Metopa abyssalis Stephensen, 1931

Tandberg, Anne Helene S. & Vader, Wim, 2009, A redescription of Metopa species (Amphipoda, Stenothoidae) based on the type material. 1. Zoological Museum, Copenhagen (ZMUC), Zootaxa 2093, pp. 1-36 : 25-33

Metopa abyssalis Stephensen, 1931

Fig. 17–18 .

Metopa abyssalis Stephensen, 1931

Material examined. Morphological examination: ZMUC-CRU 5019. Male. Ingolf Station 101 (7?). 66 23’ N, 12 05’ W. 1011m. Temp. -0.7 C. Ingolf Station 138. 63 26’ N, 7 56’ W. 887m. -0.6 C. (for comparison only, not dissected.)

Morphological redescription of type material. This description is based on the same slides that Stephensen (1931) presumably used when he created the species, they are at least made by Stephensen, have his handwriting, and are of the same specimens that Stephensen based his description on. They do, however, miss several of the pieces normally used for a description. There are no other specimens of this species present in the Copenhagen collections than the two samples from Stephensen. Due to the very small sample size of the collections, no further specimens were dissected. Stephensen presumed the slides were of a male; we have found nothing to contradict this. Stephensens slides from station 138 were not found in the collections, these have been of an adult female.

Head ( Fig. 17 ): epistome reduced and rounded, lateral cephalic lobes little prominent, rounded; eye small, indistinct, only consisting of a few scattered ommatidia. Antenna 1 ( Fig. 17 ): slightly shorter than antenna 2; basis of peduncle ad broad as half head, peduncle as long as half flagellum; flagellum 10-articulate, few short simple setae on the distal articles. Antenna 2 ( Fig. 17 ): peduncle articles 1–3 short; articles 4 and 5 each longer than articles 1–3 together; flagellum 8-articulate, shorter than peduncle, 2 short simple setae distally on the last 6 articles. Mandible ( Fig. 17 ): palp 3-articulate, cross-section circular, article 2 3x as long as article 1, with one simple seta, article 3 as short as article 1, one simple seta at apex; incisor coarsely serrate, lacinia mobilis more finely serrate, no raker setae on the mandible examined, no molar. Maxilla 1 ( Fig. 17 ): inner plate with one small conical seta; outer plate with one largish cuspidate seta, two simple setae and one serrate seta; palp 1-articulate, distal margin serrate with one short simple seta in each serration, two longer simple setae and one type A seta at distal tip. Maxilla 2 ( Fig. 17 ): outer plate in normal position to inner plate; inner plate with one short and two long simple setae; outer plate with one short and strong seta and four long and simple setae. Maxilliped ( Fig. 17 ): inner plate fused with a ridge along the midline, a row of short strong setae at distal margin; outer plate very small; palp 4-articulate, simple setae along inner margin of palp and outer plate; article 3 of palp with a cushion of flat serrate setae and two type A setae at distal end; dactylus (article 4) with a line of short flat serrate setae on inner margin, slightly curved.

Pereon: Smooth. Gnathopod 1 ( Fig. 18 ): simple to vaguely subchelate; merus with a patch/cushion of several short, flat slightly serrate setae at posterior margin, four type A setae at posterodistal corner; carpus as long as propodus, suboval with long type A setae at posterior margin and anterodistal corner, propodus oval with two short simple setae where dactylus folds in, the palm recognizable from the setation, as long as posterior margin; dactylus curved with a small tooth-like projection at tip, short simple setae along inner margin, and one long simple seta at outer margin. Gnathopod 2 - pereopod 4: no data from slide, hidden under coxa 2–4. Pereopod 5: basis slender. Pereopod 6 and 7: basis posteriorly expanded.

Urosome: smooth. Uropod 1 ( Fig. 18 ): longer than uropod 2; peduncle slightly longer than rami, 3 short setae at inner margin; biramous; rami of equal length, one short and simple seta at inner margin of outer ramus. Uropod 2 ( Fig. 18 ): slightly longer than uropod 3; two short and simple setae at inner margin; biramous, inner ramus as long as peduncle, outer ramus shorter than inner ramus. Uropod 3 ( Fig. 18 ): peduncle approximately as long as ramus; ramus 2-articulate, outer article slightly longer than inner, one short simple seta on inner article. Telson ( Fig. 18 ): entire, sharp rounded tip; two short cuspidate setae at proximal half.

Ecology: Very little is known about the ecology of this species. We only know it from the two samples in the collections in Copenhagen (see map D on Fig. 22 ), from 887 and 1011m depth, and subzero temperatures of the water.

Geographic distribution of C: Metopa groenlandica and D: M. abyssalis .

Other material. Stephensen (1931) used dissected material from both stations when he described M. abyssalis . The slide from station 138 was not found in the collections now, and is presumed lost. By agreement with the museum we did not dissect the last remaining specimen in the tube from station 138. We have not been able to find any other specimens of M. abyssalis in other museums collections.

Remarks. Stephensen (1931) dissected out a gnathopod 2 from both a male and a female, they do not show any conspicuous sexual differences. Gnathopod 2 is subchelate, with a subovate coxa; slender basis and subquadrate ischium; merus short, with a few setae at posterior margin, carpus subtriangular, carpal lobe not free, and reaching 1/4 of posterior margin of propodus; propodus suboval, little more than double length of carpus, with a clear palm, oblique, but very clearly marked by a strong tooth at palmar corner, palm crenulate; dacylus smooth, as long as palm.

Figures

FIGURE 17. Metopa abyssalis Stephensen, 1931, holotype, male, 4.5 mm, Iceland: maxilla 2, maxilliped, maxilla 1, left mandible, head. All scale bars are 0.1 mm.

FIGURE 18. Metopa abyssalis Stephensen, 1931, holotype, male, 4.5 mm, Iceland: gnathopod 1, telson, uropod 1, uropod 2, uropod 3. All scale bars are 0.1 mm.

FIGURE 22. Geographic distribution of A: Metopa clypeata and B: M. glacialis An additional record from the Sea of Japan is not on the map due to missing locality-information.