Rapturella atlas n. sp. (Figure 1)

Type material. Holotype MZSP 91018 (Fig. 1 A–B) . Paratypes: BRAZIL, Rio de Janeiro state, Cape São Tomé, 21°58.32' S 40°02.05' W, 155 m, MZSP 107499, 1 shell (18.vi.2005); São Paulo state, São Sebastião, 24°44'69" S 44°44'965" W, 153 m, MZSP 107496 (Fig. 1 C–F), 1 shell, MZSP 107497, 1 shell, MNHN 25271, 2 shells, MNRJ 30855, 2 shells; 24°42.8' S 44°42.9' W, 162 m, MZSP 107498, 2 shells (20.xi.1997); 25°14.4' S 45°03.1' W, 147 m, MZSP 107500, 4 shells (21.xi.1997) .

Type locality. BRAZIL: Rio de Janeiro, off Cape São Tomé, 22°00.708' S 40°03.380' W, 162 m (Project Frade Development, sta BC_A_E39_1, C.H. Caetano coll., 29.vi.2005) .

Etymology. In honors of Atlas, the nick name of Frank Fontaine, the character of the science-fiction video game series BioShock: an allusion to the hidden identity of the species. The noun in apposition.

Diagnosis. Small shell with walls strongly thickened. Umbilicus deep and narrow. Spiral grooves on teleoconch separated by regularly spaced intervals, each interval about 10 times wider than the grooves; last whorl has about 11 spiral grooves, but no subsutural grooves. Palatal region has no thickened area.

Description. Shell oval, profile rounded, maximum length (L) 3.1 mm; about 1.3 times longer than wide. Walls thickened. Color white; spire short, length about 1/4 of total length (Fig. 1A, B). Protoconch small, glossy, 1 whorl; width about 0.4 mm; separated from teleoconch by narrow orthocline furrow (Fig. 1F: arrows). Teleoconch up to 4 whorls, each whorl convex (Fig. 1C). Surface smooth, glossy, sculptured with narrow punctuated spiral grooves distributed on entire teleoconch; punctuations very small, oval (Fig. 1D); grooves regularly spaced, separated by gaps about 10 times wider than grooves; last whorl with about 11 spiral grooves, gradually becoming deeper and closer to each other towards umbilical area; 3 grooves on anterior half and 8 on posterior half; umbilicus rimate, partially covered by extension of columellar margin. Aperture about 2/3 of total length, antero-posteriorly elongated, length about 1.5 times width; posterior end pointed, anterior (siphonal) rounded. Inner lip concave; superior half convex, rounded, with thin callus, about 1.3 times length of inferior half; inferior half slightly concave, with shallow edge, and small thickened columellar fold in posterior region. Outer lip thick, with sharp edge.

Measurements (length by width in mm). Holotype: 2.8 by 1.6 (Fig. 1 A–B). Paratypes: MZSP 107496 (Fig. 1 C–F), 1 shell, 3.1 by 2.2; MZSP 107497, 1 shell, 2.6 by 1.7; MZSP 107498, 1 shell, 2.7 by 1.8; MZSP 107500, 3 shells, (#1) 2.3 by 1.5, (#2) 1.7 by 1.3, (#3) 2.3 by 1.6; MZSP 138843, 1 shell, 2.7 by 1.8; MNHN 25271, 2 shells, (#1), 2.0 by 1.4, (#2) 2.5 by 1.7; MNRJ 30855, 2 shells, (#1) 1.8 by 1.3; (#2) 2.5 by 1.7; MZSP 90698, 1 shell, 3.3 by 2.2; MZSP 91018, 1 shell, 2.6 by 1.7.

Distribution. Rio de Janeiro to São Paulo, Brazil.

Habitat. Muddy sand bottom, 147–162 m depth.

Material Examined. Types.

Remarks. Rapturella atlas n. sp. fits in the genus Rapturella because it presents most of the diagnostic features of the genus including a minute shell size with thick walls, rounded spire whorls with a subsutural shelf, a teleoconch sculptured by spiral grooves composed of rounded punctae, and the characteristic aperture with a rounded trapezoidal shape, a columellar region thickened with a weak fold, a narrow anal canal, and a rounded siphonal canal region.

Rapturella atlas n. sp. most resembles R. ryani, from the Florida Keys, USA. It differs by having a smaller size (length up to 2.5 mm, while R. ryani easily reaches 4 mm); thicker walls forming a more solid shell; 3 spiral grooves on the penultimate whorl (5 more closely spaced grooves on R. ryani), about 3 grooves in the periumbilical region (about 10 more closely spaced grooves on R. ryani), and a smaller protoconch (0.42 mm, against 0.47 mm of R. ryani). Despite it being a smaller species, the smaller protoconch of R. atlas is a relevant developmental parameter. Rapturella atlas n. sp. differs from R. globulina (Forbes, 1884), from the Eastern Atlantic (Salvador & Cunha, 2016), by its slightly rounded whorls forming a straighter profile (whorls of R. globulina are more rounded), the small subsutural shoulder on each whorl (absent on R. globulina), and by the more widely spaced spiral grooves on its shell surface. Additionally, R. atlas n. sp. differs from its congeners in lacking two closely spaced, punctate, subsutural grooves (Salvador & Cunha 2016: fig. 4C, F), and in lacking a faint tooth-like thickening on the palatal region.

This discovery expands the geographic distribution of the genus Rapturella southwards to the southwest Atlantic region off the Brazilian coast. However, the distribution of the genus remains restricted, like that of the genera Callostracon Repetto & Bianco, 2012 and Mysouffa Marcus, 1974, to the Atlantic and Mediterranean. This conforms to the heterogenic distribution of acteonids, with most genera confined to specific regions such as the subarctic (e.g. Neactaeonina Thiele, 1912), the Pacific (e.g. Pseudactaeon Thiele, 1925 and Maxacteon Rudman, 1971) or the Western Atlantic (e.g. Ovulactaeon Dall, 1889). Only the genus Acteon has a more widespread range, but, with species of such diverse shell shapes (e.g. Rudman 1971; Marcus 1974; Valdés 2008), further taxonomic investigation may result in its segregation into more genera.

The new species, as well as the genus Rapturella, are recognized only by a combination of few conchological characters. The lack of soft tissue samples inhibits any comparative morphological evaluations or molecular analyses. Thus, the putative relationships of the genus with other acteonids and the classification of the species, which we here assign to Rapturella, need to be tested once new quality samples become available.