Convict surgeonfish (Acanthurus triostegus) are the roaming sheep of the reef but, instead of noshing on grass, they feed on algae. Their grazing helps to balance the growth of algae and coral on the reef. Both corals and algae get their food from sunlight, but there is limited space underwater where sunlight reaches. Corals take much longer to grow, so the seaweed needs to be trimmed by surgeonfish and other herbivores regularly to keep it from growing out of control and blocking the sun from the coral. Seeing many herbivorous fish and not too much algae or seaweed is a sign of a healthy reef. But if herbivores are overfished, there are fewer checks on the seaweed and algae. Read a blog post about a Hawaiian marine reserve trying to save a reef by saving its herbivores.