(Figures 5–7)
(to synonymy before 1973 see Crosnier & Forest, 1973: 105)
Nematocarcinus rotundus — Wenner, 1979: 380; Takeda & Okutani, 1983: 53; Squires, 1990: 105; Burukovsky, 2001 b: 1440; 2003: 144; 2004: 558; 2012: 173.
Material examined. Oceanprof I: A–12, 22o 12´S / 39 o 47´W, 1640 m, 1 female (10.93), MNRJ 23449.
Habitats. Hab 12, 22o 13 'S / 39 o 53 'W, 978 m, 1 female (20.9), MNRJ 21777; Hab 13, 21o 40 'S / 39 o 57 'W, 989 m, 2 females (25.3–28.3), MNRJ 21779; Hab 13, 21o 40 'S / 39 o 57 'W, 989 m, 6 males (17.9–23.2), 12 females (27.05–29.59), 20 ovigerous females (24.8 –30.0), MNRJ 21781; Hab 14, 21o 6 'S / 40 o 8 'W, 990 m, 1 female (29.0), 2 ovigerous females (25.0– 29.9), MNRJ 21780; Hab 14, 21o 6 'S / 40 o 8 'W, 990m, 2 males (20.1, 20.7), 13 females (23.0– 28.8), 6 ovigerous females (25.9–30.9), MNRJ 21782; Hab 14, 21o 6 'S / 40 o 8 'W, 990 m, 9 males (21.0– 23.6), 28 females (18.2–31.6), 12 ovigerous females (23.3–29.3), MNRJ 21752; Hab 62, 23o 9 'S / 40 o 50 'W, 1244 m, 1 male (21.3), 2 females (19.2 –24.0), MNRJ 21744.
REVIZEE: D–0503, 19 o 39 ’S / 38 o 38 ’W, 740 m, 1 female (19.6), 2 ovigerous females (24.2–24.7), MNRJ 23450; D–0504, 19 o 42 ’S / 38 o 36 ’W, 278 m, 2 ovigerous females (23.1, 24.2), MNRJ 23451; D–0505 19 o 43 ’S / 38 o 38 ’W, 1089 m, 1 sex not identified (19.4), MNRJ 23452; D–0506, 19 o 42 ’S / 38 o 36 ’W, 1067 m, 1 male (21.7), 2 ovigerous female (23.3–23.6), MNRJ 23453; E–0499, 13 o 23´S / 38 o 37´W, 761 m, 2 males (16.6–17.5), 4 females (19.1–23.2), 1 ovigerous female (24.2), MNRJ 14710; E–0503, 14 o 37´S / 38 o 42´W, 740 m, 9 females (16.3–19.1), MNRJ 14711; E–0506, 14 o 36 ’S / 38 o 49 ’W, 1067 m, 14 males (19.0– 24.4), 34 females (18.9–24.7), MNRJ 14726; E–0507, 15 o08’S / 38 o 40 ’W, 1026 m, 11 males (18.7 –23.0), 21 females (20.2 –31.0), 3 ovigerous females (24.5–26.9), MNRJ 14723; E–0517, 13 o 22 ’S / 38 o 36 ’W, 750 m, 1 male (17.3), 5 females (17.7–21.3), MNRJ 14713; E–0519, 13 o 19 ’S / 38 o 19 ’W, 1730 m, 1 male (18.1), 2 females (19.0, 24.3), MNRJ 14718; E–0520, 13 o 21 ’S / 38 o 16 ’W, 2137 m, 1 female (24.8), MNRJ 14717; E–0522, 13 º 30´S / 38 º 38´W, 1144 m, 1 male (18.9), 23 females (22.5–26.8), MNRJ 14708; E–0523, 19 o 42´S / 38 o 32´W, 922 m, 4 females (25.4–26.4), 6 ovigerous females (24.4–26.8), MNRJ 14719; E–0524, 19 o 43 ’S / 38 o 39 ’W, 925 m, 1 female (29.5), 6 ovigerous females (24.2–29.7), MNRJ 14721; E–0527, 19 o 50 ’S / 39 o 10 ’W, 1402 m, 1 male (20.7), 2 females (23.2–24.1), MNRJ 14704; E–0528, 19 o 45 ’S / 39 o03’W, 1237 m, 3 males (20.8–22.8), MNRJ 14702; E–0535, 19 o 58 'S / 39 o 35 'W, 1002 m, 4 females (18.2–25.3), 10 ovigerous females (23.4–32.4), MNRJ 14714; E–0547, 21 o 46´S / 39 o 40´W, 1799 m, 1 female (24.2), MNRJ 14697.
