Tonicia atrata ( Sowerby, 1840 )

Schwabe, Enrico, Försterra, Günter, Häussermann, Verena, Melzer, Roland R. & Schrödl, Michael, 2006, Chitons (Mollusca: Polyplacophora) from the southern Chilean Comau Fjord, with reinstatement of Tonicia calbucensis Plate, 1897, Zootaxa 1341, pp. 1-27 : 12-14

Tonicia atrata ( Sowerby, 1840)

(Figures 9, 18)

Chiton atratus Sowerby, 1840: 294 .

Detailed synonymy in Kaas & Van Belle (1998: 25).

Size: Maximum length 85 mm ( Reid & Osorio 2000). Body outline: Elongate oval, moderately high elevated, subcarinated dorsum. Color: Variable but generally greyish, or blackish purple. Valves: 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, forward­directed, 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. Radial rows of ocelli situated on terminal valves, as well as on anterior region of lateral zones. Articulamentum: Solid, thickened, white with a hue of brown or red under mucro or apical regions. Apophyses rather long, rectangular (in intermediate valves), to trapezoidal in tail valve. Apophyses connected by well­developed, strongly serrated jugal plate. Insertion plates long, with pectinated outside. Slit formula: 8 / 1 / 11 –12. Slits deep, teeth with rather sharp edges. Slit rays present in all valves. Perinotum: Wide and fleshy, appears naked. Dorsally with small elongate oval calcareous spicules showing fine striation at distal end, minute smooth and slender spicules, and corneous processes occurring randomly. Ventrally short rectangular to squarish scales, arranged in radial rows; theses scales, slightly bent and showing distinct radial sculpture. Marginal short fringe of minute scale–like spicules. Ctenidia (ZSM Moll 20040973, specimen measuring 15.4 x 8.6 mm): 39 ctenidia on each side of foot, arranged holobranchially and adanally. Radula (same specimen): 5.6 mm long, 2.4 mm of which taken up by radula cartilages. 62 teeth rows, 49 of them mineralized. Extremely slender central tooth showing forward­bent, obtuse, simple cutting edge. First lateral tooth noticeably longer than central one, with deeply notched distal end. Base of first lateral moderately extended, with wing­like extension covering lowermost part of central tooth. Second lateral tooth long with sharply keeled shaft and having discoidal unicuspid head with sharp edge. Major uncinal tooth spoon­like.

Material: Tonicia atrata is commonly found within the fjord at virtually all stations (see Table 1 ). According to Reid and Osorio (2000), the species ranges bathymetrically from the low eulittoral to a depth of 10 m. The specimens reported here were found between 1.5–36 m depth.

Distribution: Tonicia atrata occurs between 40° and 54°S along the Chilean coast, at Tierra del Fuego, and around the Falkland Islands ( Reid and Osorio 2000, and material from ZSM).

Remarks: Plate (1901) described Tonicia atrata as hosting the simple unicellular parasite Chitonicum simplex Plate, 1898. Unfortunately, the systematic position of this parasite is unknown and no further work has been done to evaluate Plate’s findings.