Belomitra brachymitra, Kantor & Puillandre & Rivasseau & Bouchet, 2012

Kantor, Yuri I., Puillandre, Nicolas, Rivasseau, Audrey & Bouchet, Philippe, 2012, 3496, Zootaxa 3496, pp. 1-64 : 51-52

Belomitra brachymitra new species

Figures 20, 29G–N, 30A

Type material: Holotype MNHN 24495 (measurements: SL 9.5 mm, BWL 5.8, AL 4.3, SW 3.4 mm); 5 paratypes MNHN 24496

Type locality: Fiji, Lau Group, Yagasa I., 19º01’S, 178º26’W, 395–540 m, 10 March 1999 [BORDAU 1 sta. DW1486] .

Material examined: FIJI. BORDAU 1 , sta . DW1432 , 17º20’S, 178º44’W, 477–493 m, 1 dd; sta . DW1486 , 19º01’S, 178º26’W, 395–540 m, 1 lv (holotype), 27 dd; sta . DW1488 , 19º01’S, 178º25’W, 500–516 m, 13 dd (paratypes) .

Description (holotype, Fig. 29G–J): Shell small, solid, turriform, glossy, with medium high elevated spire, consisting of 1.5 protoconch and 5.25 convex teleoconch whorls. Protoconch smooth, globose, diameter 640 µm, height 560 µm. Protoconch-teleoconch transition marked by orthocline rib after which teleoconch sculpture appear. Suture shallowly adpressed. Last whorl tall, 0.61 of SL, convex. Subsutural ramp well defined, very narrow and concave. Beside inconspicuous growth lines, axial sculpture consisting of narrow, closely spaced, and very weakly recurved, nearly orthocline, ribs. Number of ribs increasing from 13 on first teleoconch whorl to 17 on penultimate, and 23 on last whorl, extending over whole whorl height on spire and becoming obsolete on shell base at onset of canal. Spiral sculpture of distinct low, narrow cords, covering entire shell surface. On adapical teleoconch whorls, subsutural cord simple, forming row of knobs at intersection with axial ribs, becoming subdivided into two very closely spaced ones on penultimate whorl; the cord forming the abapical border of subsutural ramp forms pronounced rounded knobs, while other cords form less developed thickenings at intersections with axial ribs. Seven cords below ramp on penultimate whorl and 22 on last whorl. Aperture medium high, 0.45 of SL, narrowelongate, gradually passing into short and broad siphonal canal, strongly recurved to left. Outer lip thin, concave adapically (corresponding to subsutural ramp) and convex and evenly rounded abapically. Columella slightly convex, with five poorly developed plaits, adapical smallest. Callus narrow, of thin transparent glaze overlying parietal region. Siphonal notch absent. Shell colour off-white. Periostracum absent. Operculum ( Fig. 29J) small, 0.4 of AL length, narrow leaf-shaped, with eroded terminal nucleus, recurved to left.

Radula from dehydrated body of holotype ( Fig. 30A), short, of approximately 50 rows of teeth, 0.6 mm long (0.13 AL), narrow, about 45 µm in width (1.04% of AL). Lateral teeth about 45 µm in maximum length (1.04% of AL), with narrow base, attached to the membrane at an angle of 7–13º to longitudinal axis. Lateral teeth bicuspid, inner cusp slightly (1.3 times) longer than outer one. Rachidian teeth closely spaced and cusps overlapping previous row. Tooth base trapezoidal, with slightly notched broader anterior edge. Three closely spaced, strong cusps, equal in length, and curved in side view, emanate from posterior edge of tooth base.

Distribution: Known from three nearby stations on the Lau Ridge, alive at depths of 395–540 m ( Fig. 20).

Etymology: From the Greek brachys, meaning short, and suffix - mitra, by reference to the broad, mitriform shell shape.

Remarks: Belomitra brachymitra is rather uniform in shell characters. The slight variations observed concern the degree of development of the spiral cords, which can be less developed than in the holotype. Most of the examined specimens were drilled, often several times, by an unknown predator.

Belomitra brachymitra is most similar to B. hypsomitra that inhabits similar depth range off Fiji. It differs in having a smaller (maximum SL 13.2 mm in B. brachymitra vs 18.7 in B. hypsomitra ) and broader shell (SW/SL 0.36± 0.03 in B. brachymitra , range 0.33–0.41, n= 11 vs 0.25± 0.02 in B. hypsomitra , range 0.24–0.28, n=5), in having a generally more pronounced subsutural ramp, and less developed spiral cords and thus the absence of the characteristic cancellate sculpture of Belomitra hypsomitra . The two species differ markedly in the shape of the rachidian radular teeth ( Fig. 30).