Reef Fish Feeding Roles: Who Eats What on Indonesian Reefs

Reef fish feeding roles on Indonesian reefs split the work of eating into a few clear groups, and knowing those roles tells you what you are looking at while you snorkel or dive. Some fish scrape algae from dead coral, some pick parasites off other fish, some hang above the reef and take plankton from the current, some sift the sand for worms and shellfish, and some wait on a coral head and drop onto whatever passes below. Reef fish across Indonesia sort neatly by that feeding role, and each role does something different to the reef. For the wider picture of the animals found across the archipelago, start with the Indonesian wildlife guide.

Reef fish feeding roles and why they shape the reef

Plant-feeding fish such as parrotfishes and surgeonfishes keep algae from smothering the reef, and each group keeps a different part of the reef in shape, so losing one group changes the reef in a different way from losing another.

The reason roles matter is that the reef is a set of surfaces that compete for space. Algae try to settle on dead coral, coral needs clean rock to grow onto, and small animals in the water column need a place to rest. Fish that graze, clean, filter plankton or hunt from a perch each take something from that competition, and the way they do it decides which parts of the reef stay open. Reading a reef by its feeding roles is a quick way to understand why one patch looks lush and another looks bare.

The roles also explain the shape of a dive. A group of surgeonfish moving along a reef face, a cleaning station with several visitors waiting in line, and a single hawkfish sitting on a branch are three different kinds of feeding happening in the same few metres of water. Once you can name the role, the behaviour makes sense, and you can predict where the next interesting thing is likely to happen.

Grazers: parrotfish, surgeonfish and the bumphead giant

Parrotfish scrape algae and produce sand

Most parrotfish species are herbivores that feed mainly on the epilithic algae growing on the surface of rock and dead coral. They use a beak-like jaw fused from their teeth to scrape the surface, and the material they swallow passes through a grinding gut. What they excrete is fine sand, and a large share of the white sand on reef-edge beaches traces back to that process. A few larger species, including the green humphead parrotfish, also eat living coral polyps, but live coral makes up a small share of parrotfish feeding overall. Sizes range from small species of around ten centimetres to the green humphead, which can reach about 1.3 m in total length. For habitats and how to spot the different species, read the parrotfish guide.

Parrotfish also show how sex and size interact on a reef. Most species begin life as females and later change to males, and a dominant individual can take on the male role when the existing male is gone. A visitor who sees a large brightly coloured fish on a reef flat may be looking at a male that has grown up through that change, which is why the colours in a single group can look so different from one another.

The biggest grazer in this family is the one most people want to see. The bumphead parrotfish is the green humphead, a fish with a blunt forehead that grinds through coral in large numbers. A single large humphead can turn a great deal of reef rock into sand over a year, which makes it one of the most important animals on a reef for moving calcium carbonate from rock into sediment. That makes a large grazer a builder of the beach as well as a feeder on the reef.

Surgeonfish keep algae low and carry scalpel spines

Surgeonfish, the family known by the scalpel-like spines on either side of the tail, are the other major grazing group. Their mouths are small, with a single row of teeth adapted to scraping algae from hard surfaces. The spines are folded against the body and can be raised as a weapon when the fish is threatened, which is why the spines are the first thing to avoid when you are close to a grazing school. Most species are well under half a metre long, so a school of several dozen fish can cover a reef face quickly.

Surgeonfish are one of the reasons algae does not take over a reef that has been damaged. They often feed in groups, and a group may be able to overwhelm territorial damselfish that defend small patches of algae. A damselfish that guards its patch can keep a rich growth of algae in place, while a sweep by a surgeonfish school clears it. The surgeonfish guide names the species you are most likely to see and shows how their schooling works around other reef fish.

