Onustus aquitanus, Simone & Cunha, 2012

Simone, Luiz Ricardo L. & Cunha, Carlo M., 2012, Taxonomic study on the molluscs collected in Marion-Dufresne expedition (MD 55) to SE Brazil: Xenophoridae, Cypraeoidea, mitriforms and Terebridae (Caenogastropoda), Zoosystema 34 (4), pp. 745-781 : 750-751

Onustus aquitanus n. sp.

( Fig. 2 )

TYPE MATERIAL. — Holotype: Brazil, Rio de Janeiro, off Arraial do Cabo, continental slope, 23°41’S, 42°06’’W, 430-450 m depth, MNHN 25216 ( Fig. 2 A-E) ( MD 55, stn CB 104, Bouchet, Leal & Métivier coll., 01. VI.1987). Paratype: Brazil, Espírito Santo, off Conceição da Barra, continental slope of Abrolhos, 18°59’S, 37°50’W, 637 m depth, MZSP 104032, 1 shell ( Fig. 2F, G ) ( MD 55, stn CB 76, Bouchet, Leal & Métivier coll., 27. V.1987).

TYPE LOCALITY. — Brazil, Rio de Janeiro, off Arraial do Cabo, continental slope, 23°41’S, 42°06’’W, 430-450 m depth ( MD 55, stn CB 104).

DISTRIBUTION. — Continental slope from north Espírito Santo to middle Rio de Janeiro.

ETYMOLOGY. — The specific epithet is related to the word “aquitan” in the native Tupi language, meaning short, small.

DIAGNOSIS. — Size c. 20 mm. Spire angle c. 100°. Dense riblets sculptured dorsally and smooth ventrally. Moderate density of foreign objects attached.

DESCRIPTION

Shell: size c. 20 mm; bluntly conic; c. 1.75 times broader than tall. Spire angle c. 100° ( Fig. 2A, B, F ). Colour pure white. Protoconch dome-shaped, with three uniform whorls; pale brown; surface smooth, poorly shining, weakly convex( Fig.2E, G ); limit with teleoconch clear, orthocline. Up to five teleoconch whorls, profile weakly concave; suture somewhat well marked. Dorsal sculpture of opisthocline, rather wavy riblets, about three per mm, crossed by fine prosocline growth lines ( Fig. 2A, D, G ). Peripheral flange narrow ( Fig.2B ). Inferior surface almost plane, weakly concave whorls; entirely covered by narrow, low, spiral riblets, crossed by concave growth lines parallel to aperture.Umbilicus opened, comprising c. 15% of maximum diameter; walls concave, simple, rounded; weak beige colour ( Fig. 2C ). Aperture elliptical, about twice wider than tall; comprising about half of maximum diameter and about half of total shell height. Outer lip as flap covering aperture dorsally. Inner lip weakly concave in umbilical region, widely concave in inferior region. Presence of foreign objects in entire dorsal surface, with shell surface c. 40% exposed ( Fig. 2A, B ).

MEASUREMENTS (in mm). — Holotype: 18.3 × 10.1; paratype: 13.3 × 7.8 (broken).

HABITAT. — Muddy bottoms, 430-637 m depth.

REMARKS

The generic attribution to Onustus aquitanus n. sp. is not straightforward. It has an intriguing mixture of characters in common to Onustus and to Xenophora Fisher, 1807 as defined by Ponder (1983). The relatively wide umbilicus, the irregular riblets in upper surface, the lack of sculpture of inferior surface and the porcellaneous inferior surface indicate Onustus affinity. On the other hand, O. aquitanus n. sp. lacks a wide peripheral flange and has a good quantity of foreign objects attached to its whorls, which approach the species to Xenophora . The generic attribution in favor to Onustus is purely by parsimony.

Onustus aquitanus n. sp. differs from O. longleyi and from O. caribaeus in being proportionally wider, while the other two species are tall, highly conic in the region close to the apex. Besides, O. aquitanus n. sp. lacks a deep furrow at outer edge in each whorl, which is normally present in the other two species ( Ponder 1983: fig. 31b). The spire angle of O. aquitanus n. sp. is c. 100°, while that of the other two species is c. 80°.Additional differences of O. aquitanus n. sp. are its sculpture of oblique riblets less dense (about three per mm, while the other species have about five per mm in second teleoconch whorl), the whitish colour (while the other two species are yellow to beige) and the convex shape of the whorls (for the other two species the whorls are planer).Anyway, by the shape of the aperture, O. aquitanus n. sp. appears to be more closely related to O. longleyi .

Beyond the generic features, O. aquitanus n. sp. additionally differs from Xenophora conchyliophora in being taller, by the presence of an umbilicus and in lacking brown pigmentation.

In regard to the protoconch, O. aquitanus n. sp. has an intermediary degree of multispirality. It is not as multispiral as, e.g., the Pacific Stellaria solaris (Linnaeus, 1764) and Onustus exutus (Reeve, 1842) . On the other hand, it is much more coiled than protoconchs of the Pacific Xenophora flindersi (Cotton & Godfrey, 1938) and others ( Ponder 1983: fig. 13). The O. aquitanus n. sp. protoconch, in fact, is somewhat similar to those of X. neozelanica Suter, 1908 .

The shell size is another interesting diagnostic feature. Onustus aquitanus n. sp. looks to be the smallest of the Recent xenophorids. The remaining species, mainly those from western Atlantic, are commonly over 100 mm. The sample studied here are not larger than 20 mm, although the paratype has the last whorl broken, its estimate maximum diameter is shorter than 50 mm. The small size if compared with other xenophorids is an interesting feature, mainly regarding the fossil record, which shows that the oldest species are of small size ( Cottreau 1922; Ponder 1983; Simone 2005a).

Superfamily CYPRAEOIDEA Rafinesque, 1815 Family CYPRAEIDAE Rafinesque, 1815

Figures

FIG. 2. — Onustus aquitanus n. sp. types: A-E, holotype MNHN 25216, maximum width = 18.3 mm; A, apical view; B, frontal view; C, umbilical view; D, detail of sculpture in region indicated by a rectangle in A; E, detail of apical region, apical view; F, G, paratype MZSP 104032, frontal and apical views, maximum diameter = 13.3 mm.