Metopa glacialis ( 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 : 14-19

Metopa glacialis ( Krøyer, 1842)

Fig. 10–12 , 19–21 .

Leucothoe glacialis Krøyer, 1842 , p. 159

Stenothoe clypeata Krøyer, 1842 , Stimpson, 1853

non Leucothoe clypeata Krøyer, 1842

Metopa glacialis Boeck, 1872:454 , Hansen, 1888:93, table III-6, Shoemaker, 1955:17, figs 5 g –l and 6 a–d, Oldevig, 1959:45, Just, 1980:47, fig45, Just, 1983, Vader & Beehler, 1983, Tandberg et al., in prep.

Proboloides glacialis Stebbing, 1906

? Metopa cariana Gurjanova, 1929:313 , fig 5, Stephensen, 1944:56, Dunbar, 1954:727, fig 8

Material examined. Morphological examination: ZMUC-CRU 6558 (type), Bellsound, Spitsbergen. USNMNH 97T408; Atlantic biological station, St Croix River, New Brunswick, June 20, 1927, male, 4mm. TSZCr17994, Tromsø Museum. 7840.592N 02126.230E (Hinlopen strait, Spitsbergen), 40– 70m. 2.3 C. 21.08.2006 (used for all drawings), female, 7mm.

SEM-examination: tubes 104, 112, 134, 144. St. JM 340–346. 7840.592N 02126.230E, 40– 70m. 2.3 C. 21.08.2006

Redescription. We have examined the type material for this species, but it was in poor condition, and was not fit for making slides for illustrations. All illustrations are therefore from our fresh additional material from a site a mere 200 km from the type-locality, also from the archipelago of Spitsbergen. Our examination showed that the types and the fresh material are identical.

Morphological redescription based on new material. This redescription is based on a 7 mm long female.

Head ( Fig. 10 ): epistome small and rounded, cephalic lobe rounded; eye large (half of head-length),with well-developed ommatidia, bright red in live animals. Antenna 1 ( Fig. 6 ): as long as antenna 2; flagellum 11–12 articulate, with 3 short simple setae on each article; no accessory flagellum. Antenna 2 ( Fig. 10 ): peduncle article 4 slightly longer than article 5; flagellum subequal to article 5, 5-articulate; one short simple seta on ventral side of each article. Labrum ( Fig. 10 ): bilobed, slightly asymmetrical. Mandible ( Fig. 10 ): palp 3-articulate, oval in cross section (see Fig. 19 ), palp article 2 2x article 1, article 1 almost 2x article 3, rows of simple setae ventrally on article 2 and a single seta at tip of article 3; incisor and lacinia mobilis serrate and well developed; raker setae serrate and plumose; no molar. Labium ( Fig. 10 ): normal. Maxilla 1 ( Fig. 10 ): inner plate small and rounded, no setae; outer plate with 5 cuspidate distal setae in a crown; palp 1-articulate, 5 robust setae and one acute tooth at distal margin, several thin and simple setae along outer margin, 1.5x as long as outer plate. Maxilla 2 ( Fig. 10 ): plates of subequal length, with several simple setae (outer plate with 12 and inner plate with 6 normal and 3 shorter along the inner margin). Maxilliped ( Fig. 10 ): inner plates separate, 3 small cuspidate and several thin simple setae at each lobe; outer plate completely reduced; palp 4- articulate, article 3 longer than article 2 longer than article 1, article 4 as long as article 2; article 3 with two annulate setae and a patch of short cuspidate setae at distal end; all articles with several long and strong setae along inner margin.

Pereon: smooth. Gnathopod 1 ( Fig. 11 ): subchelate; coxa square, 1.5x as broad as basis, two small setae at front corner, all corners rounded; basis slightly widening distally, with three longish simple setae at posterior margin; ischium and merus subquadrate, both with long type A setae (see above) at distal margin; carpus subtriangulate, several type A setae on inner surface, and the distal margin densely beset with type A setae; propodus rectangular, palm transverse, slightly convex, and with short simple setae, no well delimited palmar corner, several type A setae on inner surface, outer surface more smooth (see Fig. 20 ), a row of type A setae on posterior margin; dactylus smooth and curved, as long as palm. Gnathopod 2 ( Fig. 11 ): coxa oval and covering coxa 1; basis with a row of simple setae on anterior margin; ischium and merus simple and smooth except for a row of setae at each distal margin; carpus subtriangular with a row of type A setae at distal margin and a patch of flat serrate short setae in rows at distal posterior margin; propodus oval with smooth outside and several short simple setae on inside, palm oblique and serrate with a strong tooth at palmar corner, at anterior corner of palm a pair of protrusions (size varying between individual animals, most likely agedifferences) which dactylus is attached between; dactylus curved, smooth, almost as long as palm. Pereopod 3 and 4 ( Fig. 11 ): simple and slender; coxa 3 elongate, few, evenly spaced simple setae on distal margin, two slightly longer setae at posterior distal corner; coxa 4 subtriangulate, almost subsquare as distal margin has a strong curve. Pereopod 5 ( Fig. 12 ): coxa with well-developed posterior lobe, reaching 1/3 of basis; basis slender; meral lobe reduced; propodus smooth; dactylus half length of propodus. Pereopod 6 and 7 ( Fig. 12 ): basis posteriorly expanded, a row of small and strong simple setae on anterior margin; coxa 6 more produced posteriorly and ventrally than coxa 7, which is small and reduced; meral lobe small, approximately 1/4 of carpus; propodus and dactylus smooth.