Description. Body moderately slender; integument moderately soft, fragile, but not membranous, surface smooth, shiny. Rostrum horizontal, usually reaching second antennular peduncle segment distal margin; rostrum 3–4.4 times shorter than carapace (Figs. 5 A, B; 6 A); distance from rostrum apex to the rostrum distal dorsal tooth is usually similar than that between the remaining rostrum dorsal teeth (Fig. 5 A, B); rostrum dorsal margin (including postrostral crest) with 10–16 (usually 12–15) teeth; rostrum ventral margin with one well-defined tooth lying in the region of the rostrum under the distal dorsal tooth or slightly ahead of it (Figs. 5 A, B; 6 A).
Eyes normally developed its width larger than the diameter of the eye stalk.
Posterodorsal margin of third abdominal somite poorly developed, rounded. Continuation of its sides when crossing forms an angle close to 120 ° (Figs. 5 C, 6 B). Fifth abdominal somite pleura widely rounded, without spine or with a poorly developed spine, its sides intersecting at an angle of almost 120 ° (Figs. 5 D, 6 C, 7 A) and their inner sides without tubercles or rolls.
First pereopod merus unarmed, ischium with 2 spines. Second to fourth pereopods merus with one spine. Fifth pereopod merus unarmed.
Distoventral organ at sixth abdominal somite formed by two single parallel rows of long plumose setae, relatively widely spaced, usually extending to anterior margin of spots (Figs. 6 D; 7 B, C); each spot located on a well-pronounced elevation with relatively sharp back and sloping front edges; spots widely oval, about 2 times longer than wide, distance between them similar to the spots width (Figs. 6 D; 7 B–D).
Telson with 6–8 (usually 7–8) pairs of dorsolateral spines; with one accessory spine (Figs. 6 A, 7 E).
Distribution. At the Western Atlantic Ocean four regions of N. rotundus occurrence can be distinguished (Burukovsky 2001 b; 2012) (Fig. 11). The first region is in the North Atlantic between Long Island and Chesapeake Bay (at depths from 783 to 1960 m, mostly from 1000 to1500 m). The second region is between the Northwestern Bahamas archipelago and the south of Florida (at depths from 686 to 1875 m, usually deeper than 1000 m). The third region is in the Northern Gulf of Mexico (at depths from 421 to 1629m). The fourth region was mentioned by Takeda & Okutani (1983) and Gueguen (1995) that also found N. rotundus in French Guiana (07°N/ 53 °W, at 500–1000 m depth). Herein a fifth region was reconised, the South Atlantic from off Bahia to Rio de Janeiro states, Brazil (at depths from 603 to 960 m).
Remarks. Comparing Southwestern Atlantic material with Burukovsky (2001 b; 2012) data we observed that most of its features fits very well with that mentioned to N. rotundus, as follows: the rostrum length and the number of dorsal teeth; the third abdominal somite dorsal margin and the fifth abdominal somite pleura; at distoventral organ the spots are located on a well-pronounced elevation and the distance between them is similar to the spots width, and the presence of one pair of accessory setae on telson. Some features however present small differences from that observed by Burukovsky (2001 b; 2012) as the position of rostrum ventral tooth; spots length and beginning of setae rows at distoventral organ (Fig. 7 C, D).
Three Nematocarcinus species present the fifth abdominal somite with widely rounded pleura (pleura sides intersecting at an angle of almost 120 °) and a spine poorly developed or absent: N. challengeri Burukovsky, 2006; N. kaiensis Burukovsky, 2000 and N. rotundus Crosnier & Forest, 1973 (Burukovsky 2000 b; 2001 b; 2006 b). Nematocarcinus rotundus inhabit in the Western Atlantic, N. kaiensis in the Indo-West Pacific (between 13 ° 39 'N – 09° 33 'S / 120 ° 36 '– 133 ° 59 'W, at depths from 200 to 1150 m) and N. challengeri is known for a single specimen with strongly damaged rostrum sampled at Pacific Ocean (37 ° 49 'N / 166 ° 47 'W, at 5477m, the deepest record of the genus).
Nematocarcinus rotundus and N. kaiensis can be distinguished by the rostrum shape (Burukovsky 2000 b). In N. kaiensis it is like a sharp blade, raised obliquely upwards (Burukovsky 2000 b), while in N. rotundus it is horizontal, lower than in N. kaiensis, with its ventral tooth pushed back a little further from the rostrum tip, and as a rule, it is directed straight ahead and not curved upwards, like in N. kaiensis (Burukovsky 2001 b).
Nematocarcinus challengeri can be distinguished by the absence of spine on the fifth abdominal somite pleura while it is present, instead poorly developed (or absent in few specimens), in the other two species. The form of rostrum in N. chellengeri is not known. Nematocarcinus rotundus and N. kaiensis present a short rostrum, and considering the proximity between the three species probably N. challengeri presents also a short rostrum.