Metopa propoda, Wang & Sha, 2025

Wang, Yan-Rong & Sha, Zhong-Li, 2025, Discovery and taxonomic exploration of Metopa propoda sp. nov., a new species of Stenothoidae Boeck, 1871 (Amphipoda, Amphilochoidea) from a seamount of the Caroline Plate, Western Pacific, Zoosystematics and Evolution 101 (2), pp. 875-885 : 875-885

Metopa propoda sp. nov.

Figs 1 , 2 , 3

Material examined.

Holotype. Western Pacific • 1 male, 6.1 mm; an unnamed seamount in Caroline Plate ; M 6091, 10°04'N, 140°9–15'E; depth 985–2016 m; 2–11 Jun. 2019; collected by a team of ROV “FAXIAN”; MBM 286611 .

Paratype. Western Pacific • 1 male, 4.3 mm; an unnamed seamount in Caroline Plate ; M 6091, 10°04'N, 140°9–15'E; depth 985–2016 m; 2–11 Jun. 2019; collected by a team of ROV “FAXIAN”; MBM 286611 ; 9 females, 2.3–4.5 mm; an unnamed seamount in Caroline Plate ; M 6091, 10°04'N, 140°9–15'E; depth 985–2016 m; 2–11 Jun. 2019; collected by a team of ROV “FAXIAN”; MBM 286821 .

Description.

Head. Head length subequal to pereonite 1 and 2 combined. Interantennal lobe strongly projecting, broadly rounded. Eyes rounded. Antenna 1 and 2 sexual dimorphisms. In female antenna 1, subequal to or slightly longer than antenna 2, peduncular articles 1 and 2 of equal length, flagellum 12–13 articles, accessory flagellum absent; antenna 2 peduncular article 5 the longest, flagellum 3 / 4 length of peduncular article 5, with 7–8 articles. In males, antenna 1 is shorter than antenna 2, primary flagellum article 1 longest, article 2 half-length of article 1 (based on male paratype); antenna 2 with flagellum shorter than half-length of peduncular article 5.

Mouthparts. Right mandible incisor with four large teeth; lacinia mobilis serrate edge broader than that of the incisor, with 9 teeth; palp 3 - articulate, basal article quadrangular, second one the longest, very short and small third article carrying one long distal seta. Left mandible incisor with 11 teeth unequal in size, lacinia mobilis absent. Maxilla 1 palp with one article, with row of 4 robust setae in distal 1 / 4 length of medial margin and 3 setae subapically. Maxilla 2 plates in ordinary tandem position. Maxilliped inner plate fused with apical notch and small marginal setae; outer plate visible as acute tooth-shaped prolongation; dactylus subequal in length to article 3.

Coxa 2 tongue-shaped, with marginal setae; coxa 3 larger than coxa 2, rectangular; coxa 4 not excavated, about 1.3 times wider than long.

Gnathopods and pereopods. Gnathopod 1 sexual dimorphism, propodus suboval and narrow, palm not defined; carpus in female wider and shorter than that of in male, longer than propodus, proximally narrower than distally; merus projecting under carpus, all articles beset with groups of short setae. Gnathopod 2 sexual dimorphism, female propodus slightly widening distally, hind margin subequal in length to palm, which is defined by a shallow notch and not very strong tooth and has many small serrations next to dactylus insertion; dactylus somewhat shorter than length of palm; carpus in lateral view wider than long, triangular, in medial view oval-shaped; merus not lobate, in lateral view rectangular, in medial view triangular. Gnathopod 2 male propodus hind margin shorter than palm, which is defined by a very large rectangular excavation and strong thumb-shaped palmar corner and has 4 small serrations; carpus in lateral view triangular, dorsal margin with row of stridulating knobs, in medial view rectangular, with anterodistal acute projection; merus not lobate. Pereopod 3 longer than pereopod 4, all articles elongate and weak, distal three articles with setae on posterior margin. Pereopod 4 with posterior margin of merus, carpus, and propodus with a few transverse rows of small setae; merus somewhat curved. Pereopod 5 basis rectilinear; merus to propodus anterior margin with several transverse rows of small setae; merus wider than carpus, with shortly lengthened posterodistally. Pereopod 6 and 7 bases widened with rounded posterodistal lobe; merus with lengthened posterodistally hardly reaching to distal margin of carpus; merus to propodus of both armed as in pereopod 5.

Uropods and telson. Uropod 1 peduncle much longer than subequal rami, with short robust setae on peduncle and outer ramus; inner ramus unarmed with setae. Uropod 2 peduncle longer than rami, with short robust setae; outer ramus slightly longer than inner one, armed with robust setae; inner ramus unarmed. Uropod 3 peduncle subequal to ramus; article 1 of ramus slightly longer than spine-shaped robust article 2. Telson tongue-shaped, distally rounded, with three marginal robust setae on each side dorsally.

