Metopa clypeata ( Krøyer, 1842 ), Kroyer, 1842

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 : 3-14

Metopa clypeata ( Krøyer, 1842)

Fig. 1–9 , 19–21 .

Leucothoe clypeata Krøyer, 1842:157 ; Krøyer 1845:545, pl. 6 fig. 2a–f

Leucothoe norvegica Liljeborg, 1850

Montagua clypeata Bate, 1862:58 ; Goës, 1866:522

Metopa clypeata Boeck, 1870:60 , Boeck, 1872:451, pl. XVIII-4, XIX-3, Hansen, 1888:90, fig III-3, Shoemaker, 1955:15, fig. 5 a–f, Just, 1980:46

? Metopa beringiensis Oldevig, 1959:45 –46, pl.1 fig. 7–8

non Metopa clypeata Gurjanova, 1951:417

Material examined. Morphological examination: ZMUC-CRU 6050 ( Greenland), male, 6.4 mm; ZMUC- CRU 6050, female, 8.9 mm TYPE material; USNMNH 419533, male, 7 mm (Albatross station 3282, Bering Sea); Zool. Mus. AN No 22646 (114-1950);

SEM-examination: ZMUC-CRU 6050, male, 6 mm.

Morphological redescription of male type material. Head ( Fig. 1 ): epistome small and rounded, cephalic lobe not prominent, with acute tip; eye 1/3 of head length, well defined. Antenna 1 ( Fig. 1 ): long and slender, half length of body; flagellum of approximately 20 articles, one small seta on each article; no accessory flagellum observed. Antenna 2 ( Fig. 1 ): shorter than A1; peduncle longer than flagellum; flagellum of 10 articles, each with one thin seta. Labrum ( Fig. 2 ): rounded. Mandible ( Fig. 2 ): mandibular palp 2- articulate (see Discussion), circular in cross-section, 5 serrated setae with an inner smooth core (type A setae, see Discussion) on article 1, very small apical article (article 2) with 1–2 type A setae at apex; incisor and lacinia mobilis both serrate and well developed; raker setae with 3 plumose setae nearest incisor and 7–8 serrate raker setae further back; no molar. Maxilla 1 ( Fig. 2 ): inner plate with one simple distal seta; outer plate with 3 cuspidate and 3 serrated distal setae and 7 simple setae on inner margin; palp 1-articulate, 2x length of outer plate, with 3 cuspidate distal setae, 4 simple setae on inner margin and several simple facial setae. Maxilla 2 ( Fig. 1 ): outer plate in normal position to and slightly longer than inner plate; both plates with several simple setae (outer with 8 and inner with 10). Maxilliped ( Fig. 1 ): long and slim; inner plates fused, with a small notch at tip, a ridge along the middle, 3 simple setae at each side of the notch; outer plate reduced to a small widening of the distal edge of the ischium; palp 4-articulate with several simple setae at inner edge; articles 2, 3 and 4 subequal in length, and all longer than article 1; article 3 with an apical patch of densely set short simple setae and two slight cuspidate setae; article 4 with at row of short simple setae on inner edge. Pereon: smooth; pereonite 4 slightly longer and pereonite 1 slightly shorter than the rest.

Gnathopod 1 ( Fig. 3 ): simple; coxa small, semiquadrate with front distal corner rounded; basis slender, few setae at anterior margin; ischium short, few setae at posterodistal corner; merus semi-oval, with carpus inserted near basis, distal free lobe with a patch of short simple setae and a simple crown of type A setae; carpus elongate, longer than propodus, marginally wider at proximal end, posterior margin covered with long type A setae, and inner ’face’ has a single diagonal row of type A setae on proximal half; propodus long (3/4 of carpus length) and very slim, no palm, several long thin simple (Type II annulate, without setules, Watling) setae both on face and along posterior margin; dactylus pectinate, 1/6 of propodus length, a row of simple setae along posterior margin. Gnathopod 2 ( Fig. 3 ): coxa covering coxa 1, directed forwards; merus and carpus with "stridulating ridges" along anterior margin (for a discussion about stridulating ridges in stenothoids, see Krapp-Schickel 1996), carpus with a row of type A setae at posterior margin; propodus palma transverse, weakly convex, crenulate, delimited by a strong tooth, posterior margin straight, longer than palma; dactylus as long as palma, slightly curved, smooth. Pereopod 3 and 4 ( Fig. 4 ): simple and slender; coxa 3 slightly elongate; coxa 4 triangular, ventral margin rounded, dactylus of pereopod 4 with serrate posterior margin. Pereopod 5 ( Fig. 4 ): basis slender; meral lobe 1/3 of carpus, a row of paired simple setae on anterior margin of both merus and carpus; dactylus with a slight notch at tip. Pereopod 6 and 7 ( Fig. 5 ): coxa small; basis posteriorly expanded, meral lobe 1/3 (P6) and 1/2 (P7) of carpus, both P6 and P7 with a series of simple paired setae on anterior margin og merus and carpus, dactylus weakly serrate. These two legs are more "sturdy" than pereopods 3–5, with broader limbs and coarser armament. P 5–7 slightly longer than P 3–4.

