Butterflyfish, The Canary on the Reef

Flitting boldly about the world’s coral reefs, the vibrantly colored and charismatic butterflyfish are helping provide an early warning system.

This small, flat fish is named for the spots on its tail that look like eyes, and are meant to confuse predators about the fish’s direction of fleeing.

The charismatic butterflyfish that grab our attention as they flit boldly about a coral reef, is found in all tropical seas of the world. They are often the most common and conspicuous fish on the reef and there are around 120 separate species of butterflyfish in the genus Chaetodont.

Where are butterflyfish found?

Almost all coral reefs are home to these disc-shaped, highly colored fish as they flit seductively, often in synchronized pairs, in and out of the coral. Measuring around 12-30cm, the largest being the lined butterflyfish, they come in a variety of shapes but all have a flattened disc-shaped body, often adorned with extended fins. They are then, unsurprisingly, amongst the most photographed species on a reef.

What animals eat butterflyfish?

Relying on their striking markings, rapid movements, and agility, butterflyfish are well equipped to evade the many hungry predators on the reef. The fish’s compressed disc shape allows them to maneuver along the reef easily and quickly and to hide in small areas between corals. By further erecting their fins, dislodging them from their hiding place can be impossible for a would-be predator. And their remarkable colorations are also used in defense and predator evasion. Instead of swimming away, butterflyfish will often turn sideways to make their colorings even more visible.

Many have a false eye positioned towards the back of the fish. This false eye is to confuse a predator as to which part of the fish is the head and therefore its most vulnerable part. The real eye of the fish has a dark stripe to hide it, and the butterflyfish can evade injury and winning time for escape from a lurking predator. Butterflyfish are common on the reef during the day but as dusk approaches, they can be seen looking for suitable hiding places on the reef, under a coral head or overhang in which to spend the night safe and protected.

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Are butterflyfish omnivores?

They are a diverse group and feed on a range of reef inhabitants; the coral polyps, small invertebrates, and planktivores. The coral-eating species (corrallivores) tend to live in monogamous pairs living closely with the coral for both food and shelter. They have defined home ranges or territories on the reef. Spending time watching the fish move around visiting corals will quickly reveal where their home range extends. Butterflyfish make pair bonds based on size; the larger males defend the territory while the females spend more time feeding, safe in the knowledge that the men are out there protecting them and their food source.

A Canary on The Reef

It is the spectacular coloration of this group, coupled with their close association with the coral, that makes them so useful to science. That individual fish and pairs can be recognized have made it easy for scientists to follow individual pairs over several years. They have all the necessary characteristics of a coral reef “canary” that is; an early warning system or indicator of coral reef health.

Coral reef scientists have been using them as such for many years. So-called “biological indicators” are used to detect low levels of chronic environmental changes such as pollutants, or changes in nutrient or temperature levels. Catastrophic changes on reefs are obvious so such sensitive indicators are not useful. Devising any sampling method for such low-level changes can be costly and this is where our biological canaries come in. It is the coral feeding butterflyfish, most particularly the chevron and redfin butterflyfishes, and their close association with the coral, that have proved most useful for coral reef scientists.

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Many species feed exclusively on coral and further, show a preference for certain types of coral, so their sensitivity to changes can be relatively easily recorded. Their long lifespan (up to 12 years) and their strong attachment to a specific area make them ideal for longer-term study and for picking up small and gradual changes on a reef.

As the environment of the reef declines and coral food get gets poorer in quality and scarcer, changes in butterflyfish numbers and behavior will be detected. In studies where corals were removed from butterflyfish territories, pairs of fish tried to enlarge their territories resulting in increased fights with neighborhood pairs of fish. Such changes in behavior are an early indication of changes in the reef.

Once a reef becomes truly stressed, fish are likely to leave to find a more suitable habitat. The use of butterflyfish as an early warning system has been highly effective in prompting reef managers to investigate further the causes behind such low level but potentially critical, changes.

In some areas of the world these brilliantly colored, charismatic, and highly useful fish, are considered overfished for food and the aquarium trade and are now further at risk as coral reefs continue to decline globally.

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