#angelfishes

None of the angelfishes is a specialised corallivore, or coral eater. This does not mean that they are physically or physiologically incapable of eating coral tissues, and in fact it is verified that some large angelfish species do, in fact, consume corals and anemones as items within broader diets, in the wild. For example pomacanthids are adapted to accessing prey that are in and around crevices, and pomacanthids are known to extract and consume live sea anemones. It is unsurprising that such broad spectrum feeders will behave similarly when they are in captivity. Early on in their evolutionary history, pomacanthids evolved the ability to close their jaws whilst they are extended, as during normal suction feeding. This ability to bite down whilst reaching to items attached to the seabed, allows them to grip and tug efficiently at tough masses of sponges and colonial tunicates - items that are much more difficult for other fishes to eat. They are specialised in a very different way to the butterflyfishes, which also consume attached animals, but trend towards the evolution of corallivory. The difference is that between a horse and a deer on land, or maybe that between a white and a black rhinoceros. Angelfishes partition resources with butterflyfishes, because they eat in different ways; angelfishes as a whole are not inclined to be selective feeders, and they chomp indelicately at masses of tough, living tissues, whilst the butterflyfishes instead tend to nibble more selectively.

Surrogate behavior is thought to explain why individual angelfish start to bite corals and clams mantles in the aquarium. In the science of behavior, surrogate behaviors are those that arise when a natural instinct cannot find natural expressions; so that in humans, for example, shopping becomes a surrogate for gathering foods. Angelfishes in the wild, are surrounded by spongal growth and tunicate colonies, and the behavior of feeding on then consumes a great amount of their time. Not surprisingly a frustrated angelfish will not improbably begin to bite on surrogate items, in the same way, if the normal targets of such behaviors are not present. That some fish do this more predictably than others in the same species, is presumably due to personality traits such as curiousness and a tendency to act upon natural impulses - otherwise the unpredictability of coral and clam biting by angelfishes is inexplicable. The phenomenon is noted even in an unusually specialised non-coral eater (ie. Pygoplites) as well as the more typical, broad spectrum feeders.

The diets of all angelfish are not equally well known, but as a clade, their anatomy is well studied as regards eating and digestion. Individual variation in behavior must be borne in mind, alongside the fact that some angelfish species actually have are faculative coral eaters in the wild. Most worry is expressed over the dwarf angelfishes - classically the genus Centropyge, but now three different genera - which older books as late as the 1980s, used to categorise as 'reef safe' - a meaningless term that ignores the realities of ecology, but one broadly defined as something like 'compatible with and harmless to living corals, and all (or almost all) sessile invertebrates'. Many dwarf angelfishes do, in fact, have diets rich in sessile, attached animals, and such items can even predominate in their diets on the reef.

The confusion perhaps arose from comparative anatomy: Centropyge teeth resemble those of blennies, another clade that consumes much algae, but cannot be regarded as strictly herbivorous, on the whole. The same older books used to tell the reader that blennies were reef safe, without reservation: in reality many eat coral polyps as natural parts of their diet, and one genus eats only stony coral, despite not having obvious craniodental adaptations to coral eating. The comb-like teeth of blennies, and of dwarf angelfishes, are actually efficient 'multi-tools' for using on a range of foods - like the chisel-like teeth of rodents, or the toothless beaks of birds. Even as fishes with comb-like teeth are restricted so as to not consume certain items, they can have either broad or narrow feeding habits, and eat either animal or vegetable resources.

Centropyge have teeth adapted for benthic grazing, and they resemble blennies and mbuna in this respect. They are among the most herbivorous angelfishes, but their diet still includes sponges and tunicates, and again dietary isotopes indicate extensive omnivory exists in these fishes. In the aquarium they do nip at coral polyps and clam mantles, so they ought not to be regarded as 'reef safe'. The Paracenteopyge angelfishes are basically carnivorous by nature. They should not be confused, ecologically speaking, with the dwarf angelfishes of Centropyge proper, or the pygmy angels of the genus Xiphypops; but their diet is not well documented in the literature, only that they eat sessile animals, and although the can be encountered in the shallows, they are more often associated with deeper reef habitats.

