Reef Fish Diets: Just What Do They Eat?

Over tens of thousands of years, fish have evolved to be expert foragers, feeding on a huge range of different food sources, both animal and vegetable. Some have become specialists on a certain kind of food, others are opportunists, feeding on whatever they can. One thing all species have in common is that they are experts in spotting an opportunity and hunting out their next meal. Fish are the dominant vertebrates of coral reefs. About 70% of reef fish species are carnivorous, 20% are herbivores, and 10% are omnivorous, meaning they have a varied diet that includes both animal and plant matter. This simple division into meat-eaters and vegetarians can be clarified by dividing the fish into so-called feeding guilds.

What is a feeding guild?

A feeding guild is a way of grouping different species of fish according to their diet. In many cases, fish within a feeding guild are similar in appearance. Fish that browse on small reef invertebrates often have deep
bodies, making them highly maneuverable, for example. As well as body shape, the mouthparts and teeth of fish within a guild are often quite similar, yet quite diverse between different guilds. For example, the rasping teeth of algae-eating surgeonfishes and damselfish are quite different from the amazing beaks of parrotfish or the fangs of a moray eel.

Why are so many coral reef fish so specialized in their diet?

The pressure of food competition on the reef has led to many species of fish becoming in their appearance: the beaks of coral-eating parrotfish, the long snouts of plankton-eating seahorses and pipefish, and the enormous mouths of predatory groupers are all examples of this. The advantage of being a specialist is that the fish gain a competitive edge. They become expert at finding and catching their particular prey, so much so that they can easily out-compete a generalist. The downside is that their diet becomes increasingly inflexible. As an analogy, a racehorse can easily outrun any other breed of horse, but is useless for any other kind of work.

This dietary specialization presented real problems to aquarists in the early years of tropical marine fishkeeping as they struggled to supply an adequate diet. But although fish might often be specialized for a particular type of diet, this does not necessarily mean that they will turn down the chance of a change from time to time. Fishes such as spiny chromis and sergeant majors feed opportunistically in this way, switching from a diet of mostly algae to take advantage of the annual mass spawning of corals on the Great Barrier Reef to gorge themselves on the rich pickings available at this time.