Brookula exquisita Clarke, 1961

Schwabe, Enrico & Engl, Winfried, 2008, Description of two new deep­water species of the genus Brookula Iredale, 1912 (Mollusca, Gastropoda, Trochoidea), with a revision of the genus for the Subantarctic and Arctic Sector of the Atlantic Ocean *, Zootaxa 1866, pp. 187-204 : 191-193

Brookula exquisita Clarke, 1961

( Figs 3 –4)

Brookula (Benthobrookula) exquisita Clarke 1961: 356 –357, pl. 3, fig. 8, pl. 4, fig. 2.

Holotype ( MCZ: 225964), not examined, SEM images available in Absalão et al. (2001) and Zelaya et al. (2006).

Type locality: Antarctica , South Georgia (55°29’S 37°57’W), 3758 m. OD.

Material examined: Four specimens ( ZSM Mol 20070892­20070894), Antarctica , South Sandwich Islands, St. PS 61/140–08 (58°15.98’S 24°53.72’W to 58°16.13’S 24°53.87’W), 2947–2970 m, leg. Michael Schrödl, March 22 2002, ANT XIX/4 (ANDEEP II) expedition. One specimen fixed and preserved in 75% ethanol ( ZSM Mol 20070892); remainder dry shells only including SEM­mounted specimen ( ZSM Mol 20070894). One specimen ( ZSM Mol 20021582), Antarctica , South Sandwich Islands, St. PS 61/140–07 (58°16.00’S 24°53.78’W to 58°16.39’S 24°54.85’W), 2941–2945 m, leg. Michael Schrödl, March 21 2002, ANT XIX/4 (ANDEEP II) expedition, fixed and preserved in 96% ethanol. One dry specimen ( ZSM Mol 20070901), Antarctica , Northwest Weddell Sea, St. PS 61/131–03 (65°19.19’S 51°32.54’W to 65°19.99’S 51°31.23’W), 3050–3055 m, leg. Michael Schrödl, March 5 2002, ANT XIX/4 (ANDEEP II) expedition. Five specimens ( ZSM Mol 20021216; 20021170), Antarctica , Drake Passage, St. PS 61 / 041–03 (59°22.24’S 60°04.06’W to 59°22.55’S 60°04.01’W), 2359–2375 m, leg. Katrin Linse, January 26–27 2002, ANT XIX/3 (ANDEEP I) expedition, fixed and preserved in 96% ethanol.

Description of a specimen examined by SEM (ZSM Mol 20070894)

Shell minute, 1.11 mm long, 1.67 mm wide ( Fig. 3A ), SL/SW ratio 0.66, depressed, fragile, white in color, with roundly inflated whorls, clearly expanded laterally, low spired, spire angle 126°. Protoconch ( Fig. 3D ) of one whorl, 320 µm in diameter, 116 µm long, globose, sculptured with fine anastomosing threads, forming reticulate pattern. Teleoconch ( Figs 3A, C ) of up to 1.75 whorls with convex profile. Whorls connected only for a short distance, more or less at periphery of preceding one. Suture consequently deep and channeled. Last whorl clearly increasing in diameter and laterally expanded. Aperture circular, 0.68 mm high (SL/AL ratio 1.63), peritreme holostomate, with thin lip. Shell base roundly inflated. Umbilicus ( Fig. 3B ) phaneromphalous, circular, wide, surrounded by four slightly elevated spiral cords, two of them entering umbilicus. Shell surface cancellate. Axial sculpture of narrow ribs, 29 on last whorl in the holotype, deeply entering the umbilicus. Ribs regularly spaced, rounded, interspaces about four times rib width. Spiral sculpture of numerous flat threads, five on first teleoconch whorl, about 14 on the last adult whorl, distinctly narrower than axial ribs, partly overlaying axil ribs. Operculum circular, multispiral, with centric nucleus.

Remarks: This is the first record of this species from outside the type locality and the new material considerably extends the geographic and bathymetric range of this species (Fig. 4).

There are minor differences to the description of the holotype presented by Zelaya et al. (2006). Although somewhat smaller (1.11 mm vs 1.5 mm) our specimen has a slightly larger protoconch (320 µm vs 300 µm). The spire angle in our specimen is greater (126° vs 105°) and although the teleoconch whorl number is smaller (1.75 vs 2.25), the number of axial ribs on the last body whorl is similar (29 vs 28). However, too little is known about the intraspecific variation of this species to warrant the description of a new species on these differences alone.

Figure 4: The geographic distribution of Brookula exquisita Clarke, 1961 . The circle marks the literature data (type locality only), while the triangles show the new records. A 2500 m depth contour in light grey.

Figures

FIGURE 3: A – D, Brookula exquisita Clarke, 1961 (ZSM Mol 20070894) from Antarctica, South Sandwich Islands (58 ° 15.98 ’ S 24 ° 53.72 ’ W to 58 ° 16.13 ’ S 24 ° 53.87 ’ W), 2947 – 2970 m depth. A, Ventral view of the complete shell. B, View of the shell base showing the wide opening of the umbilicus. C, Apical view. D, Detail of fig. C to show the fine protoconch ornamentation. Scale bars: 100 µm.