Sea urchins have modified mouth parts on the underside of their bodies called an Aristotle’s lantern. The lantern is made up of tooth-like plates that scrape up bits of food like algae, as seen here.
We specifically don’t scrub the algae in this habitat for the urchins, and I see them on the walls all the time, presumably getting a tasty meal. But I had never seen it in action before last week! Invertebrates are such cool little aliens!