Information about the diet of any fish you keep in your tank can be crucial to their long-term health, as well as the well-being of other animals that share the tank with them. In this day and age, when many marine aquarists include invertebrates in their saltwater aquariums, it is imperative to know if a fish added to a reef tank is going to cause problems for its spineless neighbors. Fish can also be added to a reef aquarium to help control nuisance algae or pestilent invertebrates, but you have to know something about the fish’s diet to know if it will serve such a utilitarian function. Of course, knowing what a fish eats will also determine what, when and how much to feed it and, in some cases, should be a factor that dissuades you from purchasing a particular species (e.g., a butterflyfish species that only eats live corals).
In this article, we will look at the general food habits of reef fish. We will examine the primary feeding guilds and some of their unique husbandry needs. Coral reef fish can be categorized into different food or dietary categories. The four primary food habit categories or guilds are the detritivores, herbivores, carnivores and omnivores. We will start our survey by looking at one of the most overlooked feeding guilds: the detritivores.
Detritivores feed on decomposing organic particles, which include animal and plant remains, waste products and bacteria and other microorganisms associated with it. Some fish selectively ingest this material, while others incidentally consume it along with algae or encrusting invertebrates. At least three reef fish families feed heavily on detritus in what scientists call the “epilithic algal matrix”: surgeonfishes, damselfishes and blennies.
While many of the surgeonfishes (Acanthuridae) feed on algae, there are some species that specialize in detritus. For example, most members of the Ctenochaetus genus ingest detritus, bacteria, diatoms and large amounts of sediment. When these fish feed, they press their jaws against the substrate and then throw the lower jaw upward. This effectively brushes particulate matter off of rock and dead corals, and also out of turf algae. These species avoid areas with long filamentous algae, or if they happen to ingest it, they quickly spit it out because it gets stuck in their teeth.
There are also Acanthurus species that feed predominantly on detritus, but they ingest it off of sand surfaces rather than hard substrates (e.g., orange-shoulder surgeonfish, Acanthurus olivaceus). If you use live sand in your aquarium, these species can be employed to keep the surface layers of the sand detritus-free and turned over. When they feed, they take in mouthfuls of sand and associated debris.
The Pomacentridae family (damselfishes) also includes a number of species that rely on detritus as a major food source. It was once thought that many of these damsels fed primarily on algae and some of the small invertebrates that live in the algae. More recent studies suggest that it is detritus that collects in the algal mat that makes up the bulk of territorial pomacentrid diets. The same can be said about many in the family Blenniidae. For example, it is now thought that detrital aggregates are as important as algae in the diet of the Patzner’s blenny (Salarias patzneri). This may be the case for other members of the Salarias genus, as well.
Coral mucus can also be classified as detritus. This mucus is loaded with energy-rich wax esters and triglycerides, which provide a valuable source of energy. The best-known “mucus munchers” belong to the butterflyfish family. For example, some of the obligatory coral-feeders, such as the ornate butterflyfish (Chaetodon ornatissimus), rely heavily on mucus as a food source. There are other reef fish that feed on coral mucus. For example, the short-bodied blenny (Exallias brevis) ingests large quantities of mucus along with coral polyps. The sulfur (Pomacentrus sulfureus) and blue-green chromis damsels (Chromis viridis) will gently nip at live corals and ingest the mucus of the corals. I have observed captive azure demoiselles (Chrysiptera hemicyanea) nip the mucus off of hard corals and mushroom anemones. Pygmy angelfishes do the same to hard corals, soft corals and clams. This can become a problem if these fish constantly graze on the mucus on these invertebrates, as it could damage coral polyps or tridacnid mantles. Pygmy angels are also fond of dead and dying coral, and anemone tissue, as well as their waste products.
Fish feces is another waste product that is utilized by a number of detritivores. (Eating feces is known scientifically as coprophagy.) Why would a fish feed on another fish’s feces? Fecal pellets contain macro-nutrients, such as carbon, nitrogen, protein, carbohydrates and lipids. They also contain micro- and nano-nutrients that are important for growth, such as phosphorous, silicon, copper, iron, manganese, zinc, nickel and chromium. Most of the fish that feed on feces are herbivores or detritivores. In the case of herbivores, their diets are low in nitrogen, so they utilize feces as a source of this important nutrient. There are herbivorous members of the pygmy angelfish, chub, parrotfish, rabbitfish, surgeonfish and triggerfish groups that occasionally ingest fish feces.
