Metopa robusta Sars, 1892

Tandberg, Anne Helene S., 2010, A redescription of Metopa species (Amphipoda, Stenothoidae) based on the type material. 3. Natural History Museum, Oslo (NHM) 2465, Zootaxa 2465 (1), pp. 1-94 : 20-22

Metopa robusta Sars, 1892

Metopa robusta Sars 1892: 270–72 , pl. 96-1

Metopa robusta — Reibisch 1905: 177

Metopa robusta — Stephensen 1925: 5; Stephensen 1926: 68; Stephensen 1928: 168–70, fig. 32:1–6; Stephensen 1931: 180; Stephensen 1938: 168;

Metopa robusta — Gurjanova 1951: 417–420, fig. 258; Shoemaker 1955: 22

Material examined. Morphological examination: F13717 (coll: G.O.Sars, Bejan, Trondheimfjord) female. Syntype .

Photo: F13717, syntype .

Type locality: Trondheimsfjord (at Bejan), Norway .

See figures 51, 52, 53, 54 and 55.

Morphological redescription of female type material.

Head ( Figs. 51 and 52 ): small and somewhat tapering; eyes small and round. Antenna 1 ( Fig. 52 ): subequal to antenna 2; peduncle shorter than flagellum, article 1 as long as articles 2 and 3 together; flagellum with 16 articles, each with a minute seta; no accessory flagellum observed. Antenna 2 ( Fig. 52 ): peduncle more than twice as long as flagellum, articles 4 and 5 subequal; flagellum with nine articles. Mandible ( Fig. 52 ): palp 3-articulate, with several simple setae along article 2 and one simple seta at tip of article 3; incisor and lacinia mobilis serrate; raker setae thick; no molar. Maxilla 1 ( Fig. 52 ): inner lobe small and rounded; outer lobe flat at distal margin, one tooth, four cuspidate setae and a line of simple setae along inner margin; palp 1-articulate, a tooth at distal margin, serrate inner margin with simple setae in each nook of the serrations, outer margin with very thin simple setae. Maxilla 2 ( Fig. 52 ): outer plate in normal position to inner plate; inner plate with seven and outer plate with 10 simple setae. Maxilliped ( Fig. 52 ): long and slender; inner plates fully separate, a single simple seta at distal margin; outer plates very small; 0.2x palp article 1 length, one simple seta at tip; palp 4-articulate, lined with simple setae along inner margins, article 3 cushioned at inner distal margin with short simple setae, and a type A seta at tip, article 4 lined with short simple setae at inner margin.

Pereon ( Fig. 51 ): smooth. Pereopod 1 ( Fig. 53 ): simple; coxa small and subquadrate; basis strong and linear, several simple setae along anterior margin; ischium subquadrate; merus short, almost round, distally free, posterior margin cushioned with very short simple setae, distal margin with several long simple setae; carpus elongate and slim, posterior margin with simple setae; propodus thin and elongate, shorter than carpus, no palm but several simple setae at posterodistal margin; dactylus boat-shaped, lined with short simple setae along ridge, length 1.5x width, depth 0.5x width. Pereopod 2 ( Fig. 53 ): coxa suboval, directed forwards, covering coxa 1, five simple setae at posterior margin; basis straight, anterior margin flat with s few short simple setae; ischium subquadrate; merus triangular, posterodistal margin with long simple setae; carpus short and wide, posterodistal margin with long simple setae; propodus subrectangular, palm almost transverse, smooth with simple setae, palmar corner with small notch and strong tooth, hind margin 2x palm length; dactylus strong and curved, smooth. Pereopod 3 ( Fig. 53 ): coxa subrectangular, five simple setae at posterior margin; rest of leg thin and simple, basis with many simple setae along anterior margin; meral lobe less than 0.1x carpal length; dactylus very thin and long, 0.8x propodus length, almost straight. Pereopod 4 ( Fig. 54 ): coxa triangular, wide, distal margin sinuate; rest of leg simple, broader than P3; meral lobe 0.3x carpal length; dactylus strong, inner margin serrate with a notch at tip, slightly curved. Pereopod 5 ( Fig. 54 ): coxa small, posteriorly slightly extended; basis slim, linear; meral lobe 0.5x carpus length; anterior margin of merus, carpus and propodus lined with pairs of simple setae; dactylus smooth, almost straight, 0.5x propodus length. Pereopods 6 and 7 ( Fig. 55 ): coxae small, coxa 6 extended posterodistally; bases extended posteriorly, rounded; meral lobes 1.1x and 1.4x carpus length respectively; dactyli both 0.5x propodus length, slightly serrate and notched at tip, weakly curved.

Urosome ( Figs. 51 and 55 ): smooth. Epimeral plate 3 ( Fig. 55 ): posterodistal corner acute, with rounded tip. Uropod 1 ( Fig. 55 ): longer than uropod 2; biramous; peduncle longer than rami, inner margin with five simple setae; outer ramus slightly longer than inner ramus, three simple setae. Uropod 2 ( Fig. 54 ): longer than uropod 3; biramous; peduncle longer than rami, few simple setae at inner margin; outer ramus slightly longer than inner ramus, two small setae. Uropod 3 ( Fig. 54 ): uniramous; peduncle subequal in length but wider than ramus, two small simple setae; ramus 2-articulate, inner article longer than outer, one seta at distal margin. Telson ( Fig. 54 ): boat-shaped, with sides curving up, rounded, length 2x width, two pairs of cuspidate setae.

Sexual differences: no sexual differences noted.

Distribution: Norwegian coast (Trondheimfjord to northern Norway), Skagerrak, Scotland, Faroe Islands, West Greenland, Kara Sea, Tschukotsk Sea, Bering Sea, Alaska, see map Figure 68 L .

Ecology: the species is noted by Sars (1892) to have been found among hydroids.

The name robusta refers to the "robust" build of the species, and in particular gnathopod 2.

Remarks. The gnathopod 1 dactylus being boatshaped like that of Metopa palmata suggests that this species, in spite of its 3-articulate mandible palp, also possibly should be removed from Metopa , but as the mouthparts follow the definitions by both Boeck (1872) and Barnard & Karaman (1991), this is not suggested here. It is, however, clear that the phylogenetic placement of this species should be tested, and this will be done in a forthcoming paper (Tandberg & Vader in prep). The peculiar shape of the pereopod 1 dactylus is a character that should be examined further.

Figures

FIGURE 51. Metopa robusta Sars, 1892, photo of F13717, syntypes female (A), male (B).

FIGURE 52. Metopa robusta Sars, 1892, female, F13717 (syntype): head, mandible, maxilla 1, maxilla 2, maxilliped. All scale bars are 0.1 mm.

FIGURE 53. Metopa robusta Sars, 1892, female, F13717 (syntype): gnathopod 1, gnathopod 2, pereopod 3. All scale bars are 0.1 mm.

FIGURE 54. Metopa robusta Sars, 1892, female, F13717 (syntype): pereopod 4, pereopod 5, uropod 2, uropod 3, telson. All scale bars are 0.1 mm.

FIGURE 55. Metopa robusta Sars, 1892, female, F13717 (syntype): pereopod 6, pereopod 7, urosome. All scale bars are 0.1 mm.

FIGURE 68. Distribution map of K: Metopa pusilla Sars, 1892 and L: Metopa robusta Sars, 1892