(Fig. 9)
? Cyclostrema decussatum Pelseneer, 1903: 19, pl. 5, fig. 48. Brookula pfefferi Powell, 1951: 104, pl. 5, fig. 8.
Brookula rossiana Dell, 1990: 102, fig. 169.
Brookula sinusbreidensis Numanami & Okutani, 1991: 38, figs 2–6. Brookula delli Numanami, 1996: 55, figs 30 A–E.
Holotype of B. pfefferi (NHM 1961.368).
Type locality: Atlantic Ocean, South Georgia, off mouth of Stromness Harbour (54 °04’S 36 ° 27 ’W – 53 ° 58 ’S 36 ° 26 ’W), 155– 178 m. OD.
Material examined: 40 specimens (ZSM Mol 20021687), Weddell Sea sector of the Antarctic Peninsula (63 °01.10’S 61 °09.10’W), 311–365 m, ANT XVII/ 3 (EASIZ 3).
Remarks: This is the only species for which numerous specimens were available. The shell morphology of B. pfefferi is sufficiently described in Absalão et al. (2001) and Zelaya et al. (2006) but at least a first preliminary (excluding the ctenidia) overview of the external bauplan of the species may be given here (Fig. 9). Radula data were provided by Zelaya et al. (2006). A detailed histological examination is being undertaken by Thomas Kunze (LudwigMaximilians University, Munich, Germany).
The cylindrical head has a cloven, obtusely pointed snout. There is a pair of long, thick cephalic tentacles that show in their anterior half laterally dense fringes of sensory papillae. It is likely that the short rudiment on the base of the cephalic tentacle (Fig. 9 C) belongs to the eye stalk (eyes are not visble in the examined specimen), but this has to be confirmed by the histological data. The foot is fleshy and elongate, anteriorly with two wide, conical foot lappets and posteriorly, under the horny, circular, multispiral operculum (Fig. 9 D) with two shorter ones. Laterally between the anterior and posterior foot lappets a fringe of about 12 short, smooth, elongate epipodial tentacles is situated; at least on the dried animal, the tentacles do not show epipodial sense organs. There is a dense fringe of slender, hairlike cilia on the lateral foot margins. Immediately behind the right cephalic tentacle (Figs 9 A, C, E; marked with “*”) we detected a structure, which could be a penis, but also requires confirmation by histology.
Examination of the shell by SEM showed that the the inner shell layer of the aperture does not consist of a prismatic nacreous layer as typical for iridescence (see Fuchigami & Sasaki 2005).
Absalão et al. (2001) failed in locate the holotype of Cyclostrema decussatum Pelseneer, 1903, and Zelaya et al. (2006) did not take that species into account. The holotype of this species should be together with the other Pelseneer types in the Institut Royal des Sciences Naturelles de Belgique, Bruxelles, Belgium, but according to Yves Terryn (Scientific Associate), who searched the collection for it on the authors request, the type is not to be found. Based on the species’ original descriptions and figures we can not see differences between Brookula decussata (Pelseneer, 1903) and B. pfefferi Powell, 1951, and consider the latter merely a junior synonym of the former. It is important to point out that Brookula decussata (Pelseneer, 1903) does not “clearly differ from Benthobrookula pfefferi in having a smaller shell, with fewer, more separated, spiral threads” (Zelaya et al. 2006, p. 80) but is instead really similar (see also Powell 1951, p. 104). A direct comparision of Pelseneer’s illustration (1903, pl. 5, fig. 48) with the photo of the holotype of Brookula pfefferi Powell, 1951, available from Zelaya et al. (2006, fig. 5 A), does not only show the close similarity but also give the impression that the spiral threads are not “more separated”. In addition, while Zelaya et al. (2006) gave a maximum size of 1.83 mm for Brookula pfefferi Powell, 1951 (2 mm in the original description!), the maximum shell size for B. decussata (Pelseneer, 1903) is originally given as 2.5 mm and can thus under no circumstances be defined as “smaller”.