Tonicia chilensis

Tonicia chilensis (Frembly, 1827).

Complete synonymy in Ibáñez et al., 2019.

Description of Tonicia chilensis northern ecotype (Figs 11A, B; 12–14)

Body large sized, up to 81 mm in length (Table 2), oval, minimally elevated (dorsal elevation 0.25), rounded or slightly subcarinated. Side slopes straight, valves are slightly beaked in young specimens. Tegmentum streaked with various combinations of dark brown, yellow, reddish brown, black and red in small specimens with body lengths up to 20–25 mm. Large individuals uniformly black. Head valve semicircular, hind margin slightly concave. Tegmentum sculptured with low pustules arranged without a definite pattern. Growth lines noticeable. Intermediate valves broadly rectangular, front margin slightly concave in jugum and convex in pleural parts. Hind margin weakly concave on both sides of the prominent pointed apex. Lateral area slightly raised and sculptured like the head valve. Central area with 10 or more longitudinal ribs in the central part of the valve. Several low pustules run diagonally between the central and lateral area of small specimens. Tail valve is smaller than the head valve. The front margin is concave. Antemedian portion of mucro not raised, antemucronal area sculptured like the central area and postmucronal area sculptured like the head valve. Articulamentum white with lighter or dark reddish-brown parts in the apical area. Apophyses are wide and rather long. Outer half of the apophyses and teeth sharply pectinated. Sinus has a short, dentate (less 15 denticles) jugal plate. Slit formula is 8/1/10. Girdle rather wide and spotted in young live specimens, black in adult specimens. After fixation, girdle becomes dirty yellow in color and is dorsally covered with small, flattened spicules up to 23 x 18 µm. Marginal spicules twice as large as the dorsal spicules. Ventral side covered with radiating rows of subquadragular scales. The radula of a chiton with body length of 20.6 mm is 7.1 mm long and has 41 transverse rows of mature teeth. The central tooth is long with a curved blade. The major lateral tooth is rounded with a unidentate cusp. Fifty-three gills per side, extending from valve II to valve VIII in the same specimen.

Description of Tonicia chilensis southern ecotype (Figs 11 C, D; 15–16)

Body large sized, up to 77 mm in length, elongated, oval (Table 2) with solid valves moderately elevated (dorsal elevation 0.30–0.35), subcarinated and not beaked.Tegmentum color variable, from dark green to blackish, often with pale greyish blotches or streaks; occasionally with some valves orange, and black in others. Head valve semicircular, hind margin widely V-shaped. Tegmentum has rare, very small granules, and most of these are arranged near the hind margin. Growth lines evident. Intermediate valves solid and broadly rectangular.Anterior margin almost straight in the central part, distinctly slants towards the rounded side margins. Hind margin concave at both sides of the slightly protruding apex. Lateral area not raised and sculptured like the head valve. Ocelli numerous and arranged in a random manner. Central area almost smooth or vaguely granulose. When present, small granules sometimes coalesce in short wrinkles in front of diagonal lines. Dorsal ridge very low and narrow, with a smooth keel in the middle and 1–5 longitudinal grooves on both sides. Tail valve as wide as the head valve or slightly narrower, front margin convex, mucro elevated subcentral or antemedian, elevated, antemucronal area sculptured like the central areas, postmucronal area sculptured like the head valve. Articulamentum white and blotched with brown in the central part of the valves. Apophyses large, rather wide, rounded, and connected across the narrow jugal sinus by a long dentate plate (13–24 denticles). Slit formula 7–9/1/9–13. Teeth short and deeply pectinated on the upper side and outer edge. Very wide girdle (ratio of the width of valve V to the width of the girdle is 2.1–2.7), nude to the naked eye, and velveteen. Dorsal side of the girdle black or dark green in live specimens and dirty yellow after fixation. Girdle covered with numerous short bristles and small light, striated, flattened spicules up to 42 μm in length. Marginal edge with short bristles and rare spicules that are longer than the dorsal spicules.Ventral side of the girdle with radiating rows of rectangular scales up to 40 μm in length. Major lateral tooth of the radula has a unicuspid roundish head. Gills spanning from valve II to valve VIII, 38 on each side of a specimens with body length of 44 mm.

Adults of the northern ecotype of Tonicia chilensis inhabit the intertidal zone in areas with a fairly strong surf, while its juveniles live in the sublittoral, among red calcareous algae and the color of the tegmentum of the juveniles is light with redish-brown spots that hides them against the background of red algae. Apparently, as the chitons of this ecotype grow older, they move to the intertidal zone, where the tegmentum and the strongly expanded girdle become black. The low shell and the expanded belt of large chitons help to retain them during strong surf. Unlike the northern ecotype, all individuals of the southern ecotype of T. chilensis live at depths of 0–36 m [Schwabe, 2009] in bays and fjords with a calmer wave action. There is no age segregation in their settlements.

The southern ecotype of Tonicia chilensis differs from the northern ecotype of this species by having 1–5 narrow, longitudinal grooves in both sides of the jugum (vs. 10 or more grooves in both sides of the jugum in the northern ecotypes, ratio of the width of valve V to its length is 1.8 or more (vs. usually less 1.8 in the northern ecotypes). Moreover the girdle of the southern ecotype of Tonicia chilensis have longer and more numerous bristles.