Nacella MYTILINA

NACELLA MYTILINA (HELBLING, 1779)

(FIG. 6)

Patella mytilina Helbling, 1779: 104, pl. 1, figs 5, 6; Gmelin, 1791: 3698; Strebel, 1907: 113, pl. 3, fig. 44; pl. 4, figs 49, 51, 55, 57A, 59.

Patella mytiliformis Lightfoot, 1786: 42.

Patella conchacea Gmelin, 1791: 3708.

Nacella mytiloides Schumacher, 1817: 179.

Patella cymbularia Lamarck, 1819: 335; Mermod, 1950: 700, fig. 11.

Patella cymbuloides Lesson, 1830: 422.

Patella hyalina Philippi, 1845: 59.

Patella vitrea Philippi, 1845: 60.

Patella cymbium Philippi, 1845: 60.

Nacella mytilina – Dall, 1870: 274; Rochebrune & Mabille, 1889: 97; Pilsbry, 1891: 115, pl. 50, figs 32–39; Pelseneer, 1903: 6; Smith, 1905: 336; Thiele, 1912: 234; Melvill & Standen, 1914: 114; Carcelles, 1950: Vol. 2: 52; Powell, 1951: 80; Dell, 1964: 273; Powell, 1973, 192: Vol. 3, 191, pl. 173, figs 1, 2; Castellanos & Landoni 1988: pl. IV, figs 9, 10; Ríos & Gerdes 1997: 51; Mutshke et al. 1998: 8; Ríos et al., 2007: 100; de Aranzamendi et al., 2009: 1; González-Wevar et al., 2010: 116; 2011a: 1937; 2016a: 1; 2017: 863; Rosenfeld et al., 2015: 55; 2016: 76.

Nacella compressa Rochebrune & Mabille, 1889: 98, pl. 5, fig. 9.

Nacella falklandica Preston, 1913: 221, pl. 4, fig. 6.

Nacella (Nacella) mytilina – Wenz, 1938: 217, fig. 400; Carcelles & Williamson, 1951: 257; Carcelles, 1953: 164, pl. 1, fig. 11; Ramírez, 1981: 51; Valdovinos & Rüth, 2005: 499, fig. 2A.

Nacella (Patinigera) mytilina [sic.] – Linse, 1999: 400. Nacella mytelina [sic.] Adami & Gordillo, 1999: 186.

Studied material: Tamar Island, Strait of Magellan (52°53’48.04’’ S, 73°47’15.03’’ W) N = 20; Duntze Sound, Strait of Magellan (54°19’22.32’’ S, 73°48’12.11’’ W) N = 20; London Island, Pacific Patagonia (54°40’00.30’’ S, 72°03’58.67’’ W) N = 24; Carlos III Island, Strait of Magellan (53°38’55.41’’ S, 72°15’31.04’’ W) N = 50; Port Famine, Strait of Magellan (53°36’34.07’’ S, 70°55’53.40’’ W) N = 50; Carrera Point, Strait of Magellan (53°35’10.92’’ S, 70°55’24.14’’ W) N = 30; Otway Sound, Strait of Magellan (52°56’13.10’’ S, 71°11’39.07’’ W) N = 25; Chabunco, Strait of Magellan (52°59’14.66’’ S, 70°48’44.43’’ W) N = 30; Laredo Bay, Strait of Magellan (52°56’59.14’’ S, 70°48’03.96’’ W) N = 30; Possession Bay, Strait of Magellan (52°13’55.50’’ S, 69°17’50.71’’ W) N = 30; Santa María Bay, Tierra del Fuego (53°19’29.86’’ S, 70°22’34.30’’ W) N = 40; Caleta River, Tierra del Fuego (53°51’45.32’’ S, 7 0°1 9’5 4.1 1’’ W) N = 3 0; Virginia Bay, Beagle Channel (54°54’16.95’’ S, 67°56’05.35’’ W) N = 30; Puerto Williams, Beagle Channel (54°56’04.95’’ S, 67°36’48.33’’ W) N = 35; Hookers Point, Falkland / Malvinas Islands (51°42’09.60’’ S, 57°46’07.49’’ W) N = 40.

Shell: The shell shape and sculpture are relatively variable in the analysed individuals across the species’ distribution (Fig. 6A–C). The species exhibits a conical morphology, oblique with a very thin and translucent shell (Fig. 6A–F). The anterior part of the animal is laterally compressed (Fig. 6D–F). It has a medium shell size (maximum length 90 mm) and a medium profile. The apex is situated at the anterior 10% of the shell’s length, anteriorly directed and curved toward the border. The anterior slope of the shell is concave, while the posterior and lateral ones are convex (Fig. 6A–F). The aperture of the shell is oval to oblong. The surface of the shell has a sculpture of a few radial ribs, more obvious in the anterior zone, and concentric growth lines, which increase in thickness towards the shell margin. The margin of the shell is relatively even. The external coloration varies (pearly cream, yellow and light brown with darker tones towards the margin with a brown/red apex). The internal part of the shell is simple with white nacreous coloration.

External anatomy: The ventral area of the foot varies from dark grey to black and the epipodial fringe is highly recognizable (Fig. 6G). The mantle fold is narrow and cream-coloured. The mantle tentacles are in alternated series of three white shorter ones and a white longer one (Fig. 6G). Nevertheless, some individuals also exhibit a configuration of three shorter (white/black/white) and a black longer one. The cephalic tentacles are dorsally pigmented with a black line (Fig. 6G)

Radula: See Valdovinos & Rüth (2005) (Fig. 6H).

Distribution: Magellanic province. Pacific Patagonia: from Guarello Island (50°S) to Cape Horn (56°S). Atlantic Patagonia: Tierra del Fuego to Puerto Deseado (47°S). Falkland/ Malvinas Islands (Fig. 6I).

Habitat: Nacella mytilina is regularly found on macroalgae (Macrocystis, Gigartina and Lessonia; Rosenfeld et al., 2015) but in some localities individuals can also be found on inter- and subtidal rocks.

Comments: The previously recognized distribution of N. mytilina included the Magellanic province and the Kerguelen Archipelago (Powell, 1955, 1957, 1973; Cantera & Arnaud, 1985; Troncoso et al., 2001), as well as Antarctica (Castellanos & Landoni, 1988). Cantera & Arnaud (1985) treated N. kerguelenensis from the Kerguelen Islands as a junior synonym of N. mytilina. However, recent multi-locus molecular analyses have demonstrated that kelp-associated N. mytilina -like individuals from the Kerguelen Islands represent a particular morphotype of N. kerguelenensis, which is genetically distinct from N. mytilina (González-Wevar et al., 2017). At the same time, no confirmed record exists for N. mytilina in Antarctica or in peri-Antarctic islands, such as South Shetlands Islands or South Orkneys. Accordingly, the distribution of N. mytilina is restricted to southern South America and the Falkland/ Malvinas Islands. As recorded for other Patagonian marine invertebrates (Leese et al., 2008; González-Wevar et al., 2012a, 2018), N. mytilina exhibits population-level genetic differentiation between Pacific Patagonia and the Falkland/ Malvinas Islands (González-Wevar et al., 2016a).