Aclis watsoni, De, José Carlos N., De, Silvio Felipe B. & Francisco, Jonata De A., 2007

De, José Carlos N., De, Silvio Felipe B. & Francisco, Jonata De A., 2007, Two new species of Aclis (Mollusca: Gastropoda: Aclididae) from the continental slope of northeast Brazil, Zootaxa 1614, pp. 61-68 : 65-67

Aclis watsoni n. sp.

( Figures 7–9 )

Type material: Holotype, MZSP 77991 [Lt 4.4 mm; Wt 1.4 mm] and 3 paratypes, MZSP 77992, off the state of Sergipe, Brazil, D-6, 10º41.4’00’’S, 36º18.7’00’’W, 365 m, muddy bottom, 28.x.2000; 4 paratypes, MZSP 77993, off the state of Sergipe, Brazil, D-7, 11º35.5’00’’S, 37º12.3’00’’W, 510 m, muddy bottom, 30.x.2000.

Type locality: Off the state of Sergipe (10º41.4’00’’S, 36º18.7’00’’W), continental slope of northeast Brazil, at 365 m depth.

Diagnosis: Shell elongated, turreted, with strong spiral keels, varying from two to three on body whorl. Aperture elliptic, lower part of aperture being drawn out towards length axis, weakly reflected, forming angles at terminations of spiral keels. Inner lip weakly reflected. Base conical, narrow. Umbilical chink.

Description: Shell small [Lt 4.1 to 4.4 mm], white, elongate-turreted shape, polished between spiral keels ( Figs. 7, 9 ), holotype Lt/Wt 3.14. Spire high, with progressively angular contour of post-nuclear, starting at third whorl. Protoconch 1½ whorls, globose, slightly inclined nucleus, distinct from remaining whorls, terminating in orthocline axial thread ( Fig. 8 ). Teleoconch with numerous whorls, 6 1/3 to 7 1/3 whorls in adult specimens, with markedly angular contours due to presence of single median spiral keel to antepenultimate whorl, forming shoulder between second and fifth whorl. Penultimate whorl with presence of second suprasutural keel, evident in last whorl; third higher and weaker keel may be present. Suture well-marked, subsutural region wide, entirely smooth; area below subsutural keel narrower and smooth ( Fig. 9 ). Body whorl following shape of previous whorls. Aperture elliptic, lower part of aperture being drawn out towards center of shell, weakly reflected, forming angles at terminations of spiral keels, with weak posterior sinus. Inner lip weakly reflected on umbilical area. Parietal region fine, no “callus”, columella thick. Base conical, slightly convex, umbilicus in shape of fissure.

Etymology: In homage to Mr. R.B. Watson, who first identified species of Brazilian Aclididae .

Geographic and bathymetric range: Off the state of Sergipe (10º41.4’00’’ – 11º35.5’00’’S; 36º18.7’00’’ – 37º12.3’00’’W), northeast Brazil, 365 to 510 m depth.

Remarks: Aclis carolinensis Bartsch, 1911 from the North Atlantic may at times develop a keel, as observed by Warén (pers. comm., June 2006, based on types deposited at the Smithsonian Institute). Aclis floridana Bartsch, 1911 presents post-nuclear whorls with a strong sloping shoulder, and featuring a pronounced subsutural carina. However, it is evident that both species have a smaller number of keels that also present a different arrangement to those of A. watsoni .

Aclis angulata ( St. Helena island) is correlated to Aclis watsoni in its turreted shape, suture depth and the interesting convexity of the whorls, revealing a single strong spiral keel. As observed in Smith (1890: 280, pl. 23, fig. 28), the former species is clearly distinguished by the presence of conspicuous growth threads, a lower spire, a rather inflated body whorl and a protoconch that is much smaller than the first post-nuclear whorls.

Aclis minor ( Brown, 1827) is similar to A. watsoni regarding the oval to ellipsoidal aperture shape and the weakly reflected, fine, straight parietal region. The former differs from the latter regarding the presence of a broadly umbilicate base, and of an expanded, rounded body whorl with three to five spiral keels.

Aclis trilineata Watson, 1897 presents similar morphological features to A. watsoni (see Peñas et al. 2006: 77, figs. 127–128). The contour of the last whorls is correlated in both species, as are the broad subsutural slope, open, rather oblique suture and shallow, small umbilicus. Based on Watson (1897: 255), A. trilineata differs from A. watsoni in having three very strong rounded threads on each post-nuclear whorl (instead of keels), the adapical of which forms a well-marked shoulder, faint close-set microscopic striations and vaguer traces of longitudinal markings.

Aclis supranitida ( Wood, 1842) and Aclis ascaris ( Turton, 1819) are slightly angulated adapically, revealing some indication of a subsutural ramp similar to A. watsoni (see Fretter & Graham 1982: 398, fig. 282C; 400, fig. 285). The main characteristics that distinguish A. supranitida from A. watsoni are the protoconch with about four whorls, 10–11 swollen whorls, spiral ridges when obvious, three to five on the body whorl and two to four are on more adapical whorls, and growth lines as slight markings. Aclis ascaris and A. watsoni have an umbilicus chink and a shallow anal sinus in common; however, the former is distinguishable by the peripheral bulge and four to five spiral ridges on the last whorl.

Aclis watsoni and A. sarissa have conical shells, however, the latter stands out from its congener by the glassy surface, by having nine rounded whorls, and by the posterior region of the aperture that is markedly angulated and effuse on the base.

Figures

FIGURES 7 – 9. Aclis watsoni n. sp. (SEM): 7. Paratype (MZSP 77993), ventral view (Lt: 4.1 mm); 8. Holotype, detail of protoconch; 9. Holotype (MZSP 77991), ventral view (Lt: 4.4 mm); Scale bars: Figs. 7, 9 = 500 µm; Fig. 8 = 100 µm.