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

Kantor, Yuri I., Puillandre, Nicolas, Rivasseau, Audrey & Bouchet, Philippe, 2012, 3496, Zootaxa 3496, pp. 1-64 : 44-49

Belomitra gymnobela new species

Figures 6, 26, 27, 28

Type material: holotype MNHN 24499 (sequenced as MNHN IM200738541 ) (measurements: SL 25.7 mm, BWL 16.6, AL 12.8, SW 9.2 mm) , paratype MNHN 24500 (sequenced as MNHN IM200738570 ) .

Type locality: French Polynesia, Tuamotu Archipelago , Kaukura Island. 15º38’S, 146º51’W, 887–890 m, 4 October 2009 [TARASOC sta. DW3378 ] .

Material examined: MARQUESAS ISLANDS. MUSORSTOM 9, sta. DR 1244, 10º28’S, 138º42.1’W, 1015–1020 m, 1 lv; sta. DR 1272, 07º55’S, 140º44’W, 660–680 m, 1 dd; sta. DR 1276, 07º52’S, 140º37’W, 800–805 m, 1 dd; sta. DR 1278, 07º52’S, 140º39’W, 1000 m, 1 dd; sta. CP1307, 08º58’S, 140º16’W, 708–738 m, 2 dd .

TUAMOTU. TARASOC, sta. DW3377 , 15º38’S, 146º53’W, 780–825 m , 3 dd, 1 lv (sequenced, MNHN IM200739372 ); sta. DW3378 , 15º38’S, 146º51’W, 887–890 m , 1 lv (holotype, MNHN IM200738541 ); sta. DW3380 , 15º39’S, 146º56’W, 970–1060 m , 1 lv (sequenced, MNHN IM-2007-39247 ) .

SOCIETY ISLANDS. TARASOC, sta. DW3462 , 17º27’S, 149º50’W, 1000–1145 m, 8 dd; DW3474 , 17º28’S, 149º50’W, 720 m, 2 dd; sta. DW3480 , 17º32’S, 149º45’W, 880–900 m, 1 lv (paratype, sequenced, MNHN IM200738570 ); sta. DW3500 , 17º38’S, 149º17’W, 630–680 m, 1 dd .

AUSTRAL ISLANDS. BENTHAUS, sta. CP1910, 27º38.2’S, 144º15.4’W, 840–1200 m, 1 dd.

FIJI. MUSORSTOM 10, sta. CP1361, 18º00’S, 178º53.7’E, 1058–1091 m, 3 dd, 6 juv.

LOYALTY BASIN. BIOGEOCAL, sta. CP273, 21º02’S, 166º57’E, 1920–2040 m, 1 lv (radula prepared); sta . DW313 , 20º59’S, 166º59’E, 1600–1640 m, 2 dd .

NEW CALEDONIA. BATHUS 1, sta. CP661, 21º05’S, 165º50’E, 960–1100 m, 1 dd .

SOLOMON ISLANDS. SALOMON 1, sta. CP1858, 09º37.0’S, 160º41.7’E, 435–461 m, 1 dd juv. (identification tentative). SALOMON 2, sta. CP2253, 07º26.5’S, 156º15.0’E, 1200–1218 m, 1 dd .

INDONESIA. KARUBAR, sta. CP54, 08º21’S, 131º43’E, 836–869 m, 1 dd; sta. CP72, 08º36’S, 131º33’E, 699– 676 m, 1 dd; sta. CP91, 08º44’S, 131º05’E, 884–891 m, 1 dd .

PHILIPPINES. AURORA 2007, sta. CP2688, 15º08.5’N, 122º51.8’E, 2216–2253 m, 1 dd; sta. CC2702 , 14º55.3’N, 123º11.9’E, 944–1004 m, 1 dd, 2 lv (sequenced MNHN IM200734636 , sequenced MNHN IM200734638 ) .

