Slate Pencil Urchins feed on algae, turtle grass, kelp, sponges, mussels, barnacles, and dead fish/decaying matter . Another source claims they eat Pavona and Pocillopora corals - the species shown here does not fall into either group. So the urchin could be grazing the surface for algae, or just happen to be walking across. I'd be surprised if corals were susceptible to algal growth, though. The coral species seems to be something along the lines of File:Colpophyllia natans (Boulder Brain Coral) entire colony.jpg (not yet nominated because no article). Also relayed question to Nhobgood. Papa Lima Whiskey (talk) 08:05, 16 October 2010 (UTC)
Thanks for the nomination. I am far from an expert on these urchins but from personal observation, the few that I have seen do not seem to feed on coral, whereas the much more abundant Echinometra viridis urchins are frequently seen in large groups on top of partially eaten brain corals of different species. I did run across another E. tribuloides on a Porites coral but I would speculate that both specimens were just traveling over the hard corals and not feeding on them as there was no evidence of predation on the corals.Nhobgood (talk) 11:02, 17 October 2010 (UTC)