Metopa exigua, Krapp-Schickel, 2009

Krapp-Schickel, T., 2009, New and poorly described stenothoids (Crustacea, Amphipoda) from the Pacific Ocean., Memoirs of Museum Victoria 66 (1), pp. 95-116 : 107-110

Metopa exigua n.sp.

Figs. 10–11

Holotype: one male 2mm. Off Korea, 38°15'N, 128°45'E, 200m depth, coll. Schönau IV/1897. Slide ZMUC CRU-20191 .

Additional material: 2 females ov. 1.8mm same locality. Slides ZMUC CRU-20192 .

Type locality: off Korea.

Etymology: from Latin „exiguus“ meaning poor, weak, tiny, minute.

Description. Based on male 2mm, female 1.8mm

Body. Smooth.

Head. Eyes rounded. Antenna 1 peduncle article 1 length about three times the width in female, in male slimmer; article 2 in female shorter, in male much longer than article 1; flagellum 13–14 articles, accessory flagellum absent. A 2 clearly shorter than A1, peduncle article 4 the longest, flagellum shorter than peduncle, with 6–10 articles.

Mouthparts. Mandible palp with one quadrangular basal article and a long, thickened second one which is more than 3x longer than article 1, and a very short and small third article carrying 1 long distal seta. Maxilla 1 palp with 1 article; Maxilla 2 plates in ordinary tandem position; Maxilliped IP not fused; OP visible as acute tooth-shaped prolongation; dactylus long, shorter than propodus.

Peraeon. Coxae. Cx2 oval without tooth; Cx3 tongue-shaped to rectangular, Cx4 not excavated, anterior margin straight, posterior one rounded.

Gnathopods. Gn1, 2 propodi extremely different in shape and size. Gnathopod 1 propodus rectangular and narrow, palm not defined; carpus clearly longer and wider than propodus, proximally narrower than distally; merus without free distal end; all articles beset with groups of short setae. Gnathopod 2 male propodus hind margin longer than length of palm which has one deep rounded excavation near thumb-shaped palmar corner and many small serrations next to dactylus insertion; these incisions show single setae; dactylus somewhat shorter than length of palm. Gn2 carpus longer than wide, triangular, merus not lobate.

Peraeopods. P3 basis elongate and slender; all other articles elongate and weak, dactylus longer than half propodus, weak and smooth. P4 all articles much more robust, with dense setation; merus somewhat curved; dactylus longer than half propodus. P5–P7 merus wider than carpus and only shortly lengthened posterodistally; basis P6, 7 widened with rounded posterodistal lobe; all peraeopods with short setation.

Pleon. Uropods. U1 peduncle longer than subequal rami, with short robust setae on peduncle and rami; U2 peduncle also beset with small robust setae, longer than longer ramus, rami very unequal; U3 peduncle shorter to subequal ramus, article 1 of ramus shorter or subequal to the spineshaped robust article 2.

Telson . Triangular, distally pointed, with few marginal robust setae.

Habitat. 200m depth.

Distribution. Off Korea, Pacific Ocean.

Remarks. At first sight this species looks similar to Metopa wiesei Gurjanova, 1933 , as the second male gnathopod is nearly identical.

For a better comparison I provide here a detailed translation of the original description of the latter species:

„ Metopa wiesei Gurjanova 1933: 123 ; 1951; 421 fig. 260

Type locality: Jugorsky Shar, 69° 46'N, 60°35'O, 20m depth.

Translation of original description in Gurjanova 1933:

Length 3.5mm. Eyes large, roundish. Antennae long; A1 article 1 as long as 2+3 together; flagellum 13 articles. A2 somewhat longer than A1, peduncle article 3> article 2; flagellum short, 7 articles. Mxp inner plate not fused; last articles of palp with short stiff setae on inner margin and basis. Mx1 palp with 1 article, Md palp with 2 articles.

Cx 4 evenly rounded, very large. Peraeopods robust. P6,7 basis short, broad, merus broadened and lengthened.

Gn1 simple, dactylus on inner margin with short setae. Gn 2 in male strongly developed with a long acute tooth on palmar corner; palm with 5 rounded humps which are stronger versus outer margin. Ep3 with acutely lengthened posterodistal corner. T oval, with acute tip and 3 pairs of thick dorsal robust setae. U3 peduncle with 3 thick robust setae, ramus articles subequal, but shorter than peduncle.

Stands near to Metopa clypeata , but Ep3, eyes, antennae, both gnathopods and telson shape are different.“

This description, without any illustrations in Gurjanova 1933, but with some sketchy ones in Gurjanova 1951, makes clear that mainly the antennae (A1 article1 as long as article 2+3 together)and peraeopods (merus broadened and lengthened) are very different from the newly coined species. It seems also probable that M. wiesei has a more robust body living in 20m depth, while M. exigua n.sp. has the thin and delicate legs of typical mud inhabitants (it comes from 200m depth).

The new species belongs to a difficult group with often not very clear morphological character-states: Metopa abyssi Pirlot, 1933 ; M. angustimana Gurjanova, 1948 ; M. bruzelii (Goes, 1866) ; M. dawsoni Barnard, 1962 ; M. longicornis Boeck, 1871 ; M. longirama Dunbar, 1942 ; M. palmata Sars, 1895 ; M. quadrangula Reibisch, 1905 ; M. tenuimana Sars, 1895 ; M. wiesei Gurjanova, 1933 .

They all share the simple Gn1 with elongate propodus and carpus, which are similarly wide (vs. carpus much wider in the group around the type species M. clypeata ) and should be separated from the other members of Metopa . It should also be checked, if all of them have the inner plates of the maxilliped well separated like the type M. clypeata and unlike many other Metopa members.

Figures

Figure 10: Metopa exigua n.sp.: holotype male 2mm, female 1.8mm, off Korea.

Figure 11: Metopa exigua n.sp.: holotype male 2mm, female 1.8mm, off Korea.