Urosome ( Fig. 5 ): smooth. Epimeral plate 3 ( Fig. 5 ): rounded. Uropod 1 ( Fig. 5 ): longer than uropod 2; biramous, peduncle and rami subequal length. Uropod 2 ( Fig. 5 ): longer than uropod 3; peduncle slightly longer than rami, 6 simple and short setae; inner ramus longer than outer ramus, inner ramus with two short and simple setae near basis. Uropod 3 ( Fig. 5 ): uniramous, ramus with two articles; peduncle as long as ramus article 1. Telson ( Fig. 5 ): rounded, with a single pair of small marginal robust setae and a few minute setules, slightly shorter than peduncle uropod 3.

Sexual differences: there are, unlike in many other Metopa species, not many sexual differences.

Distribution: found in a variety of localities (see Fig. 22 , map A): East- and West-Greenland (Godthaab/ Nuuk is type locality) ( Krøyer, 1842; Just, 1980), Bering Sea ( Krøyer, 1842), Point Barrow, Alaska ( Shoemaker, 1955), and Gulf of St Lawrence ( Shoemaker, 1930) at depths varying from 20 to 300m. Also some curious older and not verified reports from the Bohuslän coast (south-west Sweden), Banff ( Scotland), Kristiansund (Western Norway) and Tromsø (Northern Norway) ( Boeck, 1872).

Ecology: little known. A cold-water species. All records are of apparently free-living animals, although some of the older records mentions it living "on Sertulariae " ( Krøyer, 1845) or "on Thujaria " ( Stephensen, 1931).

The name clypeata (shield-bearer) from clypeis (lat.) refers to the large coxa 4.

Other material. USNMNH (419533, male, 7 mm, Albatross station 3282, Bering Sea), (male, 7mm, station 3279 Bering sea), see Fig. 6 , 7 , 8 and 9 .

Head and antennae as in type from ZMUC, most mouthparts (except from those mentioned under) as in type-material. Mandibles: one specimen with left mandible palp 2-articulate and right mandible palp 3- articulate (see Fig. 6 ), the other specimen with both mandible palps 3-articulate. The 2-articulate palp has a short article 1 and a 4x longer article 2, whereas the 3-articulate palps has a very small article 3 added to this "formula", this article looks very much like the minuscle article 2 in the palps of the type-specimens, palps with circular cross-section. Lacinia mobilis at left mandible, both mandibles with a row of plumose raker setae, and no molar. Labrum: asymmetrically bilobed. Maxilliped: long and slim, inner plates well separate, else as in type.

Pereon: as in type. Pereopods: as in type.

Urosome: as in type.

We have also examined specimens labelled as Metopa clypeata from the Russian Academy of Sciences in St Petersburg: these were clearly not M. clypeata as the gnathopod 1 propodus was subchelate with a shape more resembling M. glacialis and M. groenlandica . We have not yet examined further what species the specimens from St Petersburg are, as they were not any longer interesting for this re-description, but we have eliminated all records of M. clypeata from Gurjanova (southern Kara Sea, Gurjanova, 1933) from the distribution, until all her material is thoroughly re-examined.

