5 small robust setae ventrally and group of setae near the dorsodistal corner; peduncular article 3 with 7 setae on ventral margin; length ratio of peduncular articles 1–3 = 1.00: 1.07: 0.68; flagellum 25-articulate, with setae on each dorsal and ventral corner; article 1 slightly elongate, about twice length of article 2; accessory flagellum 2-articulate, terminal article short, with 5 apical setae. Antenna 2 (Fig. 3C) length 0.51 x antenna 1; peduncular article 2, gland corn well developed; peduncular article 3 short, subrectangular, with 2 setae on ventrodistal corner; peduncular article 4 with 2 setae on dorsal margin and row of setae on ventral margin, peduncular article 5 with row of setae dorsal and ventral margins; length ratio of peduncular articles 3–5 = 1.00: 2.89: 2.89; flagellum 8-articulate, article 1 length 1.50 x article 2. Lower lip (Fig. 3D) well developed; inner and outer lobes with rounded patch of pubescence apically; outer lobe with row of feeble setae medially. Right mandible (Fig. 3E), incisor with 5 teeth; lacinia mobilis with 6 teeth; accessory setal row with 5 bladed setae; molar triturative; palp 3-articulate; article 1 short, unarmed; article 2 with 7 setae; article 3 with 13 setae; length ratio of articles 1–3 = 1.00: 2.33: 2.22. Maxilla 1 (Fig. 3F), inner plate with 6 plumose setae; outer plate with 9 setal teeth; palp bi-articulate, proximal article subrectangular, unarmed, distal article expanded, with 2 long and 8 robust setae apically. Maxilla 2 (Fig. 3G), inner plate with row of setae on inner and apical margins; outer plate length 1.09 x inner plate, with row of setae on apical and apicolateral margins. Maxilliped (Fig. 3H), inner plate subrectangular, slightly extending half-length of outer plate, inner margin armed 14 conical teeth and apical margin; outer plate subovate, exceeding end of article 2 of palp; palp 4-articulate, article 4 with a nail. Gnathopod 1 (Fig. 4A) smaller than gnathopod 2; coxa with 10 setae on posterior margin; basis subrectangular, with unequal setae on anterior margin and long setae on posterior margin; ischium with palmate setae; merus with numerous pubescence posteriorly; carpus proximally narrow, but broadened distally, with 7 clusters of setae posteriorly; propodus (Fig. 4B) without anterodistal hood, with row of setae on anterior and posterior margins; palm with distinct protuberance on proximal part of palmer margin, with 6 robust setae on palmer corner; dactylus falcate, not reaching palmer corner, with 1 tooth on medial margin; length ratio of articles 2–7 = 1.00: 0.35: 0.32: 0.76: 0.35: 0.09. Gnathopod 2 (Fig. 4C), coxa subrectangular, with row of setae posteriorly; basis with 3 short seta on anterior margin, unequal 9 setae on posterior margin; ischium with palmate and 2 short setae posteriorly; merus posterodistal corner acutely elongate, with 1 simple and 1 robust seta near the posterodistal corner; carpus subtriangular, with 1 robust seta on anterodistal corner and numerous pubescence on posterior margin; propodus (Fig. 4D) large, elongate ovate, width 0.36 x length, anterior margin rather straight, slightly sinuated, with groups of setal row anteriorly and pubescence posteriorly; palm elongate, gently slanted, slightly serrate, lined with unequal simple and robust setae; dactylus falcate, fitting palm, with 1 seta on anterior margin; length ratio of articles 2–7 = 1.00: 0.36: 0.48: 0.64: 1.28: 0.92. Pereopod 3 (Fig. 4E), coxa subrectangular, with rounded corners, width 0.50 x length; basis slightly recurved, with unequal setae anteriorly and posteriorly, with 1 seta and 1 robust seta on posterior corner; merus slightly expended posteriorly, with short setae anteriorly and posteriorly; propodus slightly shorter than carpus; dactylus falcate, with a nail; length ratio of articles 2–7 = 1.00: 0.18: 0.58: 0.40: 0.36: 0.11. Pereopod 4 (Fig. 4F) similar to pereopod 3, but coxa broadened, posterior margin excavate, posterodistal margin; merus with 1 robust seta on anterior corner; length ratio of articles 2–7 = 1.00: 0.31: 0.84: 0.63: 0.53: 0.21. Pereopod 5 (Fig. 5A), coxa bilobate, posterodistal margin with