According to my marine invertebrates book, collector urchins (Tripneustes gratilla) gather algae, shells, and other material on their spines. Why they do this is not entirely clear. It might be for disguise, protection from the light, or even to store food. These urchins are quite common in the shallows and their adornments are many and varied.
Posted in response to Becky’s October Squares challenge theme of ‘Past Squares – Spiky.’ See more responses here.