Coprophagy is regularly observed in the aquarium, especially if the aquarium contains a pygmy angelfish, damselfish, surgeonfish or rabbitfish. While it may seem a bit distasteful, this is a desirable behavior. Instead of being broken down by microbial action and releasing nutrients into the water, the fecal pellets will be recycled. The result is that some potential pollutants will be absorbed into another fish’s tissues before being defecated into the aquarium for a second time.
Feeding Guilds of Reef Fish
Below you will find a list of the various feeding guilds. Some families have species in more than one feeding guild. For example, there are damsels that are detritivores. There are others in this family that are omnivores, and there are even some pomacentrids that feed almost exclusively on zooplankton (that is, they are carnivorous).
Algae is not conspicuous in the majority of reef habitats, and the main factor that affects its abundance is the presence of herbivores (i.e., fish and sea urchins). For example, on shallow fore-reef slopes where herbivores are abundant, all algae have been consumed – with the exception of coralline algae, the “roots” of filamentous forms and macroalgaes that have chemical or structural defenses (e.g.,Halimeda). It has been estimated that in this reef zone, herbivores take from 40,000 to 156,000 bites per meter squared per day. Knowing this, it should not be surprising that if herbivores are removed, thealgaewould grow like mad. In fact, in reef habitats where algae-eating fish are not abundant, macroalgae is common and is a problem for sessile invertebrates. It has been suggested that modern reefs would not exist in their present form if it were not for the herbivores; these fish (and urchins) control the competitively superior macroalgaes from overgrowing other sessile organisms, such as corals. It has been suggested that on portions of the Great Barrier Reef, adult pinnate batfish (Platax pinnatus) may be the only hope for reefs being smothered by Sargassum macroalgae. This batfish, which is omnivorous, is one of the only fish known to feed heavily on this macroalgae that is overgrowing some nearshore reef areas.
All of the herbivores are diurnal, and in contrast to many carnivores, they consume large quantities of food and feed continuously throughout the day. For example, the purple tang (Zebrasoma xanthurum) feeds for up to seven hours a day. The lined surgeonfish (Acanthurus lineatus) will nip the substrate 200 times in a 10-minute period. The Cortez damselfish (Stegastes rectifraenum) takes more than 3,000 bites in a day, and it takes more than 500 bites to fill its gut once. This fish reaches a maximum weight of about 2.5 ounces and consumes about 0.4 ounces of algae per day.
Herbivores consume large quantities of food because algae does not contain a lot of digestible nutrients. If you are keeping herbivores in your aquarium, keep these facts in mind, especially if your tank is devoid of algae. Rather than giving them one big meal, it is best to provide several smaller meals throughout the day. To supplement this, add freeze-dried algae sheets to the aquarium once a day. For those of you who keepfish-onlyaquariums, I strongly suggest that you encourage a lush growth of filamentous algae, which will act as a natural food source for these fish. Some of the healthiest surgeonfishes I have ever had have been in tanks with a thick mat of green hair algae. If you have a reef aquarium or a fish-only tank, browsing fish will appreciate the occasional introduction of Caulerpa or red macroalgae. Some herbivores require special consideration when it comes to their captive diets. For example, the Rainford’shover goby(Amblygobius rainfordi) will not thrive unless some filamentous algae is present in the aquarium; combtooth blennies (Cirripectes spp.) require large amounts of detritus and diatoms to survive; someNaso surgeonfishdo best if provided with brown macroalgae.
Carnivores feed on animal prey. The way they capture and handle their quarry depends on their anatomy and the prey item. For example, some sessile invertebrate-feeders scrape parts of their prey off of hard substrate with comblike dentition (e.g., angelfishes) or fused, beaklike teeth (e.g., puffers). Other fish rip small pieces of flesh or body parts from prey items (e.g., butterflyfishes), ingest their victims whole (e.g., groupers) or tear or break them into bite-sized morsels (e.g., wrasses).
