Cornwallis Beach, Manukau Harbour, Auckland, New Zealand, 70.2mm., 2000/xii.
The « Common Cartrut » or « Cartwheel Purple » is a large rock shell ranging from Australia (Queensland to Western Australia, including Lord Howe Island and Tasmania) to New Zealand. It is a common to abundant carnivorous gastropod across the range and inhabits rock surfaces of intertidal to very shallow water less than -10m deep. Mainly a predator preying on other molluscs such as Lunella torquata (Gmelin, 1791) and Septifer bilocularis (Linnaeus, 1758) but may also scavenge; it forages up the shore during high tide and retreats to rock crevices during low tide. It is currently the only species in genus Dicathais but is extremely variable in sculpture / shape and different forms were once considered to be separate species. For example specimens from southern Australia is generally reduced in sculpture and has been named Dicathais textilosa (Lamarck, 1822); nodulous specimens from western Australia lacking strong radial ribs were given the name D. aegrota (Reeve 1846). The variation is known to be attributable to a mixed effect of temperature, diet, substrate, and exposure to wave action; a moderately strongly sculptured specimen is depicted here. The thick, heavy shell is usually white to grey but juveniles are often brown with thinner shells. Typical shell length around 60mm., extremely large specimens are known to reach even 120mm.