Dicathais orbita (Gmelin, 1791)
Description: This species varies widely in sculpture. The eastern Australian shell (Figs. 1a, 2a) usually has strong spiral sculpture; 7 to 9 strong spiral ribs on body whorl, with intervening grooves of about the same width; ribs and grooves with secondary spiral riblets. Anterior fasciole about same size as spiral ribs. Columella smooth; outer lip internally reflects external sculpture. Colour off-white or grey, aperture cream, often pale yellow on the edge.
Size: Average length 60 mm, maximum 120 mm.
Distribution: In Australia, from Fraser Is., Qld, to North West Cape, WA, including Tasmania. Also New Zealand and Lord Howe Is.
Habitat: On rock platforms and rocky shores, in the surf zone, particularly in crevices at low tide and below. Abundant.
Synonym: Dicathais vector Thornley, 1952
Remarks: The southern Australian form (Figs. 1b, 2b) is generally much less coarsely sculptured than the eastern form (Figs. 1a, 2a) and has been known as Dicathais textilosa (Lamarck, 1822). The figured specimen of this southern form (Figs. 1b, 2b) from Tasmania has about 25 spiral ribs, and the interior of the lip has about 20 lirae. West Australian specimens (Figs. 1c, 2c) are often nodulose, and have been called D. aegrota (Reeve 1846).
Phillips, Campbell and Wilson (1973) examined a large number of specimens of this species from central Queensland to central Western Australia. They concluded that a single variable species exists, with variation in shell shape and sculpture being a function of water temperature, diet, substrate and degree of exposure to wave action. They kept specimens of the NSW orbita form in an aquarium for three years, and found that the new shell growth was no longer coarsely ribbed, but first striated and then smooth.
At Rottnest Island, WA, D. orbita is a pedator on another large gastropod, Ninella torquata, normally attaching by drilling at the edge of the operculum of its victim (Taylor & Glover, 1999).
Figs. 1a, 2a: Newport, Sydney, NSW (DLB2600)
Figs. 1b, 2b: Tasmania (DLB1262)
Figs. 1c, 2c: West Wallabi Is., Houtman Abrolhos, WA (C.69353)
Copyright Des Beechey 2000