Butterflyfish and Angelfish Feeding Behavior
The limestone reef is full of holes, cracks, and crannies that hide tasty morsels, such as sponges, shrimps, and polychaete worms. Coral polyps are also nutritious, but as they are concealed within their limestone refuge during the day, many fish find them difficult to reach. Nonetheless, certain fish specialize in these apparently well-protected food sources, including members of the butterflyfish and angelfish families.
How are the fish adapted to their diet?
Fish that feed on sessile (non-moving) invertebrates such as sponges and corals share the same kind of body plan as the grazing herbivores. Again, deep bodies are ideal for precise maneuvering, supported by strong pectoral fins that provide great swimming control. But the most noticeable thing about fish that eat coral, or probe into the reef to extract sponges, worms, and small crustaceans, is the snout. The extreme example is the longnose butterflyfish (Forcipiger spp.), which probes deep into the reef to reach prey that no other fish can. The shorter snout of the banded butterflyfish (Chaetodon sthatus) enables it to tackle a wider range of prey, including worms, corals, and the occasional crustacean. Within their narrow jaws, most butterflyfish have rows of short, bristlelike teeth with which they can firmly grip their prey as they extract it from its refuge. Polyp-eating specialists such as Meyer's butterflyfish (Chaetodon meyen), and sponge-eating fish such as the regal angelfish (Pygoplites diacanthus) have shorter snouts, as they do not need to reach as far into the reef.
How easy is it for these fish to satisfy their nutritional requirements?
One problem for animals that feed on small prey is that they must spend a great deal of time foraging. Butterflyfish and angelfish are diurnal animals that gather their food in the 12 hours of the tropical day. Hunger sometimes drives them to continue feeding at night, when there is sufficient light to do so around the time of the full moon, although the increased numbers of fish predators at night makes this a risky strategy.
Both families expend huge amounts of energy in their foraging. A study on their feeding rates in the Red Sea showed that the same butterflyfish bit at the surface of the coral, on average, every five seconds throughout the day. By contrast angelfish were much more relaxed, but even they bit at the reef at about 15-second intervals. Both species feed more actively in the Red Sea summer, when warm water and the need
to fuel breeding efforts lead them to forage more than twice as actively as in winter and spring.
How does the diet of these fish affect their prey populations?
Predation is an extremely important force in promoting the highly diverse communities on the reef. Without it, some species would rapidly overcome all their competitors, outgrowing them or taking up all the available space within their habitats. An experiment that involved excluding invertebrate-eating fish from sections of reef using cages, showed that the number of different invertebrate species quickly declined as just a few took over. Predation by angelfish on certain sponges and sea squirts, for example, is known to boost the populations of poorer competitors, such as bryozoans. By the same token, corallivorous fishes prevent certain species of aggressive corals from taking over and allow their competitors to persist again boosting biodiversity on the reef.