Nannoniscus oblongus Sars, 1870

Wilson, George D. F., 2008, A review of taxonomic concepts in the Nannoniscidae (Isopoda, Asellota), with a key to the genera and a description of Nannoniscus oblongus Sars, Zootaxa 1680, pp. 1-24 : 14-16

Nannoniscus oblongus Sars, 1870

( Figs 2–6 )

Nannoniscus oblongus Sars, 1870: 164 ; Sars, 1897a: 119, pl. 50 (partim); Hansen, 1916: 92 –94, pl. VIII, fig. 4a–4f; Gurjanova, 1932: 53, table XVIII, 68; Wolff, 1962: 262.

Not Nannoniscus oblongus: Menzies, 1962: 136 –137.

Material examined. Zoological Museum of Oslo "Ex Coll. G. O. Sars, Nannoniscus oblongus G. O. Sars ", no indication of locality or collection date: ZMO #10107, intact female on slide, Fig. 4 A–D; ZMO #10108, dissected parts on slide, Figs 4 E, 5A–E. Hjeltefjord, coll. by R.R. Hessler, J.­ O. Strömberg, near 60°40'N 4°54'E (position from gazetteer): 4. vii. 1978, Rothlisberg­Pearcy sled, 260 m, 3 females, 1 female ( Fig. 6 ; AM P.74561), 1 male ( Figs 2–3 , AM P.74562); 7–8. vii. 1978, Beyer 1­net sled, 260 m, 1 female.

Remarks on material examined. Sars (1870; 1897a) did not establish types for his species, but Museum material exists that might have been used for his descriptions. Two female specimens were borrowed from the Zoological Museum of Oslo and are re­illustrated here. Although these specimens are listed as "Ex G. O. Sars collection", whether or not they were used in his description is uncertain. The ZMO specimens illustrated here ( Figs 4–5 ) do not precisely match those of Sars (1897a: pl. 50, upper female), and they lack locality data. Consequently, no lectotype designation can be made using these specimens. Without any certainty that the "Sars collection" specimens are from the type locality, naming a neotype is also unwarranted.

Diagnosis. Body broadly oval in dorsal view. Head lateral margins broadly curved and narrowing anteriorly, without projecting anterior spines. Pereonites 1–4 anterolateral angles with small setae, 2 with largest seta; 1 distinctly shorter that other pereonites, 1–4 increasing in length posteriorly; 4 lateral margin broadly convex, widest anteriorly; 5 lateral margin medially linear, not strongly angled to midline; 7 lateral margin distinctly shorter than 5–6 lateral margins; 6–7 ventral midline with rounded lobes in lateral view, posterior lobe largest. Pleotelson broader than head width; posteriorly rounded, without posterolateral spines, without indentation above uropods. Antennula article 2 distal margin with 3 blunt projections bearing penicillate setae. Antenna article 3 scale without basal articulation, spine­like, elongate, extending beyond distal margin of article 4; flagellum with 10 articles in female, 8 in male. Pereopods II–VII ventral dactylar ("accessory") claw robust, basally as wide as dorsal claw. Pleopod I of male lateral margin nearly straight, narrowing posteriorly; distal tip lateral lobes projecting beyond margin, angular, with narrower proximal neck, medial lobes broadly rounded, distally curving ventrally, margin with elongate fine setae increasing in length medially. Pleopod II of male endopod distal article length 0.85 protopod length. Pleopod II of female broadly rounded posteriorly, midline with posteroventrally­directed curved spine; midline posterior to spine concave in lateral view. Uropods inserting on ridge anterior to anus, adjacent to but not covering anus; exopod distinctly longer than protopod.

Remarks. The type species of Nannoniscus Sars, 1870 , N. oblongus is distinctive in that it has a large anterolaterally directed spine on antennal article 3 in the position of antennal scale ( Figs 2 A–B, 4A). This spine is mentioned in Sars's (1870: 164) description. Nannoniscus oblongus is most similar to N. caspius Sars, 1897 b; these two species share a broad body, the antennular article 3 spine and strong ventral claws on the pereopodal dactyli. N. caspius , however, has an angular tip to the pleotelson, a posteriorly curved ventral spine on pereonite 7 and a broader body than N. oblongus .

The concept of N. oblongus was confounded in Sars (1897a), in which a female of another species was shown as a male. Hansen (1916) corrected this by transferring Sars's "male" to Nannoniscus crassipes , which was subsequently assigned to Rapaniscus Siebenaller & Hessler (1981) . Collections made in Hjeltefjord ( Norway) by R.R. Hessler & J.­ O. Strömberg included a fully mature male of N. oblongus that shows the essential features of a male Nannoniscus ( Figs 2 , 3 ).

