Donna Carlson Jones, a doctoral candidate in the UC geology department, conducted a yearlong study in the Florida Keys looking at how spines and other shell decorations affected the number of epibionts present. Epibionts are organisms that grow on top of other organisms without seriously harming them.
Her primary focus was the bivalve Spondylus regis which has a spiny surface, but the spines are so delicate and widespread, they don't appear capable of protecting the organism directly. Jones compared Spondylus with three other bivalves: the mussel Mytilus edulis which has a smooth surface, an oyster Crassostrea sp. which has a rough surface, and the scallop Pectin sp. which has ribs. She also included an experimental group of Spondylus which had its spines removed.
"Spondylus has beautiful spines, but they're widely spaced and fragile compared with the overall shell strength," noted Jones, who added that the spines clearly don't deter some crabs which prey on the bivalve and predatory starfish can actually use spines on other types of bivalves as leverage to pry open the shells.
Jones hypothesized that the spines increase the bivalve's surface area which provide more room and a more hospitable environment for epibionts. Additionally, spines may trap material such as algae out of the water currents. Her research demonstrated a clear difference in the number of epiobionts growing on Spondylus with spines and the other four groups.
So, instead of directly warding off predators, the spines could protect Spondylus in an indirect way. The epibionts might provide a camouflage cover that could reduce overall predation on the bivalves.