by Jenny Edwards
What’s black and white and found on mid intertidal rocks? Zebras. Well Zebra Top Shells (Austrocochlea porcata) to be exact.
These small shellfish with parallel oblique bands of black and white are often unscientifically lumped with other unrelated small sea snails and all called periwinkles. The Zebra Top Shell tolerates exposed mid-tide locations but is often found in moist positions. It is also common in our estuaries.
A closely related relative of about the same size (to 25mm diameter) that is also found on our shores is the Wavy, Checkered or Speckled Top Shell (Austrocochlea concamerata). Like so many other marine creatures it has no one common name that everyone uses.
The Speckled Top Shell has a black shell with yellow-white spots regularly spaced on ridges that spiral down from the top, although the pattern varies and can look more like narrow whitish bands. It has a much thicker shell than its cousin and the lining is white, like porcelain.
During the day the Speckled Top Shell prefers to cluster in groups in sheltered positions under boulders and is usually less common than the Zebra Top Shell. Both have a door (operculum) to lock in moisture when the tide is out. The doors are made of chitin, a horn-like substance.
Top Shells have separate sexes which simultaneously expel sperm and eggs into the sea water where fertilization occurs. The larvae which develop drift in the plankton for about a week before settling as small shelled animals if they are lucky.
Like the many other molluscan grazers of the intertidal rocks, Top Shells rasp off the microscopic plants with their tongues. They are no ordinary tongues. They have a base of cartilage covered on top by a ribbon, the radula, that is coated in innumerable, backward-curving sharp teeth. The teeth are made of chitin that is hardened with oxides of iron so they can be much harder than some rocks. They still wear out but are constantly replaced by newly formed ones at the back. No wonder the algae don’t get much of a chance to grow where periwinkles are numerous.