A new stenothoid species, Metopa propoda sp. nov., is described from a seamount of the Western Pacific. The new species differs from all other congeneric species by the mature males having a large rectangular notch in the palmar corner of the gnathopod 2. One mitochondrial gene (COI) was used to analyze the validity of Metopa propoda sp. nov. An identification key of the Pacific Metopa species, including this new species, is provided.

COI, deep sea, morphology, taxonomy

The large amphipod family Stenothoidae Boeck, 1871 comprises 354 species and subspecies within 50 genera (Horton et al. 2024; Navarro-Mayoral et al. 2024) and is found at depths ranging from subtidal zones to over 3000 meters (Krapp-Schickel 2015; Krapp-Schickel and Vader 2015). This family is regarded as one of the most challenging among all amphipod families (Tandberg 2011) and is characterized by a vestigial coxa 1 that is covered by following coxae, a shield-like coxa 4 that is enlarged and not posterodorsally excavate, and a uniramous uropod 3 (Barnard and Karaman 1991). Among all stenothoid genera, the genus Metopa Boeck, 1871, has long been considered polyphyletic (Barnard and Karaman 1991; Krapp-Schickel and Koenemann 2006; Tandberg 2011; Tandberg and Vader 2023).

Following Stenothoe Dana, 1852, the Metopa is the second largest genus in the family Stenothoidae, encompassing 56 accepted species (Krapp-Schickel 2009; Tandberg 2011; Tandberg and Vader 2023; Horton et al. 2024). Krapp-Schickel and Vader (2015) categorized Metopa into three groups based on the shape of the palmar corner of gnathopod 1, noting that the palmar shape of gnathopod 2 is not meaningful for cladistic analysis (Tandberg 2011; Krapp-Schickel and Vader 2015). Fifteen Metopa species have been reported from the Western Pacific, occurring at depths up to 2300 meters (Pirlot 1933; Gurjanova 1948, 1952, 1955; Krapp-Schickel 2009). Among these, only M. abyssi Pirlot, 1933, and M. torbeni Krapp-Schickel, 2009, have been found in the tropical Western Pacific, at depths of 100 m and 54 m, respectively (Pirlot 1933; Krapp-Schickel 2009).

During a biodiversity survey of seamounts in the Caroline Plate, several stenothoid specimens were collected. After careful examination, these specimens were identified as belonging to the genus Metopa. The present study describes a new Metopa species, highlights the morphological distinctions between the new species and other congeneric species, and provides an identification key to all Pacific species of the genus.

Specimens were collected from seamounts in the Caroline Plate of the Western Pacific by the research vessel KEXUE with Remote Operated Vehicle (ROV) FAXIAN. The animals were sorted on board and fixed in 96% ethanol, then transferred to 75% ethanol in the laboratory for further study. All material is deposited at the Marine Biological Museum of Chinese Academy of Sciences (MBMCAS), Qingdao, China. External morphology and internal anatomy were examined, dissected, and photographed under a microscope (ZEISS Discovery V20). Line drawings were completed by the software Adobe Photoshop CS6 with a graphics tablet (Wacom PTH 851). Length measurement was made along the outline of the animal, beginning from the anterior margin of the head to the posterior margin of the telson.

Genomic DNA of Metopa propoda sp. nov. was obtained from one specimen using Illumina HiSeq sequencing. At least 3 μg of genomic DNA was used for sequencing library construction. Paired-end libraries were prepared following Illumina’s standard genomic DNA library preparation procedure (insert size of ~400 bp). Purified genomic DNA was sheared into smaller fragments with a desired size by Covaris, and blunt ends were generated by using T4 DNA polymerase. After adding an “A” base to the 3’ end of the blunt phosphorylated DNA fragments, adapters were ligated to the ends of the DNA fragments. The desired fragments were purified through gel electrophoresis and then selectively enriched and amplified by PCR. The index tag was introduced into the adapter at the PCR stage, as appropriate, and we did a library quality test. Finally, the qualified Illumina paired-end library was used for Illumina Nova-Seq 6000 sequencing (150 bp*2, Shanghai Biozeron Co., Ltd). The raw paired-end reads were trimmed and quality controlled by Trimmomatic with parameters (SLIDINGWINDOW:4:15 MINLEN:75) (v. 0.36 http://www.usadellab.org/cms/uploads/supplementary/Trimmomatic). Clean data obtained by the above quality control processes were used to do further analysis. One gene (COI, 1534 bp) was BLASTed and deposited in GenBank (for accession numbers, see Table 1).

