Angelfish: Bright Blues and Graceful Swims

What characteristics come to mind when one speaks of angels? Beauty and grace, white feathery wings, and perhaps surrounded by a divine glow. Then, what would an angelfish look like?

Quite similar, sans the divine glow and white body and wings.

Angelfish are a group of brightly coloured and boldly patterned tropical fish found on shallow reefs across the world. Marine angelfish belong to the Pomacanthidae family (Greek; poma = cover, acantha = thorn), and the name refers to the spine present on the operculum (bony plate covering the gills) of these fish. There are nearly 90 species of angelfish, a relatively smaller group compared to other groups of marine fish.

 

Marine angelfish, sometimes confused with freshwater cichlids, which are also called angelfish, have laterally compressed (flattened) bodies, large fins, and a small mouth. They are most closely associated (and confused) with butterflyfish in terms of physical characteristics, like their small mouths and flattened bodies. Like their butterflyfish counterparts, angelfish have bristle-like teeth, too. The easiest way to distinguish between the two fish groups is to look at the fins and overall body size — angelfish are larger and have larger faces.

Angelfish are omnivores and feed on anything from small crustaceans, worms, larvae, and filamentous algae to other small invertebrates like sea sponges, hydroids, and tunicates (sea squirts). Due to their varied diet, they fill several important ecological roles in a reef, helping keep algae and sponges in check for a balanced ecosystem. Several juveniles feed on the parasites present on other fish, carrying out the role of “cleaners”.

Angels and Deceptions

Angelfish have elongated, sometimes feathery dorsal and ventral (top and bottom) fins, which end in long, thread-like filaments, and a crescent-shaped (lunate) caudal fin. When viewed sideways with the head oriented upwards, the fish’s body resembles the silhouette of an angel’s head and wings — earning the fish its name. Their flattened bodies allow them to slip into cervices easily to avoid predation, and their large fins enable them to execute graceful twirling and undulating movements in the water.

Angelfish are some of the most conspicuous fish on the reef, with striking colouration and patterns that instantly draw the eye. Adults appear remarkably different from juveniles, a common feature across the marine realm. However, this is even more pronounced among angelfish, to the extent that the juveniles of some species hold absolutely no resemblance to the adult — often appearing like a separate species and confusing observers. These juveniles are significantly brighter and more vibrantly patterned than their adult counterparts, their appearance bordering on being psychedelic. For example, electric blue juvenile emperor angelfish do not look like the same species as adults.

There are two theories for this marked difference in appearance. One theory suggests that by looking so dramatically different, the juveniles escape competitive attacks from territorial adults of the same species. Another suggests that the bright colouration advertises a lack of palatability, and that they are merely cleaner fish (feeding on fish parasites) and pose no threat or competition to other fish in terms of food or prey. Whatever the reason, these juveniles can sometimes leave you puzzling over their identity.

Courtship and Social Structures

Courtship behaviour typically includes several elements, including colour displays, synchronised swimming in the water column, grunts or other sounds, and mates orienting their bodies to each other at specific angles. Males of some species change colours in response to a female signalling her readiness to mate, while other pairs orient their bodies at specific angles for a few minutes before commencing a “dance”.

Some pairs swim upwards in the water column, circling each other slowly with their beautiful fins trailing behind them. Others swim together in long arcs, with the male close behind the female, diverging only as they descend. Some others also swim side by side slowly, just above the sea bottom. During these swims, the male often swims with his snout, “nuzzling” the female’s vent (opening from which she lays eggs). Males also frequently put on a lateral display — flashing their sides to the female, flicking their fins repeatedly. 

Angelfishes are pelagic spawners, which means they release free-floating eggs in the water column, away from any substrate or bottom. This form of spawning exposes the eggs to predation. To compensate for the eggs that will inevitably be lost through predation and to maximise the survival of enough offspring, pelagic spawners like angelfish release up to several hundred eggs in one go.

Several angelfish species are protogynous hermaphrodites — they start life as females and change into males based on social cues. They live in harems — several females to one dominant male — and in the event the dominant male dies, the largest and most robust female transitions over time into a male and takes over. In some species, this change has been recorded to take just 14 days!

It is a universal fact that people like pretty things, and angelfish are extremely pretty. They are also robust and adaptive (and they don’t bite!), making the smaller species a popular choice in the multibillion-dollar global marine aquarium trade. Much like other marine life, angelfish are at risk due to warming oceans and other effects of climate change that impact oceans. However, being as resilient as they are eye-catching, these fish continue to mesmerise scuba divers and snorkellers with their blues and hues, swimming slowly among the corals.