Leptochiton medinae ( Plate, 1899 )

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 : 6

Leptochiton medinae ( Plate, 1899)

( Figures 2–5 )

Lepidopleurus medinae Plate, 1899: 82 –89, pl. 5, figs. 204–206.

Detailed bibliography and synonymy in Kaas & Van Belle (1985a: 80).

Size: Maximum length about 10 mm ( Plate 1899). Body outline: Elongate oval, dorsum round. Color: Cream, normally covered by a darker properiostracum. Valves: Rather thin; head valve semicircular, posterior valve margin widely V­shaped, slightly notched in middle, sculptured by approximately 80 radial ribs of fine roundish granules; intermediate valves rectangular, posterior margin straight, apex not present, lateral areas slightly elevated, with about 15 radial rows of granules, central area with about 70 longitudinally arranged, closely set, roundish granules; tail valve semicircular with mucro centrally, postmucronal slope steep and straight, postmucronal area sculptured like head valve, antemucronal area similar to central area of intermediate valves; growth marks present on terminal valves and lateral areas. Articulamentum: Thin and white. As diagnostic for genus insertion plates missing, apophyses well developed. Perinotum: Dorsally covered with flat, finely ribbed conical scales, up to 6 ribs, not imbricated. Ventrally with elongate scales arranged in radial rows. Ctenidia: Restricted to posterior part of mantle cavity, in contact with anal papillae. Radula (ZSM Moll 20050428; 7 mm body length): radula length 4.3 mm. Radula cartilage length 1.4 mm. 67 teeth rows, 51 of them with mineralized teeth. Central tooth slender and very short, centrally constricted; blade simple and directed inwards. First lateral tooth slightly larger, also with simple inward­directed blade. Second lateral tooth with elongate sharply pointed, bidentate head; outer denticle smaller than inner.

Material: A single specimen ( ZSM Moll 20050341) was found at station 4, at a depth between 20–25 m on a steep wall under a stone (see Table 1 ).

Distribution: The species occurs in the Magellanic Region south of 42°S along both sides of South America down to the Straits of Magellan ( Kaas & Van Belle 1985a). While also reported from the intertidal zone, it generally seems to prefer depths around 15–30 m (fide Reid & Osorio 2000; this report). Maximum depth is recorded from 250–300 m ( Leloup 1956). Leptochiton medinae inhabits a variety of hard substrata, such as holdfasts of Macrocystis , cobbles encrusted with calcareous red algae, or pebbles and shells on silt. Leptochiton medinae was found in areas with oceanic salinity levels, and also in hyposalinic locations with only 15–22‰, thus it seems to be tolerant of wide salinity variations. The single record of the species in Comau Fjord does not necessarily mean that it is rare. It is more likely that the species was overlooked, because of its small size and its dark properiostracum, which may provide it with camouflage.

Remarks: The species could be easily mistaken for Ischnochiton stramineus (Sowerby in Broderip & Sowerby, 1832). Compared to the latter species, L. medinae lacks insertion plates in the valves, has a much narrower perinotum, the dorsal perinotum scales are of a different form, and the ctenidia differ in their posterior position and in being in contact with the anal papillae.

Figures

FIGURES 2 – 5. Leptochiton medinae (Plate, 1899). 2. Dorsal view of the damaged specimen from station 3 (ZSM Moll 20050341). 3. Ventral view of the same specimen as Figure 2. 4. Dorsal view of a complete specimen from Isla Cailin, Chile (ZSM Moll 20000862). 5. Ventral view of same specimen as Figure 4. 6. dorsal view of Ischnochiton (Haploplax) pusio (Sowerby in Broderip & Sowerby, 1832) from station 4 (ZSM Moll 20040967). 7. Underwater photograph of Chiton magnificus Deshayes, 1827 in situ (ZSM Moll 20034101) from station 7, illustrated specimen measures 26 mm in length. Scale bars: 2 – 6 = 1 mm.