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Onithochiton quercinus (Gould, 1846)

Description: Animal broadly ovate, girdle broad, shell usually eroded. Median valves with lateral and pleural area hardly separated except for indistinct dividing ridge; both areas sculptured with coarse, wavy longitudinal ridges. Girdle appears smooth without magnification, densely spiculose with magnification. Aesthetes distinct, appearing as black spots; on head valve arranged in 9 radiating rows, on median valves a single row along the junction of lateral and pleural areas. Valve colour variable; cream, fawn or greenish, streaked or blotched with red, brown, green or black; jugum often fawn with thick dark brown line. Girdle uniformly pinkish brown, or pinkish brown with white bands.

Size: Up to 80 mm in length.

Distribution: Endemic to Australia, occurring in two non-contiguous area; Torres Strait, Qld, southwards to Sydney, NSW, and south-western WA, from Esperance to Shark Bay.

Habitat: Lives on the upper surface of rocks on exposed rocky shores around low tide level and below, among algae and Pyura sea squirts. Common, sometimes abundant.

Remarks: The aesthetes, the light sensitive elements of chitons, are particularly obvious in this species. Each one has a small transparent bead externally (Fig. 3) which is connected to the nervous system by a nerve that passes through a pore in the valve. In this species the aesthetes occur in 9 radiating rows in the head valve, and as one row in the median valves, best seen in pale specimens (Fig. 2).

Otway (1994) studied the population ecology of this species. Individuals were estimated to live at least 6 years, growing rapidly for the first 3 years and reaching reproductive maturity at 2 years of age. They were found to spawn in early autumn, by discharging eggs and sperm into the sea. They feed by scraping algae from the substrate surface.

Fig. 1: Dee Why, NSW (DLB4988).

Fig. 2: Nambucca Heads, NSW (C.271050). Lines of aesthetes on valves. 

Fig. 3: Group of aesthetes and edge of girdle (Same specimen as Fig. 1)


Copyright Des Beechey 2019