Marine scientists frequently note that every reef inhabitant has a role to play on the reef. Feeding habits help shape and define an individual's role. Herbivores eat plants and algae; corallivores eat coral; carnivores eat animals; omnivores eat plants and animals, and planktivores eat plankton. Scavengers eat whatever is dead or dying, and on it goes, preserving an intricate system of checks and balances that ensures diversity and an overall healthy marine ecosystem.
Herbivores are essential to healthy reefs because they keep algae and plants from overproducing and smothering corals. Plant- and algae-eating reef inhabitants can be divided into categories, or "feeding guilds," based on their particular feeding preferences and methods.
"Browsers" bite off bits of algae to make up a meal, while "croppers" tackle large areas of benthic algae and feed in schools. Browsers include angelfishes, blennies, damselfishes, filefishes, triggerfishes and unicornfishes.
Some browsers, such as triggerfishes, also serve as croppers. Surgeonfishes are major croppers. Encountering a cloud of manini or convict surgeonfishes busy "cropping" algae is a treasured experience for any snorkeler or diver, all the more so considering that schools of fish can do a better job of controlling algae than any one single fish.
"Grazers" focus on a specific patch of algae growth and often "rasp up sections of food." Sea urchins, sea hares, limpets, cowries, nerites and chitons fall into this category. They keep algae in check within a defined territory and usually remain there for as long as the food holds out.
"Grovelers" ingest sand or mud along with plant matter and bacteria. Sea cucumbers, sand dollars, sea biscuits and mullets make up this group.
"Scrapers" scrape algae away from the substrate and consume bits of substrate along with the algae. Green sea turtles belong to this category, as do parrotfishes that zero in on zooanthellae growing within corals.
Tropical water herbivores often are generalists that feed on a wide variety of plant-based sources. They will nibble on almost any type of seaweed, microalgae growing on sand and phytoplankton suspended in the water column.
Herbivores usually feed during daylight hours. Exceptions include certain herbivorous sea urchins that graze at night when competitors are inactive. Planktivores and carnivores dominate reefs at night, but night-feeding sea urchins tend to be well equipped to avoid predation.
Competition for plant food sources can result in vigorous territorialism. Some species of damselfish will defend prized mats of algae against all comers, discouraging even those much bigger than themselves. Prevented from feeding on corals within an established damselfish territory, frustrated corallivores soon depart and seek a meal elsewhere. Possessive damselfishes have been known to physically remove a coral predator, such as a crown of thorns sea star. A fearless combatant may seize the spine of an intruder and drag it to another location. Damselfishes also keep other species of herbivores from taking over an area.
Some corallivores do manage to breach damselfish defenses by returning at night. The four-spot butterflyfish will take advantage of a full moon to selectively feed on corals within a damselfish territory. At night, damselfishes shelter within the reef and "the door is open," at least temporarily.
Algae blooms occur naturally in the ocean and are part of a healthy, well-functioning eco-system that supports life from the very bottom to the very top of the food chain.
Too much algae growth adversely affects coral reefs and ultimately, the entire food chain depending upon those reefs. When algae supply exceeds demand over a sustained period of time, consequences can be devastating and entire reef systems will die.
The creation of herbivore management areas, such as the Kahekili Herbivore Fisheries Management Area in West Maui, is intended to protect herbivores so they can do their job of keeping algae in check. If too many herbivores are removed from a fragile area where algae already is becoming rampant, the decline of the ecosystem accelerates.
Efforts to preserve life in our oceans must address, among other threats, land-based activities that inadvertently fuel excessive algae growth, making it
impossible for herbivores to keep algae in check. Finding ways to reduce harmful impacts from human behavior is critical to ensuring that centuries-old irreplaceable coral-reef systems do not continue to collapse and disappear within our lifetimes.
* Pam Daoust is an author and volunteer marine naturalist at Maui Ocean Center. "Ka Mo'olelo Moana," or "The Ocean Story," is a monthly column submitted by the Maui Ocean Center, which is open from 9 a.m. to 6 p.m. daily in Maalaea. For more information, call 270-7000.
