The Great Barrier Reef (GBR) is a complex ecosystem where the survival of reef-building corals relies on a delicate balance between coral and macroalgae, or seaweed. Macroalgae are a natural part of the reef, serving as primary producers and a food source. However, when macroalgae overgrow, they become a major threat, competing with corals for light and space. This competition can lead to the smothering of corals, and some species of algae even release harmful chemicals that poison coral tissue. The constant removal of this fast-growing plant life by specialized herbivores prevents the GBR from transitioning into an undesirable seaweed-dominated state.
The Essential Reef Maintenance Crew
The primary responsibility for controlling seaweed and algae on the Great Barrier Reef falls to herbivorous fish, particularly the Parrotfish (Scaridae) and the Surgeonfish (Acanthuridae). These two groups are functionally distinct, and their combined actions maintain the closely cropped algal turf necessary for coral health.
Parrotfish are categorized as scrapers or excavators due to their fused, beak-like teeth. They use this powerful oral structure to rasp away filamentous algae and the underlying layer of dead coral skeleton. This scraping action removes the entire algal community, exposing the bare, hard substrate. This freshly cleared surface, called a grazing scar, is then available for the settlement of new coral larvae. A byproduct of this feeding is the production of fine, white carbonate sand, which is ground-up limestone passing through the fish’s digestive tract.
Surgeonfish, often called the “sheep of the reef,” play a complementary role to the Parrotfish as constant, non-selective grazers. Species like the Brown Surgeonfish (Acanthurus nigrofuscus) are among the most abundant herbivores and feed continuously throughout the day, primarily on the short, filamentous algal turf. Their grazing is less destructive to the underlying rock than the Parrotfish’s scraping, acting instead as a constant lawnmower that keeps the algal canopy short and manageable. In highly productive habitats, Surgeonfish biomass can account for a large percentage of total herbivore biomass, and their feeding activity removes a significant portion of the daily algal production.
Other Herbivores of the Reef
While fish are the most conspicuous grazers, several other organisms contribute significantly to seaweed control, often specializing in certain types of plant life or reef zones. Green Sea Turtles are the largest herbivores in the Great Barrier Reef and are primarily vegetarian as adults. These turtles consume vast quantities of seagrasses and certain types of algae, acting as large-scale mowers in seagrass meadows. Their grazing helps maintain the health and productivity of these beds, which serve as nurseries for many marine species.
Sea urchins, particularly species like Diadema, are intense grazers that exert localized control over algal growth. They use their rasping mouthparts, known as Aristotle’s lantern, to graze the algal turf to a very short height. In areas where herbivorous fish populations are low, sea urchins can become the dominant grazers, preventing the establishment of large, fleshy macroalgae. However, their intense scraping action can sometimes remove newly settled coral recruits, reducing the survival rate of baby corals.
Why Grazing Is Critical for Coral Health
Herbivore grazing is an ecological process that maintains the Great Barrier Reef ecosystem. Without constant pressure from grazers, fast-growing macroalgae would rapidly outcompete slow-growing corals for space. This unchecked algal growth can lead to an “algal phase shift,” where the reef transitions from a coral-dominated state to a degraded, macroalgae-dominated state. When large, fleshy seaweeds take hold, they can smother existing corals, block sunlight needed by symbiotic algae, and inhibit water flow.
The continuous removal of algal biomass by herbivores is necessary to create the bare, low-algal surfaces required for coral recruitment. Coral larvae require a clear space, free of dense algal turf, to settle and begin growing into a new colony. By maintaining a closely cropped algal lawn, the grazing community ensures that the next generation of corals has the necessary space to grow, linking the feeding habits of these animals to the long-term resilience of the reef system.