Description (holotype, Fig 26A–C): Shell medium-sized, solid, fusiform, with medium high spire, consisting of 1.25 protoconch and 7 slightly convex teleoconch whorls. Protoconch globose, covered by thin spiral cords, diameter 1100 µm, height 900 µm. Protoconch-teleoconch transition is marked by several strongly opisthocline ribs. Suture shallow, impressed. Last whorl high, 0.65 of SL, moderately convex. Subsutural ramp present on all teleoconch whorls, narrow, strongly concave on adapical ones and less concave on last whorl. Besides numerous inconspicuous growth lines, axial sculpture consisting of narrow closely spaced, prosocline ribs, obsolete on subsutural ramp except for a few weakly pronounced ones on ramp of last whorl. Number of ribs increases from 11 on first teleoconch whorl, to 16 on penultimate, and 19 on last whorl, poorly pronounced on its adapical half, and also becoming obsolete on shell base. Spiral sculpture weak, consisting of very thin spiral striation and 13 weak, unevenly spaced cords on shell base and canal, rounded on top. Row of well defined, small, rounded or oval, knobs abapically of suture, not aligned with axial ribs (20–21 per whorl), poorly pronounced on last whorl. Ramp bordered abapically by another row of knobs formed by thickened axial ribs. Aperture high, 0.49 of SL, oval, poorly delimitated from short and broad siphonal canal, slightly recurved to left. Outer lip thin, very slightly concave adapically, and evenly and strongly rounded abapically. Columella straight, smooth. Callus narrow, of thin transparent glaze overlying parietal region. Siphonal notch absent. Shell colour off-white. Periostracum thin, tightly adhering, very light olive.

The radula and anatomy were examined in the sequenced paratype (MNHN IM200738570). The anatomy is very similar to that of Belomitra nesiotica . The proboscis is relatively shorter (ca 3.4 mm), straight. Gland of Leiblein even larger, occupying most of body haemocoel, terminating in kidney. Radula ( Fig. 28A–B) consisting of approximately 70 rows of teeth, 10 nascent and 10 additional rows not completely formed, 2.5 mm long (0.22 of AL), narrow, about 100 µm in width (0.88% of AL). Lateral teeth rather short ( Fig. 28B), about 85 µm in maximum length (0.75% of AL), with narrow base. Lateral teeth bicuspid, with inner cusp only slightly longer than outer one. Rachidian teeth closely spaced and cusps overlapping previous row. Tooth base narrow, trapezoidal, lateral and anterior margins of tooth base merging into membrane. Three closely spaced stout cusps emanate from the posterior edge of tooth base.

The radula was also examined from the rehydrated body of a subadult specimen from the Marquesas (MUSORSTOM 9, sta. DR 1244) ( Fig. 28C–D). It had approximately 50 rows of teeth, 23 nascent, 1.3 mm long (0.16 of AL), narrow, about 95 µm in width (1.20% of AL). Lateral teeth rather short, about 53 µm in maximum length (0.67% of AL), with narrow base gradually fusing into membrane without a distinct anterior border. In the central part of the membrane ( Fig. 28C), the lateral teeth look long and similar in shape to those in other Belomitra species. When not overlaid by andjoining rows of teeth, it is clear that the teeth are rather short ( Fig. 28D). Lateral teeth bicuspid, with inner cusp only slightly longer than outer one. Rachidian teeth closely spaced and cusps overlapping previous row. Lateral and anterior margins of tooth base merge into membrane, rendering shape of base unclear. Three closely spaced cusps (central longest) emanate from the posterior edge of tooth base.

Distribution: Throughout the South Pacific, from the Marquesas, Tuamotu, Society and Austral Islands to Fiji, Vanuatu, New Caledonia, the Solomons in the SW Pacific, eastern Indonesia and the Philippines, alive in 780–2040 m, shells in 630–2250 m ( Fig. 6).

Etymology: Gymnobela, used as a noun in apposition, is used in reference to the superficial resemblance of the species with the raphitomid genus of that name.

