Nacella DELESSERTI

NACELLA DELESSERTI (PHILIPPI, 1849)

(FIG. 9)

Patella delesserti Philippi, 1849: 9, pl. 1, fig. 5.

Patella ferruginea Reeve, 1854: 425, pl. XVII, fig. 40a–c. Nacella (Patinigera) delesserti – Powell, 1973: 196; Cantera & Arnaud, 1985: 34.

Nacella delesserti – Blankley & Branch, 1984: 171; Blankley & Branch, 1985: 259; Hodgson & Bernard, 1989: 145; González-Wevar et al., 2016b: 77; 2017: 863.

Material studied: Trypot Beach, Marion Island (46°52’33.91’’ S, 37°51’51.40’’ E) N = 50; Ships Cove, Marion Island (46°51’16.22’’ S, 37°50’42.36’’ E) N = 40; Crozet Islands (46°25’34.25’’ S, 51°51’42.50’’ E) N = 50.

Shell: The shell shape and sculpture are relatively constant (Fig. 9A–C). The species exhibits a conical morphology, dorsally raised, with a very thick and non-translucent shell (Fig. 9A–F). The anterior part of the animal is laterally compressed (Fig. 9D–F). It has a medium-sized shell (maximum length 80 mm) and exhibits a variable profile. The apex is situated at the anterior 25–35% of the shell’s length (Fig. 9D–F). All the slopes of the shell are convex (Fig. 9A–C). The aperture of the shell is oval. The surface of the shell exhibits low radial folds and lamellar growth lines. The margin of the shell is highly crenulated. The external coloration of the shell is relatively constant in the species: grey to white, with dark (brown and black) primary radial ribs. The internal part of the shell has a soft metallic-white coloration, with dark lines corresponding to external colour patterns, bordered by a white/brown halo.

External anatomy: The ventral area of the foot is black and the epipodial fringe is highly recognizable. The mantle fold is thick and dun-brown coloured. The mantle tentacles are in alternate series of three shorter pigmented ones (white/black/white) and black longer one (Fig. 9G). The cephalic tentacles are dorsally pigmented with a black line (Fig. 9G).

Radula: The first lateral teeth are set close together on the anterior edge of the basal plates, long and very sharp, with two cusps. The second laterals are broader, wider spaced, with four very sharp cusps (Fig. 9H).

Distribution: Marion, Prince Edward and Crozet Islands (Fig. 9I).

Habitat: High and medium intertidal rocky ecosystem between 0 and 30 m depths.

Comments: Nacella delesserti was first reported at Crozet Islands by Cantera & Arnaud (1985), a finding we confirm here. These authors also reported the species from South Georgia, but our sample of>150 individuals consisted only of N. concinna. Zelaya (2005) also failed to find N delesserti at South Georgia. Hence, we consider N. delesserti to be restricted to Marion and Crozet Islands. Phylogenetic reconstructions placed N. delesserti as the sister species of N. concinna, their separation occurring around 0.35 Mya (González-Wevar et al., 2017). The presence of N. delesserti at sub-Antartctic Marion and Crozet Islands is thus the result of evolutionarily recent long-distance dispersal from maritime Antarctica. Nacella delesserti showed very low levels of genetic diversity that constitute evidence of recent postglacial expansion in Marion Island. Due to the exceptional high density and biomass of N. delesserti on Marion Island rocky shores, it plays an important role in the organization of intertidal and shallowwater benthic communities (Blankley & Branch, 1985).