About Dicathais orbita (Gmelin, 1791)
Dicathais orbita, commonly called the white rock shell, is a large predatory snail with a strong shell. The shell is typically around 6 centimetres (2.4 inches) long, and occasionally grows to twice this maximum size. This species shows extensive variation in shell sculpture across its geographic range. In populations from eastern Australia, each whorl of the shell has seven to nine deeply indented ribs, with gaps of similar width between the ribs, and additional fine riblets across the surface. This structure creates a fluted edge on the shell's lip, and the grooves from the sculpture are also visible on the shell's interior surface. The central columella of the shell is smooth, and the entire interior of the shell is white. The form found in Western Australia has a row of clearly defined nodules instead of ridges, and an unindented lip. The form found in southern Australia has much less prominent ridges and an almost completely smooth lip. These distinct forms were once classified as separate species, but it has now been confirmed that these variations in shell sculpture are caused by differing living conditions. In an experiment, specimens with deep ribs collected from New South Wales were kept in a still water aquarium for three years. Over this period, new shell growth was first shallowly indented, and later became completely smooth. The shell's base colour is usually creamy white or grey, and it often has a thin yellowish line running along the margin of the lip. Juvenile individuals have thinner shells, and are often brown in colour. This snail is abundant in intertidal and sublittoral zones, where it lives on rocks and among seaweed. It can be found around the coasts of Australia (including New South Wales, Queensland, South Australia, Tasmania, Victoria, and Western Australia), as well as New Zealand, the Kermadec Islands, and Lord Howe Island.