Remarks: Belomitra gymnobela n. sp. is variable in sculpture and shell shape, especially slenderness. In the Tuamotu and Society Islands, the shell varies from medium broad (holotype and paratype — Fig. 26A–D) to very broad ( Fig. 26E, F). Small specimens have more slender shell outline than larger ones. Intermediates are found, sometimes at the same station, and there is no doubt that a single species is involved. The axial sculpture is also rather different in the degree of axial ribs development, as well as number of ribs per whorl (to as low as 14 on penultimate whorl). Specimens from the Marquesas are morphologically undoubtedly conspecific, although their spiral sculpture is better developed; some have very fine, but distinct, spiral riblets ( Fig. 26I), while others exhibit deeper grooves, with an appearance of very flat, closely spaced, spiral cords (e.g. Fig. 26J). Such specimens with indiscernible cords are also present in the Society Islands ( Fig. 26E–F). In French Polynesia, Belomitra gymnobela resembles the sympatric B. bouteti but differs in having a broader shell; the two are confirmed to be distinct species based on molecular evidence.

In the Philippines, there is a distinct form ( Fig. 27) that can be distinguished from the French Polynesia morphotypes by having a more slender shell, with higher spire, narrower axial ribs, and less distinct knobs where the axial ribs meet the abapical border of the subsutural ramp. Similar shells (collected dead) were also found at the same or greater depths in Indonesia, the Solomon Islands, New Caledonia, and Fiji (alive in 940–2040 m, shells in 700–2250 m). Based on shell characters alone, we suspected that there were two distinct species. However, as discussed previously, several criteria of species delimitation were not met in this case, and we refrain to describe these two groups as separate species. The radula also is very similar among specimens of both groups ( Fig. 28). We interpret the morphological and molecular data as indicative of geographic variation/structuring. However, although it is fairly certain that Belomitra gymnobela in French Polynesia is not a species complex, it cannot be excluded that more than one species is present in the western Pacific part of the range. For instance, our allocating to B. gymnobela , e.g., New Caledonia specimens with more convex whorls ( Fig. 27J), or Indonesia specimens with fewer numerous axial ribs ( Fig. 27G), is not based on molecular evidence. The single specimen recorded from Vanuatu is also the largest known for this species ( Fig. 26L).

The anatomy was examined from the rehydrated body of a specimen from the Philippines (MNHN IM200734638, SL 23.9 mm, BWL 13.6 mm, AL 10.0 mm, SW 7.4 mm). Externally, the animal is similar to that of B. nesiotica , except in having much longer cephalic tentacles. The mantle is relatively much shorter (length/width ca 1). Ctenidium and osphradium correspondingly shorter, but much broader; respectively 0.3x mantle and 0.2x mantle width. Anterior foregut very similar to B. nesiotica , with short straight proboscis about 3.5 mm in length (0.37 of AL). Radula ( Fig. 28E) long, occupying most of proboscis length, consisting of approximately 80 rows of teeth, 8 nascent, 2.7 mm long (0.27 of AL), narrow, about 125 µm in width (1.25% of AL). Lateral teeth rather short, about 90 µm in maximum length (0.9% of AL), with narrow base. Lateral teeth bicuspid, with stout cusps, inner cusp slightly longer than outer one. Rachidian teeth broadly spaced and cusps very slightly overlapping previous row. Lateral margins of tooth base merge into membrane. Tooth base broadly trapezoidal, with shallowly arched anterior margin. Three closely spaced short cusps (central one nearly twice as long as lateral ones), emanating from the posterior edge of tooth base. The radula ( Fig. 28F) of the single specimen from the Loyalty Basin (BIOGEOCAL sta. CP273; shell SL 16.9, AL 7.3 mm; Fig. 27H–I) is medium long, around 1.9 mm in length (0.26 of AL), consisting of approximately 70 rows of teeth, 6–7 nascent, narrow, about 70 µm in width (0.95% of AL). Lateral teeth about 70 µm in maximum length (0.95% of AL). Lateral teeth bicuspid, with short stout curved cusps. Rachidian teeth closely spaced and cusps overlapping previous row. Tooth base trapezoidal, long, with deeply notched broader anterior edge. Three closely spaced strong cusps (central longest) emanating from the posterior edge of tooth base.