The parrotfish prevents algae from overrunning coral reefs. Its teeth are fused to form a sharp parrot-like beak with which it gouges out algae-laden limestone from the surface of the reef. Inside the parrotfish gut, the limestone becomes powdery, so that the accompanying algae are easily digested. The surgeon fish, which get its name from scalpel-like spines near the tail, also eats large quantities of algae on coral reefs. Its sharp, flat teeth are excellent tools for scraping algae off limestone rock. The surgeon fish does not eat the same types of algae as parrotfish. While parrotfish eat algae that impregnate themselves with calcium deposits, surgeon fish eat algae that protect themselves by means of chemical defenses, according to Coral Science.
Sea urchins also keep coral reef algae in check. When a species of sea urchins with the scientific name Diadema antillarum suddenly suffered a drastic population decline, the Caribbean reefs suffered serious damage from proliferating algae, according to Coral Science. Another echinoderm called the tiger tail sea cucumber eats algae that cling to grains of sand. It ingests the algae-laden sand, digests the algae and eliminates the sand as waste material. Owners of marine aquariums sometimes use tiger tail sea cucumbers to keep their aquarium sand free of algae.
Green sea turtles eat so much algae that their body fat is green, according to Earth Trust. Only the adults eat algae. Immature green sea turtles eat small animals such as jellyfish.
Some gastropods called sea slugs eat a lot of algae. For example, the lettuce sea slug not only eats algae, but also incorporates algal chloroplasts into its own tissue. The chloroplasts then continue to produce food by photosynthesis for the benefit of the lettuce sea slug. The snail, another type of gastropod, also eats algae. It has a rasp-like radula by which it scrapes algae off grass leaves and other surfaces. Some species are used to keep aquariums free of algae.
Such birds as flamingos and certain ducks are filter-feeders. They suck food-laden water into their mouths and squeeze the water out of the sides, leaving the food trapped within. The trapped foods include a considerable amount of algae. Grazing waterfowl, such as swans and geese, also include algae in their diets.
Some crustaceans, such as the hermit crab and the emerald crab, help keep coral reefs clean by eating proliferating algae. Tiny crustaceans called amphipods consume a considerable quantity of brown algae, according to Coral Science.