The shell shape and sculpture are very variable. The species exhibits a conical morphology, dorsally raised, with a thick and non-translucent shell. The anterior part of the animal is laterally compressed. It has a large shell (maximum length 120 mm) and a variable profile. The apex is situated at the anterior 30-40% of the shell's length. All the slopes of the shell are convex. The aperture of the shell is oval. The surface of the shell is sculptured with weak primary radial ribs, weaker secondary ones and concentric growth lines. The margin of the shell is highly crenulated. The external coloration is very variable (pale brownish/green/grey to dark brown and black). The internal part of the shell varies from creamy nacreous to dark bronzy-brown with dark lines corresponding to external colour patterns. 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 a single black longer tentacle and three shorter ones (white/black/white). The cephalic tentacles are dorsally pigmented with a black line. Radula: The first lateral teeth are set close together on the anterior edge of the basal plates, of medium length and with two spoon-like cusps. The second laterals are broader, wider spaced, with two spoonlike cusps. One of them is very small and the other is bigger and broader.

On rocks, King George Island, South Shetland Islands, Antarctica, 53.4mm., F, Coll. S. Maytia, 1992/iv/03
The « South Polar Limpet » is a sub-Antarctic to Antarctic nacellid mainly distributed in the Scotia Sea but extends further to reach Bouvet Island. The name of this species has been intricately confusing. First described as Patella polaris Hombron & Jacquinot, 1841; it was known by this name for a long time until Powell in 1973 recognised that the name is a junior homonym of Patella polaris Roding, 1798. Powell replaced the name with Nacella concinna (Strebel, 1908), which was originally described as a deep water form; until Engl decided the name N. polaris is available after all in his 2012 book 'Shells of Antarctica'. It is a algae-grazing herbivorous gastropod living attached on rocky surfaces from intertidal zone down to about -200m deep. The shallow water nominal form (shown here) has a rather elevated shell with weak to no ribbing as adults, whereas the deep water N. polaris f. concinna is much more flattened and usually carry strong radial ribs numbering 28-30. The nominal form is also generally larger with typical shell length around 40mm. and up to 65mm., f. concinna averages at around 25mm. but giants may exceed 50mm. Although it is locally a very common species it is uncommon on the shell market due to its remote distribution.

The Antarctic limpet represents a single genetic population along maritime Antarctica, but this population differes markedly from those from South Georgia. The inter- and subtidal morphotypes were once regarded as separate subspecies, N. polaris polaris and N. polaris concinna (Powell, 1951). Nevertheless, genetic comparisons using allozymes (Beaumont & Wei, 1991), mtDNA (Gonzalez-Wevar a/., 2011b, 2013,2016b) andAFLPs (Hoffman et al., 2010b) confirmed that these forms represent a single evolutionary unit. Accordingly, shell height, shape and sculpture differences in the Antarctic limpet represent phenotypic plasticity in the species.

Distribution: Maritime Antarctica, including ice-free rocky ecosystems of the Antarctic Peninsula, the South Shetland Islands, South Georgia, South Orkneys, Bouvet, Elephant Island, Seymour Island, Paulet Island, Wander Island, Anvers Island and Peterman Island.
Habitat: Intertidal and subtidal between 0 and 150 m depths.