Herbivorous coral reef fishes are those fish species that feed entirely or mainly on plant material found in coral reef ecosystems.
Among coral reef fishes there are a comparatively few groups that have become adapted for such a life style.
Thus, the number of this type of feeder is comparatively small among coral reef fishes. For example, in the Caribbean region, herbivorous coral reef fishes comprise only about 10% of the total number of coral reef fish species.
Although the number of species of plant eating reef fishes is relatively small, they are often present in large numbers, and some attain large size.
It has been estimated that on Caribbean reefs, herbivorous coral reef fishes dominate in terms of fish biomass at depths of less than about 30 feet (where there is abundant sunlight to promote rapid plant growth), with carnivorous fishes dominating at greater depths.
While herbivory eating may not be a particularly popular life style among reef fishes, it has some distinct advantages.
The most obvious is that plant material is easy to find, doesn’t need to be chased or otherwise hunted, and seldom is in short supply in the shallow waters of coral reef ecosystems inhabited by coral reef herbivores. As a result, such feeders do not have to spend a great deal of time or energy searching for or obtaining food.
The main disadvantage is that plant material contains comparatively little energy and takes considerable time and energy to digest. Not surprisingly then, eating and digesting plant material requires specialized feeding structures and digestive systems that differ considerably those of meat eaters (carnivores).
Because of these restrictions, herbivorous coral reef fishes need to feed more or less continuously throughout the daylight hours to satisfy their nutritional needs. This in turn makes them more vulnerable to predators than carnivores, which can spend much more time sheltered on or in the reef.
Most herbivorous coral reef fishes belong to one of three main groups in the Caribbean region, and four in the Indo-Pacific. Parrotfishes (Scaridae), damselfishes (Pomacentridae), and surgeonfishes (Acanthuridae) are common to both regions, while a fourth group, the rabbitfishes (Siganidae), is only found in the Indo-Pacific region.
Parrotfishes (pictured right) are the largest and most colorful of herbivorous coral reef fishes. As with most other reef herbivores, parrotfishes are common only in the shallow, well-lit waters of coral ref ecosystems.
Their name is derived from the brilliant hues displayed by many species, and the beak-like mouths that characterize the group.
The cutting edge of the beak has replaced teeth in these fishes, and is used to scrape microscopic and macroscopic algae from coral surfaces. In the process, some coral polyps are also taken and ingested. Still, most of the food found in the digestive systems of parrotfishes is composed of plant material, categorizing these fishes as herbivores.
Recent parrotfish beak scrapes are obvious and common features of coral colonies wherever parrotfishes are common. The unusually powerful jaw muscles and strong set of inner crushing teeth are used to pulverize the ingested material, with the plant material and some animal tissue digested and the crushed coral skeletons passed back into the water as a fine powdery sand.
Because parrotfishes are so large and numerous, a truly amazing amount of coral sand is produced in this manner. A single parrotfish may release a ton of sand per year. It follows that a substantial amount of coral sand found in back reef areas and forming tropical beaches bordering coral reef lagoons has passed through a parrotfish at some time.
At night, parrotfish secrete a protective mucous envelope that surrounds their bodies as they sleep within reef crevices. The “cocoon” is believed to deter detection by predators.
Surgeonfishes (pictured left) and the closely related rabbitfishes are the mid-size models of herbivorous coral reef fishes. They have shapes that exemplify the basic reef fish body plan; deep and laterally compressed.
Both groups browse the reef to feed during the day and retreat to reef crevices after dark. Some species feed in mainly in schools, while others individually browse the reef.
Rabbitfishes are named for their peaceful temperaments, blunt snouts, and rabbit-like appearance of the jaws.
Although they lack the retractable tail spines of the surgeonfishes, they are armed with numerous strong spines and dorsal and anal fins connected to venom glands that deliver a painful toxin.
The surgeonfishes’ name is derived from the retractable scalpel-like spines located near the base of each side of the tail. These are formidable defensive weapons.
Unlike surgeonfishes, rabbitfishes are absent from coral reefs of the Greater Caribbean region.
Unlike the parrotfishes, surgeonfishes, and rabbitfishes that are nomadic browsers moving freely over large areas of the reef ecosystem, small herbivorous damselfishes (pictured right) are more sedentary territorial “farmers” that maintain and vigorously guard small patches of algae on the reef.
They seldom stray more than a few feet from their homes and will aggressively attack any fish (or diver), regardless of size, who has the audacity to intrude upon their territories.
For whatever reason, their fierce behavior is effective and even large parrotfishes will retreat from such attacks. Surgeonfish schools of some species however have developed a clever strategy for dealing with such tactics.
The school will collectively descend en masse upon an area of defended reef, leaving the damselfishes overwhelmed by sheer numbers. While the defenders valiantly fight off the nearest intruders, other members of the attacking school move in and quickly decimate the defended algal patches.
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