Rostanga sp. from Vulcan Rock.
Close-up of the spots, showing the white spots within the darker area.
Three-spot nudibranch Aldisa trimaculata with similar dark blotch with pale spots within it.
All photos Guido Zsilavecz
This rare dorid nudibranch was seen at about 20m at Vulcan Rock. It is known only from the Atlantic side of the Cape Peninsula, in waters 10 to 20m. It feeds on a large, branching species of pale sponge, as shown.
There are numerous white-with-dark-spots dorids in Cape Town waters, all quite similar, so distinguishing the different species is not always easy. In order to identify this species look carefully at the darker blotches: they are speckled with pale spots. The three-spot nudibranch has similar blotches, but, as the name implies, there are only three, and these are always larger than the spots found on Rostanga sp.
Nudibranchs of Southern Africa - a guide to the opisthobranch molluscs of southern Africa, T Gosliner, 1987.
Nudibranchs of the Cape Penisula and False Bay. G Zsilavecz, 2007.