Visit any rocky shore around Australia and you'll find nerites. In other parts of the world, they are mostly snails of tropical waters but in Australia they occur all around the coast, including the temperate south.
Nerites (Nerita spp.) are interesting snails. In all species, the shell is rounded with a characteristic D-shaped opening. The exterior may be black, white or multi-coloured, smooth or ridged. The shell lip is always thickened with a row of tiny teeth. Those teeth aren't simply there for ornamentation. They seem to have a protective function.
Crabs love eating snails. But the tough shells present a problem. When disturbed, a snail withdraws into its shell, sealing the opening with an operculum as solid and tight-fitting as a castle door. Crabs get around this defence by grabbing hold of the shell lip and peeling it away. Of course, the thicker the shell, the more effort required to break it off. And that leads to an arms race between the nerites and their crab predators. The snails develop thicker shells and the crabs get beefier claws. And so it goes ...
This is the nerite that got away. Although a crab has given it a bit of scare, the snail has managed to not only escape (very slowly, presumably) but also repair its shell. Almost as good as new.
[Many thanks to Dark Orange for this spectacular photo of Nerita from Yeppoon, mid-east Queensland]