Comprehensive DescriptionRead full entry
"Genus Nacella Schumacher, 1817
Type Patella mytilina Helbing, 1779
This genus and its subgenus Patinigera are characteristic molluscs of Antarctic and Subantarctic seas. They have their centre of distribution in the Magellanic Province of southern South America from whence the seaweed-dwelling species in particular tend to spread eastward, being assisted to a considerable extent by the prevailing West Wind Drift that operates strongly in the Subantarctic Zone.
Although Nacella and Patinigera are not members of the Indo-Pacific fauna, the recognized species are listed and briefly described here, since many of them do occur in waters to the south of both the Indian and Pacific Oceans, and at one location, Campbell Island, in the New Zealand faunal area, both Nacella (Patinigera) and the warmer-water derived Cellana flourish side by side.
Both Nacella and Patinigera differ from all other Patellidae in the presence of an epipodial fringe, a scalloped lamellate flange that occupies a mid position between the edge of the foot and the gill cordon, except where it is interrupted by the head region.
There is a link with Patella in that the gill cordon is complete, not interrupted by the head as it is in Cellana. The dentition, on the other hand, with its pair of centrals, alternating with a pair of laterals, is comparable with that of Cellana, not short, straight and bent back upon itself at the nascent end as it is in Patella.
Thiele in 1929 proposed the subfamily Nacellinae for Nacella, Patinigera and Cellana, but the epipodial fringe, characteristic of Nacella and its subgenus Patinigera, is not found in Cellana or in any other patellid genus.
The radula, on the other hand, is very similar in all three of the above mentioned taxa, but very different from that of the Patellinae.
Description-Shell rather small to moderately large, thin and fragile, typically elliptical, high-arched, with the apex strongly curved forward and downward, sometimes almost at the anterior end. The surface is smooth, or occasionally weakly radially ridged. Colour pale-olive to brownish, the apex coppery; inside silvery iridescent to reddish bronze. The species live mostly attached to large seaweeds, and range from southern Chile and Argentina to the Kerguelen Island.
1817 Nacella Schumacher, Essai d'un Noveau Systeme des Habitations, p. 179. Type, by subsequent designation, Gray, 1847: Patella mytilina Helbling, 1779."
(Powell, 1973: 191-192)