# Dicathais

Dicathais is a genus of predatory sea snails, marine gastropod molluscs in the family Muricidae, the rock snails. This genus is monotypic; the only species in it is Dicathais orbita, common name the white rock shell or cart-rut shell, found round the coasts of Australia and New Zealand.

## Description

The white rock shell is large with a strong shell usually about 6 centimetres (2.4 in) long, but occasionally growing to twice this size. It shows great variation in its shell sculpture across its range. In eastern Australia there are seven to nine deeply indented ribs in each whorl with clefts of a similar width between them, sculptured with further fine riblets. This gives a fluted edge to the lip and the grooves can also be seen on the interior surface of the shell. The central column or columella is smooth and the interior of the shell is white.
The western Australian form has a row of well marked nodules rather than ridges and an unindented lip while the southern Australian form has much less distinct ridges and a nearly smooth lip. At one time it was thought that these different forms were distinct species but it has now been established that the variations in sculpture are a function of different living conditions. Specimens gathered from New South Wales with deep ridges were kept in a still water aquarium for three years during which time the new shell growth was at first shallowly indented and later was smooth.
The colour of the shell is generally creamy white or grey, often with a thin yellowish line round the margin of the lip. Juveniles have thinner shells and are often brown.

## Distribution and habitat

This snail is plentiful in the intertidal zone and sublittoral zone on rocks and among seaweed around the coasts of Australia (New South Wales, Queensland, South Australia, Tasmania, Victoria, Western Australia), New Zealand, Kermadec Islands and Lord Howe Island.

## Biology

The white rock shell is a predator, cannibal and scavenger. It uses its radula to bore a hole in the shell of a barnacle or mollusc and then sucks out the contents. Prey species include the sea snails Lunella torquata, Cronia avellana and Pyrene bidentata, and the mussel Septifer bilocularis. As the tide rises, the white rock shell makes foraging expeditions up the beach, travelling far away from the sheltered niches to which it retreats at low tide. This is in contrast to the mulberry whelk Morula marginalba which shares the same habitat but remains in an area where there are both crevices in which to hide and prey species on which to feed.
The sexes are separate in the white rock shell. The eggs that develop in the female gonoduct are wrapped in a number of membranes to form an egg capsule. Several of these are joined together in an egg mass and attached to the substrate. Different precursors of the pigment Tyrian purple have been found in the male and female gonoducts and in the egg capsule. Early development of the larvae takes place inside the capsule which contains yolk granules on which they feed. When they have developed to the veliger stage, the larvae emerge from the capsule and become planktonic. After drifting with the currents for a while, these settle on the seabed, undergo metamorphosis and become juvenile snails.