Brookula charleenae, Schwabe, Enrico & Engl, Winfried, 2008

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 : 194-195

Brookula charleenae sp. nov.

( Figs 5–6 )

Type material

Holotype ( ZSM Mol 20070879), leg. Michael Schrödl, March 22 2002, during the ANT XIX/4 (ANDEEP II) expedition.

Two paratypes ( ZSM Mol 20070880) from type locality, all specimens were fixed and preserved in 75% ethanol except for the SEM­mounted holotype.

Type locality: 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 ( Fig. 6 ).

Etymology: The species is named after the first author’s daughter Charleen Schwabe.

Description: Shell minute, reaching 1.42 mm long and 1.25 mm wide ( Fig. 5A ), globosely turbinate, fragile, white in color, with roundly inflated whorls, slightly expanded laterally, low spired, spire angle ranges from 70° (holotype) to 81° (larger paratype). Protoconch ( Figs 5C, D ) of 1.5 whorls, 278 µm in diameter, 220 µm high, globose, sculptured with fine anastomosing threads, forming reticulate pattern. Teleoconch ( Fig. 5A ) of up to 2.75 whorls with convex profile. Suture deeply constricted. Last adult whorl globose, slightly expanded laterally. Aperture circular, peritreme holostomate, with thin lip. Shell base roundly inflated. Umbilicus ( Fig. 5B ) narrow, circular, deep. Axial sculpture of narrow ribs, 28 on last whorl in the holotype, 27 on the penultimate whorl, deeply entering the umbilicus. Ribs regularly spaced, rounded, nearly lamellous, interspaces between them about three times rib width. Spiral sculpture of numerous flat threads, twelve on first teleoconch whorl, about 42 on last adult whorl, narrower than axial ribs, partly double­spaced, not overlaying axil ribs distally. Operculum circular, multispiral, with centric nucleus.

Comparisons: Brookula pfefferi Powell, 1951 : despite nearly the same shell proportions, the new species can be easily separated from B. pfefferi by 1) the absence of spiral cords on the shell base, 2) the higher number of spiral threads on the last adult whorl (42 vs 20–26), 3) the smaller protoconch (278 vs 330–400 µm). Brookula strebeli Powell, 1951 : this species has nearly the same size but has 1) a higher SL/SW ratio (1.20 vs 1.14), 2) a larger protoconch (300 vs 278 µm), 3) by nearly the same teleoconch whorl number (2 vs 2.75) a lower number of axial ribs (20 vs 28), 4) a much lower number of spiral threads (14 vs 42), 5) a nearly­closed umbilicus, 6) three to four spiral cords on the shell base.

Brookula calypso (Melvill & Standen, 1912) : in contrast to the new species this species has 1) a higher spire angle (90 vs 81°), 2) a larger sized protoconch (295 vs 275 µm) with only one whorl vs 1.5 whorls, 3) only 14–17 vs 28 spiral ribs on the last whorl, 4) two vs no spiral cords around the umbilicus.

Brookula conica (Watson, 1886) : the only other South Atlantic deep water species that does not have spiral cords around the umbilicus differs in 1) its larger size (up to 1.8 vs 1.42 mm), 2) the wider umbilicus (130 vs 105 µm [data from Absalão et al. 2001, fig. 2 b]), 3) the lower number of the wider spaced axial ribs (19 vs 28), 4) having very fine axial lines between the spiral threads, 5) its much smaller protoconch (145 vs 278 µm [data from Absalão et al. 2001, figs 2 f, d]).

Scale bars: A 500 µm, B, C 100 µm, D 10 µm.

Brookula paranaensis ( Zelaya, Absalão & Pimenta, 2006) : this species differs from the new species in having 1) different shell proportions (compare table in original description), 2) a similar sized protoconch (270 vs 278 µm) with only one whorl, 3) a larger spire angle, 4) a higher number (42 vs 28) of closer set axial ribs, 5) a smaller number of spiral threads on last adult whorl (20–21 vs 42), 6) three to four vs no spiral cords around the umbilicus.

Brookula kerguelensis Thiele, 1925 is the most similar species. It differs, however, in 1) its larger size (1.6 x 1.35 mm with the same whorl number), 2) the lower spire angle (c. 68°, taken from the illustration), 3) the smaller number of less closely set and flatter axial ribs on the penultimate whorl (c. 20 vs 27) on last whorl, 4) having an oval aperture. This species is known from a single slightly worn specimen (holotype, ZMB/Moll­ 64050) from the Kerguelen Islands (49°S 70°44’E) only.

None of the comparable congeners was ever reported from such depths.

Figures

FIGURE 5: A – D, Holotype of Brookula charleenae sp. nov. (ZSM Mol 20070879) 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 circular, deep opening of the umbilicus. C, Detail of fig. A, showing the protoconch and the first 1.5 teleoconch whorls. D, Detail of fig. C to show the fine protoconch ornamentation.

FIGURE 6: The geographic distribution of Brookula charleenae sp. nov. with a 2500 m depth contour in light grey.