Xiphypops, the pygmy angelfishes, are different from the Centropyge angels, because natural selection has helpfully reorganized the orientation of their faces for a lifestyle of herbivory by dislodgement. Although I have again heard of pygmy angels nipping at tridacnid clam mantles, they are by far the safest of the angelfishes in coral aquariums. They simply possess too much specialization for a specific type of herbivory, for them to destroy corals. Again I offer the caveat that these animals may sometimes nip at fleshy polyps, although I have never witnessed this. Truthfully, the pygmy angels are specialised, obligate consumers of delicate and foliose algae, which they eat using combing and shearing strokes of their downwardly inflected jaws.

Two other genera of angelfishes can be considered, on phylogenetic grounds, to be allies of the dwarf angelfishes; these are Apolemichthys and the surprisingly planktivorous Genicanthus, or swallowtail angelfishes. Members of the delightful genus Apolemichthys are understood to be omnivores, with a diet that is dominated by sessile animals; specifically, some 3/4 of their diet in the wild is spongal. The diet of this genus in the wild, is not so well studied as some of the other angelfishes, although these are wonderful fish indeed. Whereas their allies Genicanthus have abandoned their lifestyle of benthic feeding, and they display evolutionary reversals associated with open water planktivory. Genicanthus are thus the most extreme outliers of the angelfish group, in terms of their unique feeding ecology and behaviors. For this basic reason, these specialists are considered to be entirely 'reef safe' among the angelfishes. All the same, Genicanthus in the wild are known to consume some benthic algae and sessile animals. To my knowledge they never eat anything that we would think of as corals when they are in the wild, and I regard these specialists as about as appropriate for reef aquariums, as are Xiphypops.

At the other end of the spectrum of specializations that are observed in pomacanthids, is the regal angelfish, Pygoplites. For Pygoplites are the most specialised of all angelfishes for a life of gripping and tugging tunicate and spongal masses, to the point that wild Pygoplites will consume nothing else at all. Nonetheless, aquarists have sometimes noticed that they appear to bite at 'softies', a non-zoological catch all category, that contains all popular anthozoans that neither secrete a stony skeleton, nor large actinarian anemones. Obviously the behavior arises out of frustrated instincts when it occurs, and at least in some cases, the angelfish is likely biting encrusting organisms near the base owhere the 'softies' are attached. Observations that fish happen to bite at coral are not necessarily observations that they eat corals; they can be eating items on their surface or near their base, and people should bear in mind conclusions based only on superficial observations, ought not to be jumped to.

The butterflyfish-like Chaetodonplus are quite reef safe angelfishes, although they have been reported to bite at the fleshy polyps of 'LPS' type colonies, and also at clam mantles. This is even though their basic diet in the wild is spongal and algal - and stable isotopes have determined that they also consume detritus deliberately, making them scavengers. Nonetheless Chaetodonplus are safe in aquariums that feature stony corals with short polyps. Different individuals appear to vary in their destructiveness to other corals, and I had no trouble with mine in a mixed reef. But other people have reported they inflict regular nippings upon fleshy, sessile ornamentals. Aquarists tend to classify their stony corals into the two non-phylogenetic categories, SPS and LPS, but they are poor predictors of coral vulnerability, because the categories ignore the stings and other defences of polyps. However, because angelfishes have evolved to grab and tear at bulky items, large polyps will be at potential risk of angelfish attack, whilst those coral colonies bearing small polyps will not.

The 'large angelfishes' are two wonderful genera of fishes that have long enthralled aquarists - Holacanthus and the archetypal Pomacanthus itself. Before the rise of reefkeeping, the large, showy angelfishes were staples of saltwater fish retailing. When the hobby refocused on smaller systems, small fish, and living corals, they dropped from popularity. And some of the large angelfishes, as omnivores, actually do eat anthozoans by their nature. The subgenus Euxiphypops - unlike their smaller namesakes - are typical angelfishes, as regards their jaw mechanics. However they too have evolved into a herbivorous ecological space, and their faces are able do deal with tough seaweed as a basic diet; they use the same eating techniques that other Pomacanthus angelfishes more normally employ to exploit ruggedly attached, sessile marine animals.