Corals Don't Eat—They Farm Algae; Reef Residents Do the Eating

The Coral–Algae Symbiosis

Reef-building corals are not predators. They are sessile animals (polyps) that host symbiotic dinoflagellates called zooxanthellae inside their tissues. The algae perform photosynthesis, producing glucose, glycerol, and amino acids that provide up to 90% of the coral's energy needs. In return, the coral gives the algae CO₂, nitrogen, phosphorus, and a protected, sunlit home.

The Reef Food Web: Who Eats Whom

The coral structure creates habitat for thousands of species with diverse diets:

Herbivores (Algae Grazers)

• Parrotfish, surgeonfish, rabbitfish — scrape turf algae, preventing coral smothering.
• Sea urchins — key grazers; die-offs cause algal overgrowth.

Carnivores (Predators)

• Moray eels, groupers, snappers, barracuda, lionfish — hunt fish and crustaceans.
• Reef sharks (blacktip, whitetip, grey reef) — mid-level predators.

Omnivores

• Wrasses, damselfish, triggerfish — eat algae, small invertebrates, zooplankton.

Filter Feeders

• Sponges, tunicates, bivalves, sea fans, soft corals — strain bacteria, phytoplankton, detritus.

Detritivores

• Sea cucumbers, brittle stars, hermit crabs, polychaete worms — recycle organic debris.

Corallivores (Coral Eaters)

• Crown-of-thorns starfish (Acanthaster planci) — outbreaks devastate reefs.
• Butterflyfish, filefish, some nudibranchs — nip polyps; usually minor impact.

Biodiversity = resilience. Loss of any functional group (especially herbivores) can trigger phase shifts from coral to algal dominance.