Austromitra decresca, 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 : 765-766

Austromitra decresca n. sp.

( Fig. 7 A-D)

TYPE MATERIAL. — Holotype: Brazil, Espírito Santo, off Itaúnas, continental slope of Abrolhos , 18°56’S, 37°52’W, 85-105 m depth, MNHN 25239 ( Fig. 7 A-C) ( MD55 , stn DC82 , Bouchet, Leal & Métivier coll., 28.V.1987) . Paratypes: same data as holotype, MZSP 102687 , 1 shell ; 18°49’S, 37°57’W, 60 m depth, MNHN 25240 , 1 shell (stn DC83 ) .

TYPE LOCALITY. — Brazil, Espírito Santo, off Itaúnas, continental slope of Abrolhos, 18°56’S, 37°52’W, 85- 105 m depth ( MD 55, stn DC 82).

DISTRIBUTION. — Off Espírito Santo, Abrolhos continental slope.

ETYMOLOGY. — The specific epithet is derived from the Latin word “decresco”, meaning grow smaller, an allusion to the small size of the shell.

DIAGNOSIS. — Shell up to 6 mm, biconic (length about twice width). Sculpture mainly axial, strong, with interspaced equivalent to threads width; crossed by spiral threads forming reticulation, covering axial threads; shoulder absent. Protoconch blunt, cylindrical, of two whorls. Aperture c. ½ shell length. Canal narrow, relatively elongated, projected forwards.

DESCRIPTION

Shell: size up to 6 mm, outline biconic; width c. 45% of length. Colour uniform beige to pale brown ( Fig. 7A, B, D ); surface opaque. Protoconch blunt, cylindrical, comprising c. 8% of shell length and c. 22% of shell width; about two rounded, smooth, glossy whorls; suture clear ( Fig. 7A, B ); transition with teleoconch unclear.Spire c. 50% of shell length; spire angle c. 45°. Teleoconch up to four whorls; each whorl lacking shoulder ( Fig. 7A, D ); whorls profile weakly concave; suture clear.Sculpture strong, uniform axial threads, orthocline, from suture to suture ( Fig. 7A, B, D ); c. 14 in penultimate whorl; interspaces of axial threads roundly concave, as wide as threads; spiral threads narrower and lower than axial threads running uniformly covering axial threads in undulated fashion, forming small nodes on axial threads’ edge, four in penultimate whorl ( Fig. 7B ), c. 15 in body-whorl ( Fig. 7A, D ). Ventral region of body-whorl with anterior third with spiral threads predominating. Aperture elliptic, weakly oblique; comprising c. 35% of shell length and c. 32% of shell width. Outer lip simple, rounded; inner surface edged by liration, a series of nine short, transverse, small folds ( Fig.7D ). Inner lip weakly concave, callus shallow, upper third c. 50° in relation to longitudinal shell axis; four oblique folds uniformly distributed in middle region, comprising c. 70% of inner lip area; upper folds larger, slightly diminishing towards canal ( Fig. 7C ). Canal narrow, pointed, curved, projected forwards and dorsally along c. 13% of shell length; with c. 70% of aperture width. Dorsal surface of canal with four oblique, low folds as continuation of columellar folds ( Fig. 7A, C ). No umbilicus.

MEASUREMENTS (inmm). — Holotype: 5.6 × 2.6; paratype MZSP 102687: 5.8 × 2.7 (apex broken).

HABITAT. — Sandy bottoms with coralline red algae debris, 60-105 m depth.

REMARKS

The liration in outer lip and in dorsal region of canal approaches A. decresca n. sp. to the species of the genus Austromitra , mainly the South African A. hayesi Turner, 1999 , from which it differs in having a more biconic outline, by narrower and more elongated aperture, and by the stronger axial sculpture. There are no other species with which A. decresca n. sp. can be confused, as in the remaining species the axial threads are wider, with the spiral sculpture more dedicated and numerous ( Turner & Simone 1998; Turner 1999).

Austromitra decresca n. sp. is also somewhat similar to the species of Thala ; it differs from Thala foveata (Sowerby, 1874) from Florida region (1-91 m depth) in being smaller (that species is over 8 mm in length), more conic outline, aperture and protoconch slightly wider, and by spiral sculpture much less developed. Austromitra decresca n. sp. is also similar to Thala gloriae Rosenberg & Salisbury, 2003 , from Guam, which also has a rostrate projection anteriorly, a similar shape and sculpture. Austromitra decresca n. sp. differs by smaller size, fewer axial ribs, wider aperture and by less shouldered whorls.

Family MITRIDAE Swainson, 1829

Figures

FIG. 7. — Costellariid and volutomitrid: A, Austromitra decresca n. sp., holotype MNHN 25239, apertural view (L 5.6 mm); B, same, dorsal view; C, same, detail of inner lip, apertural-slightly right view; D, paratype MZSP 102687, apertural view (L 5.8 mm); E, Microvoluta corona n. sp., holotype, MNHN 25243, apertural view (L 5.5 mm); F, same, dorsal view; G, same, details of aperture, apertural-slightly right view.