Sponges are the simplest of all invertebrates. Because sponges are low in nutritive value, contain a lot of indigestible material, and are protected by chemical and physical means, they are not a particularly good food source for fishes. Only 11 species of Caribbean fishes regularly eat sponges. Species that feed on sponges have evolved relatively recently suggesting that competition forced them to evolve to eat a less-preferred food source.
Sponges make up the majority, 70 – 95%, of the diet of most angelfishes (Family Pomacanthidae). Angelfishes prefer to feed on the sponges with the lowest content of protective spicules. Sponge-feeding fishes have strong jaws that contain overlapping rows of tightly packed teeth that they use to teach through the tough sponge material. They secrete a thick layer of mucous around the pieces of food to protect their stomachs during digestion.
The larger species of angelfishes are protected from predation because their large size and thin bodies make them a difficult mouthful for predators. They typically feed by following well-used paths through territories that are defended for mating, rather than feeding purposes. Gray Angelfish (Pomacanthus arcuatus) live as mated pairs in large territories. They move and feed slowly, taking 3-4 bites per minute. Sponges make up 70% of their diet with the rest of their diet consisting of gorgonian polyps, other invertebrates, and algae. Although algae make up only 10% of their diet, foraging for algae takes up about a third of their foraging time. French Angelfish (Pomacanthus paru) forage in a similar manner, but they consume less gorgonian polyps and algae. The diet of Queen Angelfish (Holocanthus ciliaris) is made up almost entirely of sponges, 97%, with the balance made up of algae and invertebrates. Rock Beauties (Holocanthus tricolor) also eat mostly sponges, 96% of their diet, although half of their foraging time is spent scraping algae off of rocks.