Urosome ( Fig. 12 ): smooth. Epimeral plate 3 ( Fig. 12 ): corner right-angled; posterior margin straight; no setae. Uropod 1 ( Fig. 12 ): longer than U2; a row of robust setae on inner margin of peduncle; outer ramus, which is slightly longer than inner ramus, with two short setae on inner margin. Uropod 2 ( Fig. 12 ): longer than U3; peduncle with two robust setae on inner margin; inner ramus slightly shorter than outer. Uropod 3 ( Fig. 12 ): uniramous; peduncle shorter than ramus, smooth; ramus two-articulate, smooth. Telson ( Fig. 12 ): entire; rounded; two simple, medium long setae at posterior half.

The adult male is generally smaller than the adult female, with a maximum-length of 7 mm to the females 8 mm. Hansen (1888) has noted that he did not find find any sexual differences in the pereopods. This fits very well with our observations of about 300 adult specimens of both sexes. This is the only comment in the literature of any sexual differences in this species.

Distribution: The geographic range of this species seems confined to cold, arctic waters, and seems to be circumpolar/boreal (see map B on Fig. 22 ). The depth range is from 5 to 275m.

Ecology: Metopa glacialis lives both in its juvenile and reproductive stages inside the mollusc Musculus discors ( Shoemaker, 1955; Vader & Beehler, 1983; Just, 1983), where they engage in extended parental care (Tandberg et al., in prep). They are iteroparous and produce up to three cohorts of offspring, each containing up to 20 eggs. They feed on flagellates, and to some degree on diatoms (see Tandberg et al., in prep).

Other material. We have examined material from USNMNH, from the Atlantic Biological Station at St Croix River ( Shoemaker, 1955), and this material does not differ from the type in any way. It was also found inside the mollusk Modiolaria discors , together with 18 other specimens. This is the same type of habitat as the freshly collected material we have from the south end of the Hinlopen Strait at Spitsbergen, which is not very far from the type locality in Bellsound at Spitsbergen.

Remarks. Dunbar (1954) raises the difficult question of the inner plates of the maxillipeds. His material has (as had Gurjanovas original Metopa cariana ) an entire inner plate of the maxilliped, with only a small notch in the upper margin. Schellenberg (1935) claims (according to Dunbar) that the inner plate of the maxilliped in his Metopa glacialis specimens from Franz Joseph Fjord at East Greenland were fully separate. This is a character Shoemaker (1955) does not discuss when he synonymized Stenothoe clypeata s. Stebbing, 1906 and Metopa cariana Gurjanova, 1929 with Metopa glacialis ( Krøyer, 1842) ; he did, however, discuss the palp of maxilla 1, and had found in all specimens (both from Gurjanova, Schellenberg and Stimpson and his own from Alaska) this to be 1-articulate, and thus a Metopa . Just (1980) also found all specimens he examined (from Schellenberg, 1935; Stephensen, 1944 and the typematerial) to have a 1-articulate palp of maxilla 1, thus ending the long discussion on the articulation of this palp.

We have not seen any accessory flagellum in either light microscopy or with SEM. This is contrary to Just (1980), who found a small accessory flagellum in all Metopa species he examined.

Figures

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 10. Metopa glacialis Krøyer, 1842, new material from Spitsbergen, female, 7 mm: maxilla 2, head, left mandible, maxilliped, labium, labrum, maxilla 1. All scale bars are 0.1 mm.

FIGURE 11. Metopa glacialis Krøyer, 1842, new material from Spitsbergen, female, 7 mm: gnathopod 1, gnathopod 2, pereopod 3, pereopod 4. All scale bars are 0.1 mm.

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 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.