Metopa groenlandica Hansen, 1888

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 : 19-25

Metopa groenlandica Hansen, 1888

Fig. 13–16 , 19–21 .

Metopa groenlandica Hansen, 1888:94 , Tab III-7, Holmes, 1905:483, Stephensen, 1936, Stephensen & Thorson, 1936, Stephensen, 1944:56, Gurjanova, 1929:430, Gurjanova, 1951:430 –31, fig 274

Proboloides groenlandicus Stebbing, 1906:190

? Stenothoe clypeata Stimpson, 1853 , Spence Bate, 1862:61, pl. 9, fig. 5, Della Valle, 1893:569

? Stenothoe (Proboloides) clypeata Stebbing, 1906:195, 1906:725 , non Metopa clypeata Krøyer, 1842

Proboloides clypeatus Stephensen, 1931:194 , non Metopa clypeata Krøyer, 1842

Metopa hirsutimana Blake, 1929:20 , fig 10

non Metopa clypeata Krøyer, 1842

non Metopa clypeata Stebbing, 1906:175

Material examined. Morphological examination: Slide: "v/ Grønl. Af Boltenia bolteni munddele. Fem"; female from Greenland, in Type-collection. Dana Station 2408. 62 37’ N, 51 02’ W. 60m. Found in Ascidia callosa 11-VII-1925; female from south west Greenland. G.F. Bankey station 5229-4A. 67 05’ N, 54 07’ W. 80– 95m. W. Greenland. Clay and shells. 23. Aug. 1976.

SEM-examination: ZMUC-material "v. Grønl. Sarak i Waigat, 1892. Transl HJH", from West Greenland (69 57’ N, 51 57’ W), ZMUC-material "v. Grønl. Egedesminde, i Molgula groenlandica Levinsen. Modtaget II-1900. HJH", male from west Greenland (68 43’ N, 52 52’ W).

Morphological redescription. Head ( Fig. 13 , Fig. 16 ): front of head regularly convex, without a clear cephalic lobe; eye small and round, 1/5 of head length, ill defined. Antenna 1 ( Fig. 13 ): slightly shorter than A2; peduncle almost double length of flagellum (see Fig. 12 ); flagellum 9-articulate, short simple setae on each article. Antenna 2 ( Fig. 13 ): peduncle 5/6 of antennal length, short simple setae ventrally on 3 last peduncular articles; flagellum short, 4-articulate. Labrum ( Fig. 13 ): asymmetrically bilobed, broader than long. Mandible ( Fig. 13 ): mandibular palp 3-articulate, oval in cross section; article 2 with several simple setae; article 3 very short, with one long apical seta; incisor serrate with one deep notch; lacinia mobilis only on left mandible, serrate; raker setae only on left mandible, with two thick plumose-dentate setae nearest incisor, a row of simpler dentate setae further down; no molar. Labium ( Fig. 13 ): normal. Maxilla 1 ( Fig. 13 ): inner plate with one simple seta and two very thin simple setae; outer plate with two long and one short cuspidate setae, one long and thin tooth and two simple setae on outer margin and a series of very thin simple setae on inner margin; palp 1-articulate, with a sharp apical tooth, 8 short cuspidate setae set on the slightly serrate inner margin, 6 longer simple facial setae and a series of setules on the outer margin. Maxilla 2 ( Fig. 13 ): outer plate in normal position to inner plate; outer plate with 15 long simple setae on apex and along inner margin; inner plate with 7 shorter simple setae; both plates with several very thin simple facial setae. Maxilliped ( Fig. 13 ): inner plates fully separate, with short simple setae along the split, curved up anteriorly, two cuspidate and 4 shorter simple setae at distal margin; outer plate fully reduced; palp 4-articulate, both palp and outer plate with setose inner margin; article 3 with a cushion of flat serrate setae at distal inner margin, and 3 type A setae; article 4 with inner margin covered by flat setae.

Pereon ( Fig. 16 ): smooth. Gnathopod 1 ( Fig. 14 ): subchelate; coxa rounded quadrate, small; basis linear, a row of longish simple setae at anterior margin; ischium subquadrate, 4 simple setae at posterodistal corner; merus with a small patch of flat serrate setae at posterodistal margin, with a row of type A setae at distal margin; carpus inner side setose, with simple setae at inner surface, and type A setae and a seta-type very like to type A, but with much coarser serration around the inner smooth core (type B) at posterior margin; propodus subtriangular/subrectangular, shorter than carpus, with an almost transverse palm with rounded palmar corner, as long as posterior margin, setose inner surface (all type A setae), the setae along anterior and posterior margins longer than the other setae; dactylus smooth, curved, as long as palm. Gnathopod 2 ( Fig. 14 ): coxa subtriangular, covering coxa 1, smooth with three short simple setae at posterodistal corner; basis linear, with straight margins; ischium and merus short, a few simple setae at posterior margin; carpus triangular, a row of type A setae at distal margin, reaching further towards anterior margin at inner side, where there also is a patch of flat serrate setae; propodus oval, palm very oblique, irregularly convex, with a deep notch and small tooth at palmar corner, palm weakly serrate, about as long as posterior margin, inner surface setose (some intsersperced type B setae), margins with longer setae than both inner and outer surfaces; dactylus smooth, curved, as long as palm. Pereopod 3 and 4 ( Fig. 14 ): simple and slender, no serrations, very few small and simple setae; coxa 3 subrectangular; coxa 4 large and subquadrate (posterodistal corner rounded to make the coxa look almost rounded triangulate). Pereopod 5 ( Fig. 15 ): coxa small and oval; basis slender, meral lobe minute, reaching less than 1/6 of carpus; dactylus shorter than 1/2 propodus, smooth. Pereopod 6 and 7 ( Fig. 15 ): slender; coxa small; basis posteriorly expanded, a row of short simple setae on anterior margin; meral lobe short (approximately 1/3 of carpus; dactylus approximately 1/3 of propodus, smooth.

