Ischnochiton (Haploplax) pusio (Sowerby in Broderip & Sowerby, 1832)

(Figure 6)

Chiton pusio Sowerby in Broderip & Sowerby, 1832: 105.

Detailed bibliography and synonymy in Kaas & Van Belle (1994: 67).

Size: Maximum length 21 mm (Kaas & Van Belle 1994). Body outline: Oval, rather elevated, dorsum subcarinated. Color: Usually olive­brown to blackish brown, with brighter spots. Valves: Rather solid; appear smooth (microgranulated) except for commarginal growth lines on terminal valves and lateral areas of intermediate valves; head valve less than semicircular, posteriorly widely V­shaped, slightly notched in middle; intermediate valves rectangular, posteriorly faintly convex on both sides of slightly protruding apex, lateral areas hardly raised; tail valve with mucro in anterior third, postmucronal slope nearly straight and rather steep. Articulamentum: Bluish white to greenish blue, sometimes with darker streaks in central part, apophyses subtrapezoidal and rather short, slit formula: 11–14 / 1 / 11, slit rays present in all valves, teeth sharp and rather smooth at outside. Perinotum: Dorsally covered with smooth, solid, imbricated scales obtusely pointed; ventral side with radial rows of opaque rectangular to elongate scales. Ctenidia (ZSM Moll 20040966; 12 mm body length): 25 holobranchial ctenidia on each side of foot, posterior ctenidia longer than anterior ones. Radula (12 mm specimen): about 3.2 mm in length; radula cartilage 1.6 mm in length; 87 teeth rows, 72 showing mineralized teeth. Central tooth slender, extended at its base, with simple roundish blade; first lateral tooth twice length of central one, leaf­shaped, second lateral tooth with bicuspid head, inner denticle longer than outer.

Material: Four specimens were collected at station 1, in a depth between 5 and 30 m and two additional specimens at station 4, between 5 and 20 m (see Table 1).

Distribution: Ischnochiton pusio occurs from Tumbes, Perú, southwards to the Atlantic coast of Tierra del Fuego, Argentina. It is also known from offshore of the Chilean Juan Fernández Archipelago (Kaas & Van Belle 1994). According to Reid & Osorio (2000), I. pusio inhabits the underside of stones and other hard substrata from the intertidal zone down to 90 m, with a depth preference around 5–30 m in fjord areas. This is supported by our study and we speculate that Ischnochiton pusio is less tolerant of salinity fluctuations and therefore lives below the halocline.