Etymology.

The species is named for the large rectangular notch on the palmar corner of gnathopod 2.

Distribution.

Presently known only from a seamount of the Caroline Plate at a depth of 985–2016 meters.

Remarks.

According to the diagnostic key by Barnard and Karaman (1991), M. propoda sp. nov. definitely belongs to the genus Metopa for having the basis of pereopods 6 and 7 similarly expanded, the palp of maxilla 1 only 1 - articulate, and the mandibular palp 3 - articulate. Krapp-Schickel and Vader (2015) classified members of the genus Metopa into three groups based on the shape of the palmar corner on the gnathopod 1. M. propoda sp. nov. belongs to the N group, characterized by a palmar corner angle of 150–160 °. Therefore, the present new species is in the similar group to the following four Pacific species: M. abyssi Pirlot, 1933 ; M. exigua Krapp-Schickel, 2009 ; M. layi Gurjanova, 1948 ; and M. uschakovi Gurjanova, 1948 ( Krapp-Schickel 2009; Krapp-Schickel and Vader 2015).

Metopa propoda sp. nov. can be distinguished from these four species by the large rectangular notch in the palmar corner of the gnathopod 2 in mature males. Additionally, the new species differs from M. abyssi in having antenna 1 that is shorter or equal in length to antenna 2, rather than longer; the inner plate of the maxilliped is fused; and the merus prolongation of pereopods 5–7 does not reach the distal margin of the carpus ( Pirlot 1933, fig. 54). It differs from M. exigua by having antenna 1 shorter or equal in length to antenna 2, a fused inner plate of the maxilliped, the merus prolongation of pereopods 6 and 7 nearly reaching the distal margin of the carpus, and the rami of uropod 2 being equal in length ( Krapp-Schickel 2009). It differs from M. layi in that the peduncle of uropod 3 is equal in length to the ramus, whereas in M. layi , the peduncle is distinctly shorter than the ramus ( Gurjanova 1948, fig. 49). It differs from M. uschakovi in that the articles of the ramus of uropod 3 are equal in length, whereas in M. uschakovi , article 1 is much shorter than article 2 ( Gurjanova 1948, fig. 55). A key (modified from Krapp-Schickel and Vader 2015) to the Pacific species of Metopa is provided below.

The phylogenetic trees produced by ML analyses (Fig. 4 ) show that Metopa species cluster together with four unidentified Stenothoidae species and Wollastenothoe minuta Gouillieux & Navarro-Mayoral, 2024 , with moderate support (BP = 66 %). The Stenothoe species form a clade with one unidentified Stenula species and Parametopella cypris Holmes, 1905 , also with moderate support (BP = 85 %). The remaining two unidentified Stenothoe J. L. Barnard, 1962 species. and one Stenothoidae species are grouped into another moderately supported clade (BP = 87 %). The species delimitation based on ABGD, ASAP, and bPTP methods has confirmed the identification of the new species (Fig. 5 ).

Figures

Figure 1. Metopa propoda sp. nov. male holotype (MBM 286611). UL. upper lip; LL. lower lip; Md L. left mandible (not showing molar and palp); Md R. right mandible; Mx 1. maxilla 1; Mx 2. maxilla 2; Mxp. right maxilliped; A 1. antenna 1; A 2. antenna 2.

Figure 2. Metopa propoda sp. nov. male holotype (MBM 286611). G 1 R. right gnathopod 1; G 2 R. right gnathopod 2 (arrow showing the medial view of merus and carpus); P 3 R. right pereopod 3; P 4 R. right pereopod 4; P 5 R. right pereopod 5; P 6 R. right pereopod 6; P 7 R. right pereopod 7; U 1 R. right uropod 1; U 2 R. right uropod 2; U 3 R. right uropod 3; T. telson.

Figure 3. Metopa propoda sp. nov. female paratype (MBM 286821). G 1 R. right gnathopod 1; G 2 R. right gnathopod 2 (arrow showing the medial view of merus to dactylus).

Figure 4. The maximum-likelihood (ML) tree shows the relationships between Metopa propoda sp. nov. and other Stenothoidae COI sequences. The numbers at each node represent bootstrap values (BP). M. propoda sp. nov. is highlighted in bold and red, and the other Metopa species are highlighted in bold and blue.

Figure 5. Bayesian inference (BI) Phylogenetic tree based on COI showing the phylogenetic relationship between Metopa propoda sp. nov. and related stenothoids. Numbers adjacent to nodes refer to BI posterior probability (> 70). Putative species identified by DNA-based species delimitation methods [Bayesian implementation of the Poisson Tree Processes (bPTP), Automated Barcode Gap Discovery (ABGD), and Assemble Species by Automatic Partitioning (ASAP)] applied on the COI tree / distance matrices are indicated by bars on the concatenated tree.