These crawl over the decor to graze on the algae which form their diet. Be careful, because they are highly prized by Balistids, puffers, and some Labrids, which have teeth strong enough to break the internal calcareous skeleton of sea urchins, and do not seem to be put off by their stings. The names of the extremely few species available are not known with any precision. Size: 10 cm.