The Importance of Algae on the Reef

The bright sunshine of the tropics is great news for the group of mainly microscopic plants called algae. They prosper, especially in shallow waters, and as they photosynthesize, they produce sugars that attract herbivores to feast on them. Some species of algae grow as a covering on underwater substrates, but others, such as diatoms, live freely in the water column. Each type is food for reef animals, some of which have become highly specialized to reap this rich harvest. Compared with other marine environments, coral reefs have more than their fair share of plant-eating fish, and although only around one in five of these species is herbivorous, as many as half of all fish on a reef, measured by weight, may be herbivores.

Which coral reef species eat the algae?

There are plenty of different types of herbivorous fish on coral reefs, including surgeonfish and representatives from other families — there are algae-eating damsels, blennies, butterflyfish, and angelfish. Compared to other food sources, algae is fairly low in nutrients, so grazing fish have to spend a proportionately greater amount of their day eating. Fish are not the only herbivores on the reef, either. Among the invertebrates, sea urchins are perhaps the most important of the grazers, along with a range of snails, shrimps, and even some hermit crabs.

What adaptations do these herbivores have for their diet?

Grazing fish usually have deep, laterally compressed bodies and strong pectoral fins that allow them to maneuver around the reef and to hold station while they graze on the algal turf with their tails raised. These fishes usually have fairly small mouths and bite their food off the substrate with small, incisorlike, flat-edged teeth. Many carnivorous fish, on the other hand, suck in their food and then grind it up with the pharyngeal teeth found at the back of the mouth. To extract the most from their food, many herbivores have bacteria in the gut that help to break down the otherwise indigestible parts of the algae. Sea urchins rasp at the algal turf in a similar way to the fishes, although their mouth and tooth structure is very different. Snails, too, rasp at the algae with a specialized tonguelike structure called a radula, which scrapes against the substrate like a biological belt sander.

What is the effect of herbivores on the algae?

The pressure on algae is intense. On shallow reefs, it has been calculated that herbivores make over 100,000 bites every day on every square foot of the substrate. In the process, the grazers can remove virtually all the algae that grows there. It might seem to be bad news for the algae but this is not entirely true. On reefs where for one reason or another there are few herbivores, the algae grows unchecked into thick covering layers. In these conditions, a handful of algae species take over; driving all others out of the habitat. Grazers have the effect of preventing these dominant algae species from taking over, allowing the other algae species to keep a foothold. The grazers benefit as well; the dominant algae species are among the least nutritious, so by keeping them in check and creating the conditions that allow other species to grow, the algae as a whole provides them with a better meal.