Cleaners and the mimic that copies them

How a cleaning station works

The cleaner wrasse is the most recognisable cleaner on Indonesian reefs. The bluestreak cleaner wrasse, the species most often meant by the name, is a small blue and black fish that removes parasites and dead skin from larger fish. It eats parasites from the skin, gills and sometimes mouths of its clients, including the small crustacean larvae known as gnathiid isopods. Clients gather at cleaning stations on the reef, and a client that wants to be cleaned adopts a species-specific pose that gives the cleaner access to its body surface and gills.

The arrangement gives both sides something. The cleaner gets a meal every time a client comes in, and the client gets relief from parasites. Large fish that could swallow a small wrasse in a moment will hold still for a cleaner working over their gills, which is one of the more striking sights on a reef. For its appearance and how to watch a station without disturbing it, see the cleaner wrasse page.

Cleaning stations are good places to take a slow look around, but they are fragile. Sudden movement near a station can interrupt the visit, and a client that is spooked may leave before it has been cleaned. Keep a steady distance, let the fish come to you, and avoid touching anything. The fish that wait in line at a station are often the same ones you see moving across the reef at other times of day, so a station is also a good way to learn the local residents.

The sabre-toothed blenny copies the cleaner

Not every fish that looks like a cleaner is one. The sabre-toothed blenny imitates the cleaner wrasse closely enough to approach clients, then bites off a mouthful of skin or scales. This is called aggressive mimicry, and it works best when clients cannot easily tell the copy from the real cleaner. Instead of removing parasites, the mimic tears small pieces of flesh or skin from larger fish.

If you see a small fish at a station that seems to be taking more than it should, look closely at its mouth and behaviour, since a true cleaner picks at parasites while a blenny takes bites. The sabre-toothed blenny page shows how to separate the two, and why the copy is so convincing from a short distance.

Plankton feeders: anthias, damselfish and anemonefish

Plankton feeders are the reef fish that take their food from the water rather than from the bottom. They are easy to overlook because many of them are small and stay close to a single hiding place, but they are among the most visible residents of a healthy reef. Many damselfish take plankton from the water and are also strongly territorial, so a patch of coral with a damselfish on it is usually a patch where that fish will chase away intruders. The humbug damselfish is one of the species you will meet on a shallow reef, and its behaviour around its home is a good example of the group.

Anemonefish belong to the damselfish family, and they live among the stinging tentacles of sea anemones. Their relationship with the anemone is one of the most familiar partnerships on any reef, and it works as a feeding role too: the fish picks at small food items around its host and defends the anemone from some of the animals that would otherwise eat it. The anemone partnership is worth a closer look in the anemonefish guide.

Anthias take the open-water end of the plankton feeding role. They feed on zooplankton above the reef and gather in large schools, often where the current carries food past them. A school of anthias above a reef top can look like a shimmering cloud of colour from a distance, and the fish will move as one when a larger animal passes. For the schools and the habitats where anthias gather most, see the anthias page.

Sand and invertebrate feeders

Many reef fish do not feed on the reef itself but on the sand and rubble around it. Goatfish are the clearest example. They have a pair of chin barbels that carry chemosensory organs, and they use the barbels to probe the sand or the edge of a reef hole for food. Their diet is made up of worms, crustaceans, molluscs and other small invertebrates, which they stir out of the sediment and dig up with the mouth. The goatfish guide shows the barbels in use and the sandy habitats where goatfish work.

Sandy patches on a reef can hold more life than they first appear to, and the shrimp goby is one of the small residents worth searching for at the edge of the sand.

Larger invertebrate feeders work the reef edge. Pufferfish and boxfish work the bottom habitats they share with other feeders, and both are slow enough to watch at length. The Napoleon wrasse is a large predator of the same general kind, feeding on hard-shelled animals such as molluscs and crustaceans.

Triggerfish feed on hard-shelled animals too, and the largest of them can be a serious problem for divers. The titan triggerfish guards its nest in sand or rubble, and the guarding fish charges and bites anything that enters the cone-shaped zone above the nest. If one rises its first dorsal spine, leave the zone sideways rather than upward, and never try to move it off a nest.

