Tonicia atrata (G.B. Sowerby II, 1840)

Araya, Juan Francisco & Araya, Marta Esther, 2015, The shallow-water chitons (Mollusca, Polyplacophora) of Caldera, Region of Atacama, northern Chile, Zoosystematics and Evolution 91 (1), pp. 45-58 : 45

Taxon classification Animalia Chitonida Chitonidae

Tonicia atrata (G.B. Sowerby II, 1840) Plate 2, Fig. 3; Table 2

Chiton atratus G.B. Sowerby II, 1840: 294. Tonicia atrata Aldea & Valdovinos, 2005: 368, Table 1, fig. 5I; Schwabe et al. 2006: 12, figs 9, 18; Gordillo and Schwabe 2009: 267, fig. 2D. A more detailed synonymy can be found in Kaas and Van Belle (1998: 25).

Description.

Animal of medium to large size, reaching 51 mm in examined specimens. Shell elongate-oval, moderately elevated, subcarinated dorsum. Color reddish brown to blackish purple. Head valve semicircular, with straight posterior valve margin unnotched in middle; intermediate valves broadly rectangular, with distinct protruding apex and concave posterior valve margins. Lateral area hardly elevated, but clearly indicated by faint depression in front of diagonal ridge. Jugal area weakly developed, but showing partly a more or less distinct keel, especially in the more posterior intermediate valves. Tail valve with centrally situated, forward-directed, and slightly elevated mucro, straight antemucronal area and rather steep, straight postmucronal slope. Jugal area of tail valve extending towards anterior. Tegmentum without sculpture, except for growth marks and faint radial striations on terminal valves and lateral areas. Perinotum wide and fleshy (After Schwabe et al. 2006).

Material examined.

Specimens collected in a single location; Playa Mansa, in submerged rocky platform (Table 1), MPCCL 011928A, 1 specimen.

Distribution.

According to Reid and Osorio (2000), this species distributes in Chile between 40°S and 54°S, at Tierra del Fuego and around the Falkland Islands. The species ranges in depth from the low eulittoral to a depth of 36 m ( Schwabe et al. 2006). The record presented here (at 27°S) is now the northernmost record for this species.

Remarks.

Two specimens of this species were found in a single location; among a community of Tonicia chilensis , from which it distinguishes in attaining larger sizes and in having a darker body and almost smooth valves with minute granulation at the sides. It is interesting to note also that the valves of the examined specimens were widely separated, almost as in Tonicia disjuncta (Frembly, 1827). The presence of this species extends considerably the northernmost record of this species in about 1040 km (from 40°S to 27°S).