Metopa pusilla 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 : 19-20

Metopa pusilla Sars, 1892

Metopa pusilla Sars 1892: 256–57 , pl. 90–1

Metopa pusilla — Stephensen 1926: 68; Stephensen 1928: 167–68, fig. 31:9–12; Stephensen 1931: 188; Stephensen 1938: 174

Metopa pusilla — Schellenberg 1942: 121–22; Gurjanova 1951: 421, fig. 261; Oldevig 1959: 44

Metopa pusilla — Bushueva 1977: 288–89; Lincoln 1979: 188, Fig. 85a–h; Just 1980: 48, fig. 47

Material examined. Morphological examination: F13712 (coll: G.O.Sars, Haugesund), female, 1.5 mm, syntype .

Photo: F13712 Syntype .

Type locality: Haugesund , west Norway .

See figures 46, 47, 48, 49 and 50.

Morphological redescription of female type material.

Head ( Figs. 46 and 47 ): small and rounded, cephalic lobe rounded; eye very small and round. Antenna 1 ( Fig. 47 ): almost half body length, slim; peduncle article 1 as long as articles 2 and 3 combined; flagellum with 10 articles and very thin setae at tip. Antenna 2 ( Fig. 47 ): subequal in length to antenna 1 and even more slender; peduncle longer than flagellum, articles 4 and 5 subequal in length; flagellum with eight articles and very thin setae at tip. Mandible ( Fig. 47 ): palp 2–3 articulate (discussion below), circular in cross-section, simple setae at distal margins; incisor and lacinia mobilis stongly serrate; raker setae with a plumose, serrate, flat tip; no molar. Maxilla 1 ( Fig. 47 ): inner lobe small, one simple seta; outer lobe with flat distal margin, two serrate thick setae, one thick cuspidate seta, one seta type A and a tooth, simple setae along inner margin; palp 1-articulate, with simple setae along inner margin. Maxilla 2 ( Fig. 47 ): outer plate in normal position to inner plate; inner plate with 10 and outer plate with eight simple setae. Maxilliped ( Fig. 47 ): slim; inner plates fully separate, two small setae at distal margins; outer plate 0.3x palp article 1 length, one long seta at tip; palp 4- articulate, article 3 with a cushion of short simple setae at inner distal margin, article 4 lined with simple setae at inner margin.

Pereon ( Fig 46 ): smooth. Pereopod 1 ( Fig. 48 ): simple; coxa small, subquadrate; basis slim and straight, as long as merus proximal margin to propodus distal margin; ischium subquadrate; merus suboval, distal margin free, posterior margin cushioned in short simple setae; carpus as long as propodus, slim, several long simple setae along posterior margin; propodus not as wide as carpus, narrowing towards distal margin, posterior margin straight; dactylus slightly curved, 0.6x propodus posterior margin. Pereopod 2 ( Fig. 48 ): coxa suboval, directed forwards, covering coxa 1; basis slim and straight; ischium subquadrate; merus spoon-shaped and subtriangular; carpus subtriangular, spoon-shaped lobe cushioned in short simple setae at posterior margin, lined with setae type A along distal margin; propodus subtriangular, palm oblique, crenulate, with simple setae at the nooks of the crenulations, palmar corner clearly defined by a small tooth and a thicker seta, hind margin slightly longer than palm; dactylus curved, smooth, as long as palm. Pereopod 3 ( Fig. 49 ): coxa subrectangular, three setae along posterior distal margin; rest of leg thin and simple; meral lobe 0.2x carpal length, dactylus thin and curved, smooth, 0.6x propodus length. Pereopod 4 ( Fig. 49 ): coxa subtriangular, rest of leg simple, not as thin as that of P3 and with more setation, especially posterior margins of merus, carpus and propodus; meral lobe 0.3x carpal length; dactylus smooth and curved, 0.5x propodus length. Pereopod 5 ( Fig. 50 ): coxa small, slightly extended posteriorly; basis slender and straight; rest of leg simple, with short simple setae along both anterior and posterir margins; meral lobe 0.25x carpal length; dactylus slim and curved, 0.6x propodus length. Pereopods 6 and 7 ( Fig. 50 ): coxae small; bases extended posteriorly; rest of legs simple with simple setae along both anterior and posterior margins; meral lobes o.4x and 0.6x carpal lengths respectively; dactyli smooth and curved, 0.6x and 0.5x propodus length, respectively.