Coral and sponge feeders: butterflyfish, angelfish and the moorish idol

Butterflyfish are the best-known coral feeders. Many butterflyfish feed on coral polyps, and for some species living coral is almost the whole diet, while others take a wider range of food. Coral-feeding species are often territorial, and they tend to form pairs that defend a patch of reef together. The butterflyfish guide shows the species most often seen on Indonesian reefs and where they live.

The emperor angelfish works in a different part of the same habitat. It feeds on sponges and other small animals among the coral, and it is most often seen by people who move slowly and keep to the edges of a reef. Its colours change as it grows, and a juvenile looks completely different from an adult.

The moorish idol has a long dorsal fin and a striking pattern, and it feeds mostly on sponges, along with algae and other small animals. Its diet sets it apart from the algae grazers and the coral polyp feeders, which is one of the reasons it can share a reef with them.

Ambush hunters: hawkfish, trumpetfish and the longfin batfish

Ambush hunters use patience rather than speed. They sit still on a perch, or drift among branching coral, and strike when something comes within reach. Hawkfish are the classic perch hunters. They rest on their thickened lower pectoral fin rays on coral or rock, wait without moving, and then dive onto bottom-dwelling crustaceans, small invertebrates or other fish. Most hawkfish are small and brightly patterned, and the group as a whole is tied to shallow reefs. The hawkfish guide shows how to spot one on a coral head.

The trumpetfish is a more elaborate ambusher. Its long, tube-shaped snout and slender body let it hang vertically among branching coral, where it looks like a twig. It drifts closer to small fish that have not noticed it, then strikes with a fast snap. Its colour can change quickly, which makes it difficult to see even when you know it is nearby.

The longfin batfish is a flat-bodied fish that is more often seen by divers who look carefully at the sand and rubble edges of a reef than by people who only stare at the coral.

How to see the feeding roles on a dive or snorkel

The easiest way to learn the roles is to pick one patch of reef and watch it for several minutes. Start with the grazers, which are usually the most active fish on a reef top, and notice how they move along the surface and where they leave bare rock behind them. Then look for a cleaning station, which you can usually find by watching a fish that seems to be waiting, and note how clients behave when they arrive. Finally, look at the bottom and edges, where the goatfish, boxfish and batfish work through the sand and rubble.

Keep your distance and stay still when a fish is feeding. Many reef fish change their behaviour when they feel watched, and a feeding fish may stop altogether if you move in too close. Avoid touching any fish or coral, and be especially careful around spined fish such as surgeonfish and triggerfish, and around pufferfish, whose defences are not something to test. A slow, patient approach will show you far more than a quick swim along the reef.

The roles also tell you something about the health of a reef. Grazers are only useful when there is enough live coral and clean rock for them to keep clear of algae, and a reef that has lost most of its grazers can be overrun by algae faster than it can recover. Cleaners are only useful when the reef has enough fish to visit them. Each role depends on the others, so a reef with a healthy mix of feeders is usually a reef that is recovering well.

Matching a fish to its role

If you want the broad family picture, begin with the reef fish families guide, which sets out the main groups and where to look for each. Then choose a feeding role that matches what you saw on your last dive. A grazer you cannot name is most likely a parrotfish or surgeonfish, a fish that stays at a station and is cleaned is a cleaner wrasse client, and a fish that sits on a coral head and waits is most likely a hawkfish. Pages on individual species go deeper into each fish’s habits, and together they span the full range of feeding roles you will meet across Indonesian reefs.

The roles are also a practical way to plan. A reef flat with plenty of algae and surgeonfish will suit a visitor who wants to watch schools move. A reef edge with sand, rubble and goatfish will suit someone who wants to see hunting behaviour. A cleaning station with several clients and a cleaner in the middle is the best spot for watching a partnership at work. Start with the role you most want to see, and you will find the rest of the reef around it.