notch; basis posteriorly expanded, margin weakly serrate, with row of robust setae along anterior margin; merus broad, with 5 robust setae on anterior margin, with 3 robust setae on posterior margin; propodus rectangular, dorsal and ventral margin with 3 clusters of robust setae; dactylus falcate, with a nail; length ratio of articles 2–7 = 1.00: 0.24: 0.58: 0.39: 0.53: 0.11. Pereopod 6 (Fig. 5B) similar to pereopod 5, but coxa shallower; merus more expanded posteriorly; length ratio of articles 2–7 = 1.00: 0.18: 0.69: 0.51: 0.72: 0.15. Pereopod 7 (Fig. 5C) similar to pereopod 6, but coxa semicircular; basis subovate, posterior margin broadly expanded; merus expanded; length ratio of articles 2–7 = 1.00: 0.22: 0.75: 0.56: 0.75: 0.19. Uropod 1 (Fig. 5D), peduncle subrectangular, length 1.60 x inner ramus, with basofacial seta, with 4 dorsolateral and 2 dorsomedial robust setae; outer ramus slightly shorter than inner ramus, with 1 dorsal and 3 distal robust setae; inner ramus with 2 dorsal and 3 distal robust setae. Uropod 2 (Fig. 5E), peduncle slightly longer than inner ramus, with 4 dorsolateral and robust setae; outer ramus length 0.90 x inner ramus, with 3 dorsal and 2 distal robust setae; inner ramus with 3 dorsal and 2 distal robust setae. Uropod 3 (Fig. 5F), peduncle short, length 0.40 x outer ramus, with 1 robust seta on laterodistal margin, 2 robust setae on posteroventral corner; outer ramus elongate, narrow, 8.60 x longer than wide, weakly arched, with 4 robust setae dorsally and 6 robust setae ventrally, 8 short robust setae distally; inner ramus minute, scale like, 0.12 x outer ramus, with 1 robust seta near the apex. Telson (Fig. 5G) longer than broad, length 1.14 x width, completely cleft, each lobe with 2 or 3 robust setae laterally and 3 robust setae apically. Paratype, adult female, cat no. DKUAMP202403. Body (Figs. 2B, 5H) about 3.6 mm long; similar to male; coxae 1–4 more expended than those of male. Gnathopod 2 (Fig. 5I) similar to that of male; propodus smaller than that of male; palm oblique. Coxa 6 (Fig. 5J) ventral margin slightly concave, anterior lobe truncate distally, weakly hooked. Salinity range. 0.56 – 34.3 psu. Molecular data. CO1 gene sequences (GenBank accession numbers PP620152, PP658244, PP664116) were obtained from three specimens of M. aestuarina sp. nov.. These sequences were aligned and compared with sequences from M. choshigawaensis (LC371923–371925) and M. shimizui (LC371926–371928, LC637603, LC637604, PP600315, PP663868) (Table 2). Intraspecific variation of the CO1 gene sequence of M. aestuarina sp. nov. ranged between 0.0 and 0.2%, while interspecific variation ranged from a low of 11.6% (between M. aestuarina sp. nov. and M. shimizui) to a high of 14.9% (between M. aestuarina sp. nov. and M. choshigawaensis) (Fig. 8, Table 3). Remarks. The new species Melita aestuarina sp. nov. is similar to M. shimizui and M. choshigawaensis in having the following characteristics: 1) antenna 1, accessory flagellum bi-articulate; 2) uropod 3, outer ramus uni-articulate; 3) coxa 6, anterior lobe deep, length equal to width in female. However, M. aestuarina sp. nov. is distinguished from its congeners based upon the following features (different characters of M. choshigawaensis and M. shimizui in brackets): 1) gnathopod 2, propodus elongate-ovate (subovate in M. choshigawaensis and M. shimizui); 2) uropod 3, outer ramus weakly arched, elongate, about nine times longer than wide (five times in M. shimizui and seven times in M. choshigawaensis). To confirm this distinction, molecular sequences of the COI gene from both previously identified and newly identified samples of M. aestuarina sp. nov., M. shimizui, and M. choshigawaensis in Korea and Japan were compared and analyzed, along with related genus data from the NCBI (Fig. 8). These three species also exhibit genetic differences in COI (11.6–17.6%), surpassing the 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 that M. aestuarina sp. nov. represented a novel species. Additionally, Melita aestuarina sp. nov. is the most common melitid species found in the Korean brackish waters. Distribution. Korea.