Maxillipius rectitelson Ledoyer, 1973

Lee, Jeong-Hyeon, Kim, Kyung-Won & Kim, Young-Hyo, 2025, First record of the family Maxillipiidae (Crustacea: Malacostraca: Amphipoda) from Korea, including a new species and a new record species of the genus Maxillipius, Zootaxa 5717 (2), pp. 277-290 : 285-289

Maxillipius rectitelson Ledoyer, 1973 ( Figs. 6–8 )

(Korean Name: Gin-da-ri-yeop-sae-u, new)

Maxillipius rectitelson — Ledoyer 1973: 32, figs. 4–5— Barnard & Karaman 1991: 544, fig. 96— Coleman 2009: 703 , figs. 1–2

Material examined. Adult male, 2.5 mm, MABIK CR00259501 , adult female, 2.3 mm, MABIK CR00259502 , Udo island, Jeju, Korea, 33°31′32"N, 126°54′54"E, 29 May 2024, collected with SCUBA by S.J. Lee. The remaining samples (50 inds., DKUAMP202513) in the collection of the corresponding author .

Description. Adult male, MABIK CR00259501.

Body ( Figs. 6A , 7A ) small, 2.5 mm long, pereonites dorsally smooth, pleonites 1–2 carinate dorsodistally; slightly depressed dorsoventrally; head subquadrate, subequal in length to pereonites 1–3 combined; eye large, subcircular, lateral cephalic lobe indistinct; pereonites 1–6 short, pereonite 7 longest; pleonites 1–2 ( Fig. 7B ) with posterodorsal teeth, posterolaterally ridged with shallow process; preceding tooth blunt, divided into two by a shallow notch, succeeding tooth shallow; epimeral plates 1–3 posteroventral corners minutely pointed.

Antenna 1 ( Fig. 7C ) elongate, sexually dimorphic; peduncular articles 1–2 short, peduncular article 2 shorter than peduncular article 1; peduncular article 3 subquadrate laterally, while with a distomedial portion extends into a pointed projection, elongate medial surface, positioned dorsally, expanded into a triangular lobe tapering toward the apex, with several rows of medial callynophores, fused with flagellum article 1; accessory flagellum absent; flagellum elongate.

Gnathopod 1 ( Fig. 7D ) subchelate; coxa vestigial; basis subrectangular, anterior margin with 4 short setae, posterior margin nearly bared, with 1 simple seta distally; merus subrectangular, extending posterodistally, with 5 simple setae posterodistally; carpus subtriangular, expanding distally, 10 simple setae posteriorly; propodus subovate, palm oblique, with rows of simple setae; dactylus falcate; length ratio of articles 2–7 = 1.00: 0.24: 0.35: 0.71: 0.50: 0.34.

Gnathopod 2 ( Fig. 7E ) subchelate, much longer than gnathopod 1; coxa subrectangular, tapering anteroventrally with rounded corner; basis subrectangular, elongate, nearly bared; carpus elongate, with several setae posteriorly; propodus characteristic in form, longish elliptical, distal half of posterior margin with irregularly arranged setae, palm short, obtuse; dactylus falcate; length ratio of articles 2–7 = 1.00: 0.18: 0.26: 1.00: 0.70: 0.26.

Pereopod 3 ( Fig. 7F ), coxa subquadrate, wider than long; basis elongate, subrectangular, anterior margin with a pinnate seta and 5 short simple setae; merus subrectangular, slightly widening distally; carpus subrectangular; propodus elongate, subrectangular, both margins with simple setae; dactylus falcate, with a nail, posterior margin with serrulation; length ratio of articles 2–7 = 1.00: 0.27: 0.45: 0.85: 1.22: 0.69.

Pereopod 4 ( Fig. 7G ) similar to pereopod 3; length ratio of articles 2–7 = 1.00: 0.30: 0.47: 0.89: 1.30: 0.67.

Pereopod 5 ( Fig. 7H ) coxa subquadrate, wider than long; basis subrectangular, slightly expanded posterodistally, with subacute process, midposterior margin with a pinnate seta, laterodistal surface with a robust seta.

Pereopod 6 ( Fig.7I )characteristic in form,extremely elongate; coxa trapezoidal; basis subcircular,posterodistally extending beyond end of ischium with triangular corner, midposterior margin with a pinnate seta; ischium short; merus subovate, broad, subequal in length to basis.

Pereopod 7 ( Fig. 7J ), coxa small, subtriangular; basis subovate, broad, posterodistal corner acutely produced, posterior margin with a pinnate seta, laterodistal surface with 2 robust setae.

Paratype, adult (ovigerous) female, MABIK CR00259502 .

