Grazing Fish
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Grazing fish include those that forage or graze close to the coral reef substratum. Not all are herbivores (i.e. wrasse, goatfish, trigger and pufferfish), but their behaviour in foraging is similar.
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Fox-faced rabbitfish |
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Blue tangs |
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Pufferfish |
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Emperor Parrotfish / Scarus rubroviolaceus / Family Scaridae |
Rabbitfish
Family Siganidae
Rabbitfish are highly compressed, deep-bodied fish with venomous, grooved spines. Wounds from these spines are very painful, but not as serious as injures from stonefish. The pelvic fins of rabbitfish are unusual with a spine at each end and three soft rays between. All species have 13 spines on their dorsal fin which is preceded by an embedded forward-protecting spine. Rabbitfish are diurnal herbivores feeding on algae and seagrasses.
Surgeonfish - Tangs
Family Acanthuridae
This group derive their common name from the scalpel-like spine on each side of their tail base, which is sometimes used as a defensive weapon during territorial disputes. These fish can either be solitary or form schools. Most surgeonfish (Genus Acanthurus) graze on benthic algae, however, there are a number of species like the unicornfish (Genus Naso) which feed on zooplankton. The brown tangs (Genus Ctenochaetus) feed on detritus that falls upon algae. Surgeonfish show a great range of digestive systems. Species of brown tang (Ctenochaetus sp.), and several other surgeonfish have thick-walled, gizzard-like stomachs, while others use chemicals such as acids to aid in the breakdown of algae. Recent research shows that some species have a similar system to termites and use specialised symbiotic bacteria living in their digestive system to break down the cell walls of algae.
Triggerfish
Family Balistidae
Triggerfish derive their common name from a stout first dorsal spine which can be locked into potion by a small second spine (the 'trigger', for if it is pressed down the first spine can be 'unlocked'). This trigger apparatus is used by the fish to wedge themselves into coral crevices at night. These fish are also characterised by a leathery skin and a small mouth with powerful, crushing jaws. They swim by undulating the second dorsal and anal fins, bringing their tail into action only when speed is needed. Some of the larger species are a menace to divers at certain times of the year when breeding. The male parent viciously guards his nest and may aggressively charge other large fish and humans which may venture too close. Triggerfish feed on a wide variety of invertebrates including sponges, gorgonians, crabs, shrimps, molluscs and echinoderms. They are often seen head down 'blowing' water into the sand to excavate food.
Pufferfish
Family Tetraodontidae
Pufferfish are known for their ability to inflate themselves by drawing water into their abdomen. They have a tough, scaleless skin (often with spines), beak-like dental plates, slit-like gill openings, no pelvic fins and no ribs. The flesh, especially the internal organs, contains a potent toxin that has caused many humans deaths when eaten. However, they are considered a delicacy (known as 'fugu') in Japan where they are prepared by specially trained and licensed cooks.
Wrasses
Family Labridae
Wrasses are perhaps the most diverse group of fish in terms of size and form. They are the second most abundant group of fish, exceeded only by gobies. Wrasses have a second set of jaws located in
their throat called a pharyngeal jaw apparatus. These jaws are primarily used for the processing of food (just like having a blender in the head). Sex-reversal seems to be universal, with wrasses changing from female to male. These sex changes are normally accompanied by distinct colour changes. Most wrasses are carnivores, feeding on invertebrates or fish; others are planktivores, corallivores, or cleaners that feed on the external parasites of other fish (eg. Cleaner wrasse).
Parrotfish
Family Scaridae
Parrotfish are closely related to wrasses in that they also have a pharyngeal jaw, however, their dental plates are fused to form a distinctive beak-like structure used for scraping algal food from the surface of the reef. Parrotfish often form large schools grazing over the reef and have often been likened to grazing cattle. The algae is crushed by pharyngeal jaws and processed in a very long intestine. They do not have a stomach. Parrotfish are a main contributor to the creation of sediment on a reef, as they expel fine particles of limestone that are consumed during feeding. Parrotfish exhibit the same reproductive and colour patterns as wrasses. Many species form a cocoon-like mucus structure at night to help mask their scent, making them harder for nocturnal predators to find.
Parrotfish are reef engineers
The parrotfishes are unique among fishes in that they consistently remove reef carbonate when feeding. Researchers (David R Bellwood, Andrew S Hoey and J Howard Choat, Ecology Letters (2003) 6:281-285) believe almost all erosion on oceanic reefs can be attributed to just one species - the giant coral-eating parrotfish Bolbometopon muricatum. Removing 13.5kg m-2 of live coral per year, B. muricatum are a major coral predator, and a significant influence on coral growth rates, mortality, and the shape, fitness and distribution of coral colonies. Given the mean abundance of the species on the outer reef crest, giant parrotfish are responsible for removing 279.3 tonnes ha-1 annually. Because their impact is continuous, the scientists say giant parrotfish cause far more damage to reefs, and accumulation of coral sediments, than periodic storms or cyclones.