Urosome ( Fig. 15 ): smooth. Epimeral plate 3 ( Fig. 15 ): posterior corner rounded rectangular. Uropod 1 ( Fig. 15 ): longer than uropod 2; peduncle longer than rami, short simple setae on inner margins; outer ramus 1/4 longer than inner ramus, two short simple setae on inner margin; inner ramus smooth. Uropod 2 ( Fig. 15 ): longer than uropod 3; peduncle subequal to rami, few short simple setae on inner margin; inner ramus marginally longer than outer ramus, one short simple seta at inner margin; outer ramus smooth. Uropod 3 ( Fig. 15 ): uniramous; peduncle subequal to ramus, smooth; ramus two-articulate, smooth. Telson ( Fig. 15 ): sub-rounded triangular, entire; two very small conical setae at distal half.

Distribution: This species has, except from a few records off and around the Bay of Fundy ( Stimpson, 1853; Holmes, 1905; Blake, 1929), only been found around Greenland, on both coasts, see map C on Fig. 22 . It seems to be a common species around Greenland.

Ecology: The depth-range of M. groenlandica is from 6 to 300m. In many of the samples in the Copenhagen collection, it has been found inside ascidians ( Boltenia bolteni (Linnaeus, 1771) , Ascidia prunum Müller, 1776 , Ascidia callosa Stimpson, 1852 and Molgula retortiformis Verrill, 1871 ), and it has also been found inside Boltenia ovifera (Linnaeus, 1776) ( Blake, 1929) . It has also been found to live inside the mollusc Pandora glacialis Leach, 1819 , in a manner that resembles what we have found for M. glacialis ( Stephensen & Thorson, 1936) ; these authors noted that the amphipods use the antennae to "hold on" to the gills of the mollusc. There are also some records from sandy clay or plain clay, often with shells. Its colour is cited as conspicuously pink ( Blake, 1929), or bright yellow (with juveniles pale blue) ( Holmes, 1905).

The only sexual differences mentioned in the literature, is that antenna 1 is a little shorter in females, and that gnathopod 2 is a little more well-developed. Stephensen (1936) also mentions that in gnathopod 1 in females carpus and propodus are of about the same length (as in our drawings), whereas the carpus is longer than the propodus in males.

Other material. This redescription is based partly on the slides from the type-collection in Copenhagen (Hansens slides), but most of the drawings are based on a new slide prepared from material from the Dana expedition in 1925, identified by T. Wolff in Copenhagen. One specimen from an expedition in 1976 was also included for two of the mouthparts (mandible and labium), as these were badly positioned for drawings on the new slide from the Dana samples. They are however identical when examined. The new slides we produced showed no differences to the type-slide, but we managed to make better drawings from these new slides. We looked through the rest of the material of M. groenlandica in the collections, but did not dissect more. None of the specimens we examined differed from the ones we dissected, and all were from Greenland.

Remarks. This seems to be a very common species in Greenland waters, reported from both coasts, and both north and south. It is therefore very strange that Just (1980) does not mention it at all. (We have not seen any accessory flagellum in any of the examined specimens, but since Just does not mention this species, there are also no records that there should be.) Its rather confusing nomenclatural history shows that it is a common species around Greenland. Stephensen (1931; 1936; 1944) seems to be the one to have worked most thoroughly with this species, and his drawings and dissections do not show any differences from what we have found, except that we find the inner plate of the maxilliped completely cleft, and not only with a notch in the upper margin. This is, however, very difficult to see, as it can be both a sharp ridge in an entire plate and a cleavage in the plate, and we had to use SEM to be entirely sure.

Figures

FIGURE 12. Metopa glacialis Krøyer, 1842, new material from Spitsbergen, female, 7 mm: pereopod 5, pereopod 6, pereopod 7, telson, urosome, uropod 1, uropod 2. All scale bars are 0.1 mm.

FIGURE 13. Metopa groenlandica Hansen, 1888, female, Dana st. 2408, 5 mm, Greenland: left mandible, maxilla 1, maxilla 2, maxilliped, labium, labrum, head. All scale bars are 0.1 mm.

FIGURE 14. Metopa groenlandica Hansen, 1888, female, Dana st. 2408, 5 mm, Greenland: gnathopod 1, gnathopod 2, pereopod 3, pereopod 4. All scale bars are 0.1 mm.

FIGURE 15. Metopa groenlandica Hansen, 1888, female, Dana st. 2408, 5 mm, Greenland: pereopod 5, pereopod 6, pereopod 7, urosome, telson, uropod 1, uropod 2, uropod 3. All scale bars are 0.1 mm.

FIGURE 16. Metopa groenlandica Hansen, 1888, female syntype, 5 mm, Greenland: whole animal, SEM picture.

FIGURE 19. Comparisons of mandible morphology of Metopa clypeata (left mandible) (A, B), M. glacialis (left mandible) (C, D), and M. groenlandica (left mandible) (E, F).

FIGURE 20. Comparisons of gnathopod 1 morphology of Metopa clypeata (A (outside), B (inside )), M. glacialis (C (outside), D (inside )), and M. groenlandica (E (inside )).

FIGURE 21. Seta-morphology. A: Metopa clypeata gnathopod 1, B: M. clypeata maxilla 1 outer plate apex, C: M. glacialis gnathopod 1, D: M. groenlandica gnathopod 1.

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.