Body ( Figs. 6B , 8A ) about 2.3 mm long; similar to male; coxae 2–5 more expended than those of male.

Gnathopod 1 ( Fig. 8B ) similar to that of male, but basis posterior margin with 3 long setae.

Gnathopod 2 ( Fig. 8C ) similar to that of male; but coxa subrectangular.

Pereopod 6 ( Fig. 8D ) similar to that of male; propodus extremely elongate, whip-like form, gradually narrowing distally; dactylus indistinct; length ratio of articles 2–6 = 1.00: 0.22: 0.78: 1.52: 15.61.

Uropod 1 ( Fig. 8F ), peduncle slender, rectangular, with 3 robust setae dorsolaterally, 3 robust setae dorsomedially, and apicolateral corner with 1 process.

Uropod 2 ( Fig. 8F ) subsimilar to uropod 1, but peduncle shorter than that of uropod 1, marginally bared.

Uropod 3 missing

Telson ( Fig. 8F ) subrectangular, entire, wider than long, width 1.9 × length, with 4 short setae distally.

Variation. Body length at maturity of male is 2.5 mm and females are 2.8–2.9 mm, based on an ovigerous specimens. Morphological variations among different life stages were observed. In these 2 mature females, pleonite 1 ( Figs. 8H, I ) with 3–4 dorsal teeth and pleonite 2 posterodorsal tooth without notch. Smaller female ( 2.3 mm), however, pleonite 1 ( Figs. 8E ) with 2 dorsal teeth and pleonite 2 posterodorsal tooth without notch. On the other hand, small juvenile ( 1.8 mm), pleonite 1 ( Figs. 8G ) dorsal tooth underdeveloped and pleonite 2 posterodorsal tooth without apical notch.

Molecular data. CO1 gene sequences (GenBank accession numbers PV535652, PV535653, PV540242) were obtained from three Maxillipius rectitelson specimens from Korean waters. These sequences were aligned and compared with CO1 sequences from M. koreanus sp. nov. (PV535648, PV535649, PV535650, PV535651, PV540241, PV540243) collected from Korea ( Table 1). Intraspecific variation of the CO1 gene sequence of M. rectitelson ranged from 0.0 to 0.1%, while interspecific variation ranged from a low of 21.4% to a high of 22.7% ( M. rectitelson and M. koreanus sp. nov.) ( Fig. 9 , Table 2). These genetic differences provide additional support for the recognition of M. rectitelson and M. koreanus sp. nov. as distinct species.

Remarks. As previously mentioned, Maxillipius rectitelson is distinguishable from M. koreanus sp. nov. based on several morphological features. Additionally, M. rectitelson can be differentiated from its congener, M. commensalis based on the following features: 1) gnathopod 1, carpus triangular with posterodistal lobe ( vs. subrectangular without lobe); 2) gnathopod 1, propodus ovate ( vs. subovate to elliptical); and 3) pleonite with dorsal process ( vs. without process) ( Thomas 1996). Our specimens are consistent with the descriptions provided by Ledoyer (1973) and Coleman (2009). However, dorsolateral process of pleonites appears less distinct compare to Coleman’s (2009) description. These morphological differences may be attributed to size related variation or local population differences.

Distribution. Australia, China, Korea (Udo Island), Madagascar.

Figures

FIGURE 6. Maxillipius rectitelson, A, paratype, male, 1.9 mm, habitus; B, paratype, female, 1.5 mm, habitus. Scale bars = 0.5 mm.

FIGURE 7. Maxillipius rectitelson, adult male, 2.5 mm, cat no. CR00259501: A, habitus; B, pleonite 1–2; C, antenna 1; D, gnathopod 1; E, gnathopod 2; F, pereopod 3; G, pereopod 4; H, pereopod 5; I, pereopod 6; J, pereopod 7. Scale bars: A = 0.5 mm, B–C= 0.2 mm, D–J = 0.1mm.

FIGURE 8. Maxillipius rectitelson, adult female, 2.3 mm, cat no. CR00259502: A, habitus; B, gnathopod 1; C, gnathopod 2; D, pereopod 6; E, pleonite 1–2; F, Urosomite; adult female, 1.8 mm, cat no. DKUAMP202513-1: G, pleonite 1–2; adult female, 2.8 mm, cat no. DKUAMP202513-2: H, pleonite 1–2; adult female, 2.9 mm, cat no. DKUAMP202513-3: I, pleonite 1–2. Scale bars: A = 0.5 mm, B–C = 0.1 mm, D–I = 0.2mm.

FIGURE 9. Maximum-likelyhood (ML) tree based on the mitogenome sequence (COI) of Korean Maxillipius species.