Urosome ( Figs. 46 and 48 ): smooth. Epimeral plate 3 ( Fig. 48 ): posterodistal corner rounded right angle. Uropod 1 ( Fig. 49 ): longer than uropod 2; biramous; peduncle shorter than rami, naked; rami subequal. Uropod 2 ( Fig. 49 ): longer than uropod 3; biramous; peduncle shorter than rami, naked; outer ramus slightly shorter than inner ramus. Uropod 3 ( Fig. 49 ): uniramous, slender; peduncle shorter than and almost as narrow as ramus, naked; ramus 2-articulate, articles subequal in length. Telson ( Fig. 49 ): rounded tip, sides curved up, as in a weak boat shape, length 1.5x width of flattened telson, two pairs of cuspidate setae.

Sexual differences: no sexual differences noted.

Distribution: Norwegian coast, Skagerrak, Bohuslän, Wales, Scotland, Spitsbergen, Franz Joseph-land, East Greenland, see map Figure 68 K .

Ecology: specimen is fluorescent blue in glycerol under dissecting scope, and almost fluorescent purple in vivo (observation from sampling at Spitsbergen, 2005 and 2007). Sars (1892) also commented on the species being very easy to spot in its live form by the special colouring. Sars (1892) mentions this species from "comparatively shallow water", and among algae and hydroids, Lincoln (1979) mentions the depths to be 5– 50 m.

The name pusilla refers to the specimens being very, very small.

Other material. Material gathered from sampling at Spitsbergen in 2005 and 2007 has been examined. This new material fits the morphological description of the type-material, with the mandible-palps varying between 2-articulate and 3-articulate. The specimens were very small, adult specimens were found of 1–2mm length. The colour in live animals was a shimmering almost fluorescent purple, which made it quite easy to spot the animals despite their size, this also fits with the comments of Sars (1892).

Remarks. The mandible palp being 2–3 articulate in this species can have many reasons, one of them being that the inner article on the left mandible palp was not seen due to its small size. When this is not proposed as the reason it is because smaller characters were easily seen from the same specimen, and even from the same parts, and because of the small size the pieces were translucent, and such it would be even less likely that an article was hidden under the main part of the mandible. It has also been shown in Tandberg & Vader (2009) that the mandible palp in Metopa clypeata had in some specimens two mandible palp articles on one side and three on the other, whereas in other specimens it was three on both sides. Scanning electron microscope examinations of this character in M. clypeata showed that the innermost articulation of the palp clearly was visible in the interior structures, but the cuticle was entire, and thus no articulation present. This character of the inner articulation of the mandible palp might therefore be viewed as plastic.

Figures

FIGURE 46.Metopa pusilla Sars, 1892, photo of F13712, syntypes, female and male.

FIGURE 47. Metopa pusilla Sars, 1892, female, 1.5 mm, F13712 (syntype): head, mandible, maxilla 1, maxilla 2, maxilliped. All scale bars are 0.1 mm.

FIGURE 48. Metopa pusilla Sars, 1892, female, 1.5 mm, F13712 (syntype): gnathopod 1, gnathopod 2, epimeral plates. All scale bars are 0.1 mm.

FIGURE 49. Metopa pusilla Sars, 1892, female, 1.5 mm, F13712 (syntype): pereopod 3, pereopod 4, telson, uropod 1, uropod 2, uropod 3. All scale bars are 0.1 mm.

FIGURE 50. Metopa pusilla Sars, 1892, female, 1.5 mm, F13712 (syntype): pereopod 5, pereopod 6, pereopod 7. All scale bars are 0.1 mm.

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