Hansen (1916, pl. VIII, 4a–f) illustrated specimens from Ingolf samples, which agree with specimens illustrated here. The taxon N. oblongus appears to be widespread in the North Atlantic and Arctic Basins, ranging from coastal fjords of Norway and Spitzbergen to the Arctic Ocean and Iceland ( Wolff 1962; Svavarsson et al, 1997). Subtle differences between the male illustrated here and those of Hansen (1916), e.g., tip of the pleopod I and shape of the pleotelson, may signal the presence of a species complex, of which N. caspius would be a member. Menzies (1962a: fig. 31I –K) records a female specimen from off Argentina, (LGO Biotrawl 212, 44 °53.3'S, 51°26.5'W, 5843 m) but only copied Hansen's (1916) figures and didn't illustrate his specimen. Notably, Menzies (1962: 136) does not mention the distinctive antennal spine in his species diagnosis. This specimen from the South Atlantic is almost certainly not N. oblongus , although it should be checked.

The diagnosis above is based on the study of the Sars collection specimens (two females and the Hjeltefjord specimens). Given the detailed similarity between the females from the Sars collection and those from Hjeltefjord ( Figs. 4 , 6 ), I am confident that they all at least represent the same general taxon, N. oblongus . The diagnosis includes characters that were found to vary in Nannoniscus species. The male pleopods are illustrated only irregularly in the literature, but they almost certainly provide rich detail for distinguishing species. The male pleopod I presents two different lateral outlines in ventral view ( Fig. 3 A–B). When the pleopod is in situ, the medial margin of the second pleopod overlaps a thin lateral border of the first pleopod, so that the latter is convex­sided. The pleopod II of the mature male ( Fig. 3 C–D) has an elongate stylet so that the entire article is 85% the length of the protopod. Other species can be seen to have distinctly longer or shorter stylets, and the distal lateral and medial lobes of pleopod I vary considerably. As in the Desmosomatidae , nannoniscids vary in the setation and relative size of the pereopods, even within the genus Nannoniscus as currently defined. For example, some species, N. oblongus included, have a plesiomorphic form of the pereopod II–VII dactylar claws wherein the ventral claw is more robust although shorter than the dorsal claw. Deep­sea species, such as N. meteori , show a ventral claw reduced to practically a thin seta in species like N. cristatus Mezhov, 1986 or N. inermis Hansen, 1916 , or a thin flat triangular plate ( Fig.7 B arrow) as in Rapaniscus dewdneyi Siebenaller & Hessler, 1981 . The full range of variation is unknown so to define homologous states probably requires a synoptic survey of all species. Body shape and ventral spination also need to be thoroughly studied across Nannoniscus species to fully appreciate the phylogenetic patterns represented in this genus.

Figures

FIGURE 2. Nannoniscus oblongus Sars, 1870 (Hjeltefjord Norway, AM P. 74562), adult male, all parts drawn in situ. A – B, body, dorsal and lateral view. C, left antennula, dorsal view. D, right pereopod I. E, right pereopod VII. Scale bars: A – B, 0.5 mm, C – E, 0.1 mm.

FIGURE 3. Nannoniscus oblongus Sars, 1870 (Hjeltefjord Norway, AM P. 74562), adult male. A, pereonite 7 and pleotelson, ventral view. B, pleopod I, ventral view; dotted lines indicate medial sperm tube. C – D, pleopod II, ventral and dorsal views respectively; dotted lines in endopodal stylet indicate sperm tube. Scale bar 0.1 mm.

FIGURE 4. Nannoniscus oblongus Sars, 1870 (slides ex Sars Collection, Zoological Museum of Oslo). A – D, female, ZMO F 10107. A, dorsal view. B – C, antenna and antennula, dorsal view. D, pereonite 7 and pleotelson, ventral view. E, pereopod, ZMO F 10108. Scale bars: A, D, 0.5 mm; E, 0.1 mm.

FIGURE 5. Nannoniscus oblongus Sars, 1870 (slide ex Sars Collection, Zoological Museum of Oslo). A – D, mouthparts from ZMO F 10108. A, left mandible, dorsal view, arrow indicates dorsal tooth of incisor process. B, paragnaths, ventral view. C, right mandible, dorsal view. D, maxillula, ventral view. E, maxilliped, ventral view. Scale bars 0.1 mm (upper for A – C, lower for D – E).

FIGURE 6. Nannoniscus oblongus Sars, 1870 (Hjeltefjord Norway, AM P. 74561), adult female. A – B, pereonite 7 and pleotelson, lateral and ventral views respectively. C, right uropod, enlargement. D, left antennula, dorsal view. Scale bars: A, B, 0.5 mm, C – D, 0.1 mm.

FIGURE 7. Rapaniscus dewdneyi Siebenaller and Hessler, 1981, Male, AM P. 74555, WHOI 209 (type locality). A, left, pereopod I. B, right pereopod II, arrow indicates small triangular plate. Scales bar, 0.1 mm.