Plaxiphora aurata ( Spalowsky, 1795 )

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 : 18-20

Plaxiphora aurata ( Spalowsky, 1795)

(Figures 13, 19)

Chiton auratus Spalowsky, 1795: 88 , figs. 6a–b.

Detailed bibliography and synonymy in Kaas & Van Belle (1994: 266).

Size: Maximum length 110 mm (ZSM Moll 20060497). Body outline: Oval, moderately elevated, dorsally subcarinated. Color: Varying from greenish to blackish brown, with longitudinal streaks on most specimens, commarginal marks or other different coloration of brighter tones. Jugal area whitish in most species. Valves: Eroded or encrusted in most specimens; tegmentum rather smooth with fine streaks on younger animals. Head valve less than semicircular, with unnotched, widely V­shaped posterior margin. In subadult specimens more or less distinct radial ribs visible. Intermediate valves rectangular with slightly protruding apex and concave posterior margin. Lateral areas, although not elevated, clearly indicated by distinct diagonal ridge and smaller posterior rib. Tail valve rather depressed, oval with nearly terminal mucro, with vague posterior sinus in a few specimens. Growth marks arranged commarginally in all valves. Articulamentum: Strong, bluish green, forms rounded to trapezoid apophyses, slit formula: 8 / 1 / 0 (callus), slit rays weakly developed, teeth sharp and long. Perinotum: Fleshy, dorsally covered with randomly (but in all sutures) arranged tufts of long (up to 8 mm) corneous hairs. Otherwise densely beset with minute, reddish­brown, blunt­tipped calcareous spicules. Ventrally beset with minute straight spicules, longitudinally striated in upper half. Ctenidia (ZSM Moll 20034100, approximately 40 mm body length) 42 ctenidia on each side of foot, arranged holobranchially and largest ctenidia situated posteriorly. Radula (same specimen): 10.9 mm long, with 50 teeth rows, 43 already mineralized. Radula cartilage 5.4 mm in length. Central tooth rather short, rectangular, frontally keeled, and with deeply notched blade. First lateral tooth extended at its base; head very narrow, with an inward­directed simple blade. Second lateral tooth with squarish head with three large denticles of equal length, but central one wider than outer ones. First seven anterior rows of examined specimen have simple blades, without denticles, just sharp cutting edge. Few heads with smaller fourth denticle on outside.

Material: Eight specimens were collected at station 7 between 0–10 m (see Table 1 ).

Scale bars: 20 = 5 mm; 21, 22, 24–26 = 2 mm; 23 = 1 mm.

Distribution: This circum­subantarctic distributed species is reported from Valparaíso southwards to the Straits of Magellan, the Falklands, the Tristan da Cunha and Gough Islands, and from the Neozelanic Subantarctic Islands ( Kaas & Van Belle 1994). Plaxiphora aurata is known to inhabit rocky shores where it lives on and under rocks, accepting sheltered and sometimes muddy situations ( Reid & Osorio 2000, this study) Due to its dorsal coverage of several sessile marine organisms (most often algae) it is well camouflaged. It has also been recorded from holdfasts of Macrocystis . Bathymetrically it occurs from 0–12 m ( Reid & Osorio 2000).

Remarks: Reid & Osorio (2000) mentioned a certain tolerance regarding decreasing salinity (salinity ranging from 15–31‰) for this species.

The sample (ZSM Moll 20034106) contains two specimens; the smaller one has a polychaete deeply buried in the specimen’s cuticle between the valves.