Hemimarginula hemitoma, Simone, Luiz Ricardo L. & Cunha, Carlo M., 2014

Simone, Luiz Ricardo L. & Cunha, Carlo M., 2014, Taxonomical study on the mollusks collected in Marion-Dufresne (MD 55) and other expeditions to SE Brazil: the Fissurellidae (Mollusca, Vetigastropoda), Zootaxa 3835 (4), pp. 437-468 : 463-464

Hemimarginula hemitoma new species

Figures 140–151

Emarginula pumila: Métivier, 1972: 412 ; Rios et al, 1987: 11 (fig. 37); Leal, 1991: 41 (pl. 2, Figs D-E); Rios, 2009: 26 (fig. 48) (part) (non Adams, 1851).

Types. Holotype MNHN 25324. Paratypes: MNHN 25325, 1 shell, MZSP 102947, 2 shells from type locality. Brazil. Espírito Santo (MD55; Bouchet, Leal & Métivier col); off Itaúnas, continental slope of Abrolhos, 18°56’S 37°52’W, 85–105 m, MNHN 25326, 4 shells, MZSP 102948, 3 shells (sta. DC82; 28/v/1987); Jaseur Seamount, 20°27’S 35°54’W, 54 m, MNHN 25327, 1 shell (sta. DC34; 15/v/1987); Trindade Island, Enseada dos Portugueses, 20°30’15”S 29°18’46”W, 10–12 m, MNHN 25881, 3 shells (sta. 1; Bouchet col., v/1987), Andradas beach, 20°30’50”S 29°18’20”W, 0–1 m, MZSP 101247, 1 shell (Simone col. 24/vii/2011).

Type locality. Brazil. Espírito Santo; off Itaúnas, continental slope of Abrolhos 18°50’S 37°57’W, 60 m (MD55, sta. DC83; Bouchet, Leal & Métivier col. 28/v/1987).

Diagnosis. Apex blunt, located in middle, turned postero-dorsally. Height 40–60% of length. Sculpted by 50–60 strong radial cords, mostly alternating wide and narrow cords, interspaces regularly filled by minute transverse cords.

Description. Shell size ca. 5 mm; conical, limpet-like; height 40–60% shell length. Color beige to pale brown, sometimes with some radial white spots ( Figs 144, 145 ). Protoconch mostly eroded, located laterally, situated on right ( Figs 145, 147 ). Slit anterior, rectangular, width ca. 30% of length; opened anteriorly; 6% of shell width and 8–16% of shell length; edges rather irregular (Figs 140, 144, 146). Profile highly curved dorso-ventrally, sagittal profile slightly conical; general angle ca. 90°; apex blunt, curved ventrally, located in posterior 1/5–1/3 (Figs 141, 145, 147, 150). Strong sculpture of radial threads (Figs 140–148, 150), 50–60 surrounding apex; gradually increasing in width towards aperture, intercalated wide and narrow cords, varying from ca. 1/3 (Figs 140–141) to ca. 1/2 of threads (Figs 143, 146); in ca. 10% of specimens some radial threads much larger and pale pigmented ( Figs 144–145 ); interspaces between cords one half cords width, filled by transverse narrow cords, ca. 1/6 of threads width, ca. 40 scales along cords close to median line (Figs 140, 143, 146, 147). Selenizone weak, low, edges low and inconspicuous; filled by sparse lunules (Figs 140, 143, 144, 146). Aperture flat, outline elliptical, width ca. 75% of length; edges undulating due to radial cords. Inner surface smooth, glossy; horseshoe-shaped scar of shell muscle, located between middle and ventral thirds from apex to edge (Figs 142, 149); with series of minute pits, regularly spaced, radially aligned ( Figs 149, 151 ), ca. 40 pitted lines approximately organized in pairs.

Measurements (in mm). Holotype: 4.6 by 2.8; paratypes MZSP 102947 #1: 3.7 by 1.3 ( Fig. 144 ); #2: 3.1 by 1.7 ( Fig. 146 ).

Distribution. Brazil; Espírito Santo, Vitória-Trindade seamount ridge.

Habitat. Coralline and rocky bottoms, 1–105 m (dead specimens).

Material examined. Types.

Etymology. The specific epithet is related to the appearance to the slitless fissurellid genus Hemitoma Swainson, 1840 .

Remarks. The strong sculpture and the short slit of H. hemitoma are the main characters for the generic attribution, rather than to Emarginula , which mostly encompasses species with higher and more delicately sculptured shells. Hemimarginula hemitoma is similar to H. dentigera ( Heilprin, 1889) , from the Caribbean, the type species of the genus, in having some specimens with irregular colored ribs and by the strong radial sculpture ( Figs 144, 145 ). It differs from the latter in being taller (height/length ratio: 40–60%, versus 27% in H. dentigera ), and by the double-lined, radial pits on outer surface (Figs 143, 146, 147). The delicate radially arranged pits and the narrower radial ribs also distinguish this species from other north Caribbean congeners such as H. pumila ( A. Adams, 1851) and H. pileum ( Heilprin, 1889) ( McLean 2011) ; H. hemitoma has a comparable height to H. pileum , which has a height/length ca. 1/2. The sculpture of H. hemitoma is only comparable to that of H. biangulata ( Sowerby, 1901) from Japan, differing by the more rounded shape and more posterior apex. Hemimarginula dentigera has been referred as occurring in Brazilian coast (as Emarginula pumila : see McLean 2011, for a discussion). Analyzing the figures and voucher material in MZSP, it is here suggested that those reports actually belong to H. hemitoma (see synonymy above). However, some specimens, e.g., from Cairu, Bahia (5 m) (MZSP 100991), possibly belong to H. dentigera , as the specimen has a low shell and lacks radial pits between radial sculpture. The real distribution of H. dentigera and H. hemitoma requires further investigation.

The presence of aligned series of pits in the inner shell surface ( Figs 149, 151 ) appears to be a novelty for the genus. The pits possibly are related to aesthetes ( Salvini-Plawén & Haszprunar 1987 on other vetigastropods), which need to be confirmed in live-collected specimens.

Figures

FIGURES 144 – 151: Hemimarginula hemitoma n. sp. Fig. 144: paratype MZSP 102947 # 1 (L 3.7 mm), dorsal view. Fig. 145: same, left view. Fig. 146: paratype MZSP 102947 # 2 (L 3.1 mm), dorsal view. Fig. 147: same, left view. Fig. 148: same, SEM, dorsal-slightly left view. Fig 149: same, ventral view. Fig. 150: same, left view. Fig. 151: same, detail of Fig. 149 showing pits, scale = 0.1 mm. FIGURE 152: Diodora mirifica, MNHN (L 3.4 mm), dorsal view.