The sequences obtained were aligned using MEGA 6 (Tamura et al. 2013). The phylogenetic tree was constructed with maximum likelihood (ML), and the ML analyses were conducted online using W-IQ-TREE (http://iqtree.cibiv.univie.ac.at/) (Trifinopoulos et al. 2016), with clade support evaluated via 10,000 ML bootstrap replications. A total of fifty described and undescribed species within Stenothoidae and two outgroup taxa of Iphimediidae Boeck, 1871, were used for assessing the hypothesis that the specimen is a distinct new species by multiple species delimitation methods. The Bayesian implementation of the Poisson Tree Processes (bPTP) species delimitation model was employed as per Zhang et al. (2013), conducted on the web server of the Heidelberg Institute for Theoretical Studies, Germany (http://species.h-its.org/), using BI phylogenetic trees as the input data. The alignment from the fast-evolving COI gene was uploaded to the online server of Assemble Species by Automatic Partitioning (ASAP) method (https://bioinfo.mnhn.fr/abi/public/asap) with the model of Jukes-Cantor (JC69) with default settings (Puillandre et al. 2021). The Automated Barcode Gap Discovery (ABGD) analysis using a web-based interface (https://bioinfo.mnhn.fr/abi/public/abgd/abgdweb.html) as described by Puillandre et al. (2012). The analysis was conducted using the Kimura 2-parameter substitution model (TS/TV = 2.0), with a prior range for maximum intraspecific divergence set between 0.001 and 0.1, encompassing 10 recursive steps, and a relative gap width (X) of 1.0.

Order Amphipoda Latreille, 1816

Suborder Amphilochidea Boeck, 1871

Superfamily Amphilochoidea Boeck, 1871

Family Stenothoidae Boeck, 1871

Subfamily Stenothoinae Boeck, 1871

Genus Metopa Boeck, 1871

 Metopa propoda sp. nov.

https://zoobank.org/F5C694B2-6134-4A5D-8482-DDCFCA933B98

Figs 1, 2, 3

Material examined.

Holotype. Western Pacific • 1 male, 6.1 mm; an unnamed seamount in Caroline Plate; M6091, 10°04'N, 140°9–15'E; depth 985–2016 m; 2–11 Jun. 2019; collected by a team of ROV “FAXIAN”; MBM 286611.

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; Mx1. maxilla 1; Mx2. maxilla 2; Mxp. right maxilliped; A1. antenna 1; A2. antenna 2.

Paratype.Western Pacific • 1 male, 4.3 mm; an unnamed seamount in Caroline Plate; M6091, 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; M6091, 10°04'N, 140°9–15'E; depth 985–2016 m; 2–11 Jun. 2019; collected by a team of ROV “FAXIAN”; MBM 286821.

Figure 2. 

Metopa propoda sp. nov. male holotype (MBM 286611). G1 R. right gnathopod 1; G2 R. right gnathopod 2 (arrow showing the medial view of merus and carpus); P3 R. right pereopod 3; P4 R. right pereopod 4; P5 R. right pereopod 5; P6 R. right pereopod 6; P7 R. right pereopod 7; U1 R. right uropod 1; U2 R. right uropod 2; U3 R. right uropod 3; T. telson.

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.

Figure 3. 

Metopa propoda sp. nov. female paratype (MBM 286821). G1 R. right gnathopod 1; G2 R. right gnathopod 2 (arrow showing the medial view of merus to dactylus).

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).

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.

Nineteen Metopa species, including M. propoda sp. nov., have been reported in the Pacific, with all of them found in the northern Pacific (Gurjanova 1948, 1952, 1955; Barnard 1962, 1966, 1969; Krapp-Schickel 2009; Krapp-Schickel and Vader 2015). Five of these species are found in deep waters, including M. angustimana Gurjanova, 1948 (Peter the Great Gulf, 351 m), M. bulychevae Gurjanova, 1955 (Japan Sea, 414 m), M. koreana Gurjanova, 1952 (off Korea, 1100 m), M. mirifica Gurjanova, 1952 (Kuril Islands, 2300 m), and M. propoda sp. nov. (Seamount in Caroline Plate, 985–2016 m) (Gurjanova 1948, 1952, 1955). However, only the present new species was found in the deep waters of the tropical Pacific (4–5 °C).

The results of the present study validate M. propoda sp. nov. and reveal a close relationship between the new species and M. boeckii G.O. Sars, 1892, given the existing Stenothoidae molecular sequences. Metopa boeckii was redescribed by Tandberg (2010) and was distributed from the Arctic to the Atlantic at depths of 10–170 m. The present study reveals that M. boeckii showed intraspecific variation considerably exceeding the values commonly used for amphipod species delimitation (Fig. 5), indicating potential cryptic diversity (Jażdżewska et al. 2018) or misidentification, as M. boeckii had been confused with M. borealis G.O. Sars, 1883, and M. bruzelii (Goës, 1866) (Tandberg 2010).

The genus Metopa has long been considered polyphyletic (Barnard and Karaman 1991). Phylogenetic studies based on both morphological characters and molecular sequences have shown that it is not monophyletic (Krapp-Schickel and Koenemann 2006; Tandberg 2011; Navarro-Mayoral et al. 2024). Morphologically, Metopa has a close relationship with Metopoides Della Valle, 1893; Proboloides Della Valle, 1893; Prometopa Schellenberg, 1926; and Stenothoe Dana, 1852 (Barnard and Karaman 1991). However, the genus Metopa does not show a close relationship with Stenothoe species in the present study (Fig. 4) and Navarro-Mayoral et al. (2024). The Stenothoidae shows to be a controversial amphipod family, calling for further taxonomic and phylogenetic studies.

This work was supported by the National Natural Science Foundation of China (42306110), the National Science Foundation for Distinguished Young Scholars (42025603), the NSFC Innovative Group Grant (42221005), and the Biological Resources Programme of the Chinese Academy of Sciences.