Tonicia swainsoni, (SOWERBY, 1832)

TONICIA SWAINSONI (SOWERBY, 1832) View in CoL

( FIG. 4 View Figure 4 )

Chiton swainsoni Sowerby View in CoL in Broderip & Sowerby, 1832: 27.

Tonicia swainsoni View in CoL ; Leloup, 1956: 82, figs 50–52 (bibliography). – Leloup, 1973: 19. – Kaas et al., 2006: 299, fig. 121, map 55.

Type material repository: British Museum of Natural History ( BMNH) .

Type locality: Iquique , Chile .

Description of T. swainsoni

Body medium sized, ≤ 35 mm in length ( Table 2), oval, moderately elevated (dorsal elevation 0.29–0.31), subcarinate, and valves not beaked. Tegmentum pinkish buff to pale brown; typically ornamented with numerous reddish or reddish-brown lines angularly concentric to the apices. Head valve semi-circular; tegmentum sculptured with numerous radiating ribs in the interspaces. Intermediate valves broadly rectangular, with a ratio of the width of valve V to its length of 3.4–3.9. Posterior margin with pointed apex. Central area with numerous longitudinal ribs, approximately eight near both sides of the jugum, sharp, and another 20 are obscure. Lateral area with four radiating ribs. Tail valve slightly narrower than the head valve; mucro antermedian, not raised. Ratio of the length of the postmucronal area to the length of the antemucronal area is 2.5–3.0. Antemucronal area sculptured with longitudinal ribs like that of the central areas. Postmucronal area with radiating ribs like the head valve. Articulamentum white and sometimes with small reddish spots in the apical area. Sutural laminae very wide and short; connected by a short, dentate jugal plate. Slit formula 8/1/10– 12. Slits finely indicated. Teeth very short; sharply denticulated. Girdle narrow; ratio of the width of valve V to the width of the girdle is 3.0. Dorsal spicules of different sizes ≤ 24 µm long. Marginal spicules round-topped, ≤ 43 µm long. Ventral side of the girdle with radiating rows of rectangular scales. Radula of a specimen with body length of 20 mm is 8.0 mm long; 42 transverse rows of mature teeth. Central tooth narrows with long blade; major lateral tooth has a unicuspid head. Gills spanning from valve VI to valve VIII; 33 on each side of specimens with a body length of 28 mm.

TAXONOMIC KEY

Diagnostic key to aid the identification of species of Tonicia in the vicinity of the southeastern Pacific (Peruvian and Magellan provinces). 1 Girdle connects between valves. Cusp of major lateral teeth of radula is bidentate ......... Tonicia disjuncta

1′ Girdle does not connect between valves. Cusp of major lateral teeth of radula is unidentate ......................2

2 Colour of sutural laminae is brown. Tegmentum is smooth, without longitudinal grooves on both sides of the jugum ................................................................................................................................. Tonicia lebruni

2′ Colour of sutural laminae is white. Tegmentum is granulated or rarely smooth, with grooves near the jugal area ................................................................................................................................................................. 3

3 The head valve, lateral and postmucronal areas have radiating ribs. The ratio of the width of valve V to its length is 3.4–3.9. The ratio of the length of the postmucronal area to the length of the antemucronal area of the tail valve is 2.5–3.0 .................................................................................................. Tonicia swainsoni

3′ Radiating ribs on the head valve, lateral and postmucronal areas are absent. The ratio of the width of valve V to its length is <3.0. The ratio of the length of the postmucronal area to the length of the antemucronal area of the tail valve is ≤ 1.9 .......................................................................................................................... 4

4 The tegmentum is smooth, without granules or pustules except several narrow, short, weak longitudinal grooves on both sides of the jugum ................................................................................. Tonicia calbucensis

4′ The tegmentum has grooves, granules or pustules ........................................................................................ 5

5 All valves or part of them are coloured with numerous undulating, concentric lines. The ratio of the length of the postmucronal area to the length of the antemucronal area of the tail valve is 1.9. Both sides of the jugum have few short, narrow, bent grooves .................................................................. Tonicia fremblyana

5′ The colour of the valves is variable. The ratio of the length of the postmucronal area to the length of the antemucronal area of the tail valve is ≤ 1.8. Both sides of the jugum have ten or more narrow, longitudinal grooves .................................................................................................................................. Tonicia chilensis

Remarks

Tonicia swansoni differs from other species of the genus by having radiating ribs in the head valve, in the lateral areas of intermediate valves, and in the postmucronal area of the tail valve. Also, it has a short antemucronal area and short intermediate valves.

Distribution: Tonicia swainsoni is found from Callao ( Peru, ~ 12°S) to Arica ( Chile, ~ 18°S) in shallow water.