Antarctophiline alata

Antarctophiline alata (Thiele, 1912) Diagnosis Body oval, arched dorsally, white or ivory coloured. Cephalic shield square-oval. Shell internal, subquadrate, slightly angled dorsally. Radular formula 12–13 × 2.1.1.1.2. Gizzard plates (3), elongated, oval, symmetrical; internal surface, chitinous, flattened, holes or slits absent, with concentric amber and brown bands; external surface flattened. Material examined North South Sandwich Islands, SS1A/30, 56 ○ 42 ′ 60.6 ′′ S, 27 ○ 1 ′ 35.8 ′′ W, 134–142 m depth: 1 specimen, sequenced and dissected, SIO-BIC M13656 (3 September 2011), COI barcode MN486291, L = 11 mm, W = 7 mm; SS2A/38, 58 ○ 22 ′ 42 ′′ S, 26 ○ 17 ′ W, 134–260 m depth: 23 specimens, 11 sequenced and 1 dissected, SIO-BIC M12941 (6 October 2011), L = 10 mm, W = 7 mm; SS3/42, 56 ○ 42 ′ 60.6 ′′ S, 27 ○ 1 ′ 35.8 ′′ W, 103–221 m depth: 4 specimens, 1 sequenced and 1 dissected, SIO-BIC M12993 (7 October 2011), barcode MN486273, L = 15 mm, W = 8 mm. South Sandwich Islands, St. A 6-2: 12 specimens, 1 dissected (not sequenced), MCZ393943 (February 1995), L = 15 mm, W = 9 mm; St. 136-1, 70 ○ 50 ′ 12.0 ′′ S, 10 ○ 35 ′ 24.0 ′′ W, 271 m depth: 6 specimens, 1 dissected (not sequenced), ZSM20010079 (10 April 2000), L = 13 mm, W = 7 mm. External morphology (Fig. 2a) Maximum L = 11 mm, W = 7 mm. Body oval, arched dorsally; white or ivory coloured. Mantle thin, translucent white. Cephalic shield oval, extending approximately half of body, slightly extended over shell; extending longitudinally from second third until posterior edge. Parapodial lobes triangular, slightly extended over shell. Foot smooth, extending from mouth to approximately three-quarters of animal. Shell (Fig. 2b) Maximum L = 7.2 mm, W = 5.7 mm. Internal, subquadrate, white, thin, slightly angled dorsally, flattened. Periostracum thin, pellucid. Aperture almost full extension of shell, with thin parietal callus. Outer lip straight, in right angle posteriorly. Columellar wall concave. Apex superficial, umbilicated, left-sided. Growth lines marked. Radula (Fig. 2c) Radular formula 12–13 × 2.1.1.1.2. Rachidian teeth not detected. Later teeth hook-shaped, inner lateral thick, with broad base, tip rounded or pointed, inner edge with faint dentition not always detected. Outer lateral teeth hook-shaped, with broad base, tip rounded. Digestive system (Figs 2d & 3) Buccal mass very muscular. Salivary glands short, entering pharynx from posterior end, extending over crop to beginning of gizzard. Crop globose, saccular, thin-walled. Gizzard elongated, surrounded by circular muscle fibres. Gizzard plates three (maximum L = 1.5 mm, W = 0.6 mm), elongated, oval, symmetrical; internal surface, chitinous, flattened, holes or slits absent, with concentric amber and brown bands; external surface flattened (Fig. 2e,f); spines present in gizzard (Fig. 2d). Oesophagus thin, internally plicate. Stomach embedded in digestive gland. Digestive gland large, brownish, granulose; occupying most of viscera. Intestine rising posteriorly towards the anterior part of the shell, creating a loop, turning backwards to right side; ending in external anal papilla situated in mantle cavity. Reproductive system (Figs 2g,h & 4) Male penial sheath short, pyriform. Penial papilla short. Ejaculatory duct displayed in an S shape, surrounded by musculature tissue. Prostate granulose, saccular, shorter than penial sheath, attached to seminal vesicle. Seminal vesicle saccular, semitranslucent, slightly smaller than prostate, containing autosperm. Female gonad orange in colour (ovules), presenting white marks (sperm), lumpy, surrounding the digestive gland posteriorly. Ampulla cylindrical, isodiametric, displayed in zigzag. Seminal receptacle oval, short, connected by a thin duct. Albumen gland (= capsule gland; Klussmann-Kolb 2001) rugose, relatively small, covering end of ampulla and seminal receptacle duct. Membrane gland bean-shaped, internally attached to mantle cavity. Mucous gland white, large, folded, extending ventrally above viscera towards left edge. Bursa copulatrix rounded, distally connected to vagina through a thin, long duct. Vagina muscular, with ample atrium. Excretory, circulatory and respiratory systems Kidney flattened, in anterior right side of viscera, attached to mantle, internally highly ramified, connected to pericardium anteriorly. Pericardial complex arranged transversally in an angle of 45 ○. Auricle thin-walled. Ventricle triangular, muscular. Aorta thick, bifurcating into two branches that run parallel to edge of shell. Ecology Occurs in sandy bottoms at depths from 4 m (this study) down to 640 m (Hain 1990). Sponge spicules, diatoms and foraminifera were found in the gut contents of the specimens dissected in this study (Fig. 5a–c). Distribution Known from the Davis Sea and Adélie land in East Antarctica (Thiele 1912, Vicente & Arnaud 1974); South Orkney Islands, South Sandwich Islands in the Scotia Sea and Palmer Archipelago in the western Antarctic Peninsula (Powell 1951, Aldea & Troncoso 2008, Moles et al. 2021); Deception, Livingston and King George islands in the South Shetland Islands (Powell 1951, Troncoso et al. 1996, Engl 2012, Moles et al. 2021); Peter I Island, Bellingshausen Sea (Aldea & Troncoso 2008);? eastern Weddell Sea (Hain 1990, Engl 2012); and Bouvet Island (Moles et al. 2019). Remarks This species was described by Thiele (1912) based on a single shell record from the Davis Sea characterized by an almost squarish, smooth shell with growth lines, a straight outer lip and a somewhat straight base. This type of shell has been found all over Antarctica and resembles our material. Although slight intraspecific differences in the shape of the gizzard plates exist (e.g. more convex, sometimes slightly pointy, the colour on the bands), they are oval, chitinous and flattened in the interior and exterior parts, with concentric amber and brown bands on the interior side. Most records in the distribution area have been revised by Moles et al. (2019), and here we add specimens from the northern and southern South Sandwich Islands. There, we found specimens showing no remarkable morphological differences from those from the remaining areas of distribution. Overall, although molecular data recognize several lineages (Fig. 1; Moles et al. 2021), the dissected specimens from Bouvet Island, the South Shetland Islands and the South Sandwich Islands present no clear morphological differences (Moles et al. 2019; this study) and thus are retained here as a single species.