Spathoderma bulbosum, Ivanov & Scheltema, 2008

Spathoderma bulbosum View in CoL n. sp.

( Figures 1E View FIGURE 1 , 19 View FIGURE 19 , 20 View FIGURE 20 , 23A, B View FIGURE 23 , 24A View FIGURE 24 , 25 View FIGURE 25 ; Tables 1–3a, 4a, b)

Type material: Holotype USNM 1112620 View Materials . Paratypes: USNM; TAMU ; ZMUM Le1–171 (DGoMB C4/1).

Type locality: Gulf of Mexico, 28°16.78'N, 87°20.10'W, 2,627 m (DGoMB stn. S38 rep. 3, 14.06.2000) GoogleMaps .

Material examined: Off North Carolina, 1 specimen ( Table 3a); Gulf of Mexico, 89 specimens ( Table 1); Guiana Basin, 9 specimens ( Table 4a); and from off Recife, 6 specimens ( Table 4b) .

Diagnosis: Small, to 2.3 mm, opaque, widest anteriorly (bulbous), tapering posteriorly with no clear demarcation between trunk and shank in most specimens, posterium length 1/3–1/2 trunk length; without distinct fringe at knob but with clusters of long sclerites protruding from knob,>2 times length of knob. Sclerites large, thick relative to body size; bases adpressed, longer on shank than trunk. Oral shield sclerites in 3 rows.

Etymology: L. bulbus & Greek bolbos, L. bulbosus -a, -um = a swelling.

Description: External appearance ( Figs 1E View FIGURE 1 , 19A, B View FIGURE 19 ): Length to 2.3 mm, width to 0.7 mm anteriorly; when able to distinguish shank from trunk, posterium/trunk index appears to be 0.35 to almost 0.50; holotype length 2.3 mm, anterium 0.1 mm, trunk 1.5 x 0.7 mm wide anteriorly, gradually tapering to <0.4 mm posteriorly, shank 0.5 x 0.4 mm, knob 0.2 mm, posterium/trunk index 0.47; sclerite bases adpressed on both trunk and shank, blades bent outwards, arranged from anteroventral to posterodorsal on both trunk and shank. Oral shield 0.7–0.9 mm in height. Oral shield sclerites increasing in size from inner 1 st row to outer 3 rd row, with 2– 3 large dorsal sclerites with small sclerites between them.

Sclerites ( Figs 19G View FIGURE 19 , 20 View FIGURE 20 , 23A, B View FIGURE 23 ): General: large, blade bent outwards at waist ( Fig. 20K View FIGURE 20 ), length increasing regularly posteriorly to nearly 400 µm on shank, and thick, to nearly 10 µm, proximally rounded, distally with sharp point; except for size, morphologically similar from anterior trunk to knob, with blade length increasing from ~1/2 base length to equal to base length; thickest medially either on base at waist or crossing from base onto blade; axis straight ( Fig. 19G View FIGURE 19 ), but rotation of blade from plane of base gives appearance of an abrupt change in direction of axis at waist ( Fig. 20 View FIGURE 20 ). Isochromes appearing asymmetrical owing to rotation; blade and base with fine, irregularly spaced longitudinal striations ( Fig. 23B View FIGURE 23 ). Oral shield ( Fig. 20A View FIGURE 20 ): ovoid with one side concave, without distal point, thick, to 70 x 30 x 7 µm, thickest proximally. Dorsal to oral shield ( Fig. 20B View FIGURE 20 ): thin, pointed scales <50 x 20 x 2 µm to flame-shaped, 60 x 20 x 2 µm. Anterium ( Fig. 20C View FIGURE 20 ): either short, broad, with very short blade, to 80 x 30 x 2 µm, or narrow, slightly curved, with distinct waist almost in middle of sclerite, blade width equal to base width, to 90 x 20 x 3, thickest at mid-base. Ventral anterior trunk ( Fig. 20D View FIGURE 20 ): similar to narrow anterium sclerites but larger, 120 x 25–30 x> 10 µm. Dorsal trunk and lateral ( Figs 19G View FIGURE 19 , 20E, G View FIGURE 20 ): as in general description, ranging from 80 x 40 x> 10 µm to>300 x 60 x 9 µm, with blade lengths ~1/2 to equal to base length. Ventral trunk ( Fig. 20F View FIGURE 20 ): most shorter, but not much narrower, than those from dorsal trunk. Shank ( Fig. 20H View FIGURE 20 ): similar to those from dorsal trunk with same base length but thinner and with longer blade equal to or longer than base, to 380 x 75 x 7, thickest near waist. Fringing: lacking; shank sclerites covering proximal part of knob. Knob: two types: short, tapered proximally and distally, straight except proximal end with slight curve ( Fig. 20I View FIGURE 20 ), and very long clusters, 360 x 30 x 7 µm ( Fig. 20J View FIGURE 20 ).

Radula and jaws: 1 examined. Jaws 570 x 275 µm ( Fig. 19E View FIGURE 19 ); radula teeth 120 µm long, serrated membrane about 70 µm, medial to distal; central plate almost symmetrical, curved, to 50 µm ( Fig. 19F View FIGURE 19 ).

Distribution: Continental slope and rise: off North Carolina ~ 850 m; Gulf of Mexico 622–3,000 m; Guiana Basin 2,487 –2,842 m; and off Recife 943 m ( Tables 1–3a, 4a, b) ( Figs. 24A View FIGURE 24 , 25 View FIGURE 25 ).

Remarks: Spathoderma bulbosum is distinguished from most Atlantic species of Spathoderma by the bulbous anterior trunk, indistinct demarcation between trunk and shank, knob with clusters of long sclerites, and very thick mid-trunk sclerites in comparison to their length. It can be confused with S. quadratum , however, particularly juvenile specimens; see Remarks under S. quadratum .