Many carnivores show changes in their food habits as they grow. This is usually a function of physical limitations (i.e., their ability to capture and ingest larger prey) and differences in habitat usage between size classes. For example, juvenile coral trout (Plectropomus leopardus) feed more on invertebrates, as well as some smaller fish species, while adults have a diet that consists almost entirely of fish prey.
The frequency of food intake also varies between species. Fish with diets that consist of sessile invertebrates feed often and throughout the day. For example, the French angelfish (Pomacanthus paru), which feeds mainly on sponges, takes an average of about three bites per minute during the day. Like herbivores, these species need to be fed several small meals during the day rather than one large meal. Some sessile invertebrate-feeders that need to be fed several times a day include angelfishes, butterflyfishes, triggerfishes, filefishes and puffers.
In contrast, fish-eaters consume relatively few meals in a day on the reef and should be fed similarly in the aquarium. For example, the variegated lizardfish (Synodus variegatus) eats an average of two fish per day, which is approximately 12 percent of its total body weight. Piscivorous fish should be fed from once every day in the case of more active forms (e.g., groupers, jacks) to several times a week in the case of more sedentary types (e.g., morays, frogfishes, scorpionfishes). Make sure that their prey items are not too large. If they swallow something too large, it may decompose faster than it is digested. Decomposition will fill the alimentary tract with gas, and if the fish does not expel this, it can die. Also, do not feed freshwater feeder fish (e.g., goldfish, rosy red minnows) to marine fish, as they do not contain all the nutrients that marine fish require.
There is also a group of highly specialized carnivores that make their living by picking off tiny planktonic animals: thezooplanktivores. The primary food of zooplanktivores are planktonic crustaceans (i.e.,copepods), but some species are opportunistic, consuming large numbers of larvaceans, errantpolychaeteworms, coral spawn and fish eggs, as well. I have observed schools of fusiliers (Pterocaesio spp.) and chromis damselfish, both of which prefer planktonic crustaceans, feeding on the eggs of spawning surgeonfishes and wrasses. Because their prey is small, they must feed for a number of hours each day. The key to keeping these fish healthy in captivity is to feed them frequently. I would suggest that most zooplankton-feeders should be fed at least three times a day. Some of the best foods for zooplankton-feeders are mysid shrimp (which are readily available in a frozen form), oyster eggs, fish eggs and frozen Cyclops.
Omnivores feed on both plants and animals. They comprise the second-most-specious guild in coral reef fish communities. The amount of each food group consumed depends on the species and the age of the fish, and it may vary from one location to another. For example, juvenile blue (Holacanthus bermudensis) and queen angelfish (H. ciliaris) feed more heavily on algae than the adults. In contrast, juveniles of the herbivorous brown-spotted spinefoot (Siganus stellatus) and the omnivorousbicolor angelfish(Centropyge bicolor) consume more invertebrate prey than their adult counterparts. This seems to be the trend in most herbivorous and omnivorous reef fish – they include more animal prey in their diets as juveniles. There is more nitrogen in animal prey, which is necessary for growth, than in plant material.
The importance of algae in an omnivore’s diet may also vary between individuals. There are species of butterflyfish that feed heavily on sessile invertebrates that also regularly ingest plant material (e.g., speckled,Chaetodon citrinellus; saddleback,C. ephippium; Klein’s,C. kleinii; Pacific double-saddled,C. ulietensis; and vagabond butterflyfish,C. vagabundus). Like herbivores, omnivores feed frequently and only during the day.
This is a brief survey of the food habits of coral reef fishes. Remember that this type of information can be critical when it comes to keeping your saltwater pets. There are many scientific papers available on the web (try Google Scholar) that give specific details on the food habits of coral reef fishes. Happy fish-watching! AFI
Scott W. Michael has kept marine fish for more than 25 years. He is the author of Reef Sharks and Rays of the World; Reef Fishes: A Guide to Their Identification, Behavior and Captive Care and more.