Remarks. The most curious deviation between the type material and the other material we have examined, is the number of articles in the mandible palps. In Krøyers material from Greenland, all mandible palps are 2-articulate, with a long first article, and a minimal second article. The SEM-pictures show, however, that there has been an additional articulation further to the base of the now first article, but there is an entire unbroken cuticulum over this articulation in both mandibles we have SEM of, and in lightmicroscopy this is not possible to see at all. Even more interesting it is then, when one of the dissected M. clypeata from NMNH had right mandible palp 3-articulate and left mandible palp 2-articulate, while another specimen has both mandible palps 3-articulate. The articulation of the palp might be a plastic character that should not be used for identification or phylogenetic analyses.

Another interesting aspect is the accessory flagellum reported by Just (1980). He has observed the same type-material we have, but he observed a tiny 1-articulate accessory flagellum on Metopa clypeata ( Just, 1980) . We have not been able to observe an accessory flagellum. Just also found this very small accessory flagellum in other Metopa species he examined from Greenland. From all the Metopa specimens from the ZMUC-collections we have examined, none have had accessory flagella, or they are so tiny that they can be called “vestigial”.

Just (1980) also emphasizes the epistome and median cephalic front margin complex as an area of species specific morphological differences, but in his Greenland-material he did not encounter any M. clypeata , and he did thus not examine this species in more detail for the aforementioned characters, which were thought by him to be of genus-defining capacity.

Barnard & Karaman (1991) defines the inner plate of the maxilliped to be "mostly fused together or well separated (type)". As is seen from our examinations of both type-material and the USNMNH material, that Barnard most likely examined, the condition of the inner plates seems to fit the same pattern as the articulation of the mandible palp, with fused inner plates in the type-material and well separated plates in the USNMNH-material. This character is discussed further later in this paper.

Both Hansen (1888) and Shoemaker (1955) mention that the male and female are very alike in this species. This is what we find as well, and Shoemaker’s comments that the female gnathopod 2 "is not so strongly built and the characters not so pronounced, but it closely resembles that of the male" still holds as the best description of any sexual differences. Boeck (1872) mentions that the third article of antenna 1 peduncle is shorter in females than in males. We have not been able to verify this, but that might be due to the small number of females we have observed, and the fact that the female we observed was slightly larger than the males. We have not seen any specific sexual difference in size; this is also supported by Shoemaker.

The insufficiently described taxon Metopa beringiensis Oldevig, 1959 looks, as far as the scarce description and illustrations permit a conclusion, very similar to M. clypeata .

Figures

FIGURE 1. Metopa clypeata Krøyer, 1842, female syntype, 8.9 mm, Greenland: head, antenna 1, antenna 2, maxilliped, maxilla 2. All scale bars are 0.1 mm.

FIGURE 2. Metopa clypeata Krøyer, 1842, female syntype, 8.9 mm, Greenland: maxilla 1, labrum, left mandible. All scale bars are 0.1 mm.

FIGURE 3. Metopa clypeata Krøyer, 1842, female syntype, 8.9 mm, Greenland: gnathopod 1, gnathopod 2. All scale bars are 0.1 mm.

FIGURE 4. Metopa clypeata Krøyer, 1842, female syntype, 8.9 mm, Greenland: pereopod 3, pereopod 4, pereopod 5. All scale bars are 0.1 mm.

FIGURE 5. Metopa clypeata Krøyer, 1842, female syntype, 8.9 mm, Greenland: pereopod 5, pereopod 6, pereopod 7, urosome, uropod 1, uropod 2, uropod 3, telson. All scale bars are 0.1 mm.

FIGURE 6. Metopa clypeata Krøyer, 1842, from USNMNH, male, 7 mm, Bering Sea: left mandible, right mandible, labrum, labium, maxilla 1, maxilla 2, maxilliped. All scale bars are 0.1 mm.

FIGURE 7. Metopa clypeata Krøyer, 1842, from USNMNH, male, 7 mm, Bering Sea: gnathopod 1, gnathopod 2, pereopod 3. All scale bars are 0.1 mm.

FIGURE 8. Metopa clypeata Krøyer, 1842, from USNMNH, male, 7 mm, Bering Sea: pereopod 4, telson, uropod 1, uropod 2, uropod 3. All scale bars are 0.1 mm.

FIGURE 9. Metopa clypeata Krøyer, 1842, from USNMNH, male, 7 mm, Bering Sea: pereopod 5, pereopod 6, pereopod 7, urosome. All scale bars are 0.1 mm.

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.