The Variegated scallop Chlamys varia is quite common here; it is often purple, but it can be orange, red or yellow as well. Below a purple-red one I found under a rock covered in some Spirorbis spirorbis (calcified worms) and surrounded by Cushion stars, Grey and Flat top shells and Periwinkles:

A big one:

This scallop attaches itself to rocks with byssus threads but is able to move around quite a bit as well. I have had two in the aquarium for a couple of months, longer than any other bivalve I’ve tried, but in the end they died anyway. It is hard to keep filter feeders; algae tend to thrive in aquaria, but usually in their attached- rather than planktonic form. I have not had luck with tunicates either and I won’t try any new filter feeders until I have found a proper way to feed them. On one forum, I was recommended a form of artificial plankton that is commonly used for corals which could be interesting. However, this will require a lot of skimming to get rid of the non-eaten food. I’ll therefore focus on experimenting with seaweeds for now. Below two close-ups of a Variegated scallop in the aquarium. Scallops have rows of beady eyes along their mantle, this can be seen in the the second picture: