Mantled keyhole limpet. Below are a few more, with different colours and patterns.
These two photos by Diane May
Photo by Helene Zsilavecz
Photo by Guido Zsilavecz
Detail of siphon. Photo by Guido Zsilavecz
Shell of the saddle-shaped keyhole limpet, Dendrofissurella scutellum Photo by Guido Zsilavecz
Keyhole limpet shells are often found on our beaches. All have a distinct hole at the top of the shell which many people mistake as being faults in the shell, often assuming that the shell was abraded by rubbing over rocks, or maybe by a predator boring through it. The last image shows a typical keyhole limpet shell (not that of the mantled keyhole though), and if one looks carefully at the shell one can see that the hole is regularly shaped, that is, laterally symmetrical - not necessarily something one would expect if it were produced though any other means. The hole is meant to be there - through it a siphon protrudes, as shown in the fourth image. This is the anus, and its location ensures that any faeces are kept far away from the gills, which are located under the shell.
The shell of keyhole limpets is often substantially smaller than that the animal - in the case of the mantled keyhole limpet the shell reaches a mere 35 mm with the animal reaching 80 mm - more than double the length! The shell itself is covered by the mantle, and as such is not visible on any of the animals shown. As can be seen, there is a certain amount of colour and pattern variability. These large limpets are not common. They can be found singly on most of Cape Town's reefs, and are often well camouflaged and not particularly conspicuous.
For more details on this species, see "A field guide to the Marine Animals of the Cape Peninsula" available directly from SURG or better dive and book stores in Cape Town.
A field guide to the Marine Animals of the Cape Peninsula, G Jones, 2008
Two Oceans, A guide to the marine life of Southern Africa, GM Branch, CL Griffiths, ML Branch and LE Beckley, 1994, Revised Edition 2010