Butterflyfish in a Coral Garden

Butterflyfish are among the most brightly coloured and common fish occupying coral reefs in tropical and warm temperate waters globally. Marked by their yellow, black, white, and sometimes red, blue, and orange colouration, they are a distinct sight.

Feeding habits

Butterflyfish mouths are well adapted to reach into crevices to pick small prey. All butterflyfish have protruding mouths with close-set, comb-like teeth which are a defining character of the butterflyfish and bannerfish family, Chaetodontidae (chaeta means bristle and odonto means tooth). The mouth length varies between species, based on feeding habits. For instance, the yellow longnose butterflyfish (Forcipiger flavissimus) has a very long snout and small mouth, making it easy to pick out small prey from deep crevices or pedicellariae (tiny pincers) off sea urchins without getting pierced by their spines. Others, with shorter snouts, can easily pick off polyps and other invertebrates from coral surfaces.

Many butterflyfish are facultative corallivores which means they derive most of their nutrition from a wide array of food, including corals. Some butterflyfish are obligate corallivores, i.e., they feed primarily on corals, with live coral polyps comprising as much as 80 per cent of their diet.

The dietary habits of these fish vary from generalists (feeding on a diverse range of food) to specialists (exclusively feeding on a few restricted corals). The generalists’ diet usually comprises live coral polyps, algae, plankton, sponges, worms, sea anemones, and, sometimes, small crustaceans.

Interestingly, butterflyfish make up nearly half of all the corallivore (coral-eating) fish recorded worldwide. Having evolved to adapt to such a diet, some butterflyfish species have also developed a mechanism to metabolise toxins some coral species may contain.

Anti-predator adaptations

These tropical fish, like butterflies, have brightly coloured bodies with various markings. On land, the eye tends to follow a tiny flash of bright colour that is a butterfly. Underwater, one might be momentarily stunned by the generous splash of butterflyfish yellow. Butterflyfish have a striking appearance with spots, bars, checks, and lines, on bright yellow and white base colours.

Many butterflyfish have dark spots towards the back of their body on each flank, called false eyespots or ocelli. In most cases, these spots serve to confuse predators into thinking the tail is the head. The predator, likely to attack the head or fins would then be tricked into attacking the tail, allowing the fish to escape unharmed. Common butterflyfish predators include sharks, eels, and large fish like snappers and groupers. It is also common to see butterflyfish with black stripes or bands running over their eyes. These bars or stripes camouflage the eyes, hiding them from predators.

These fish are diurnal (active during the day) and take refuge in crevices and under coral at night. While extremely conspicuous during the day due to their bright colouration, their colours appear duller at night, as dark blotches appear when the light fades.

Additionally, butterflyfish can alter their colours and appearance in response to social interactions and threats. When threatened, they extend their fins to appear intimidating, and turn their flank towards the threat, often with their head pointed downwards (called lateral display). Some studies have found a few species of butterflyfish also produce small sounds as part of this defensive response. 

Monogamous pairs

Butterflyfish adults can be observed in mated pairs or foraging aggregations ranging from a few to several dozen individuals, while juveniles are mostly solitary. The coral-dependent ones are often observed in monogamous, mated pairs that hold and maintain small territories together. Males can sometimes be slightly larger and do most of the territorial guarding, while females tend to feed more to meet reproductive energy requirements. Pairs may last from a few months to six to seven years. However, if a fish loses its mate, it finds another. Partners in a butterflyfish pair stay within sight of each other and tend to coordinate movements. If they get separated, one may swim above the reef to be more visible and spot the other.

Researchers have observed the social and feeding behaviour of butterflyfish, but some aspects are still not entirely understood. Since butterflyfish do not provide care to their young, theories suggest that monogamous pairing evolved in response to other external challenges, like the need to defend territories more efficiently or guard each other (from other potential mates or predators). 

Threats

Butterflyfish are rarely caught for consumption but are extremely popular fish in the exotic aquarium and pet trade because of their bright colours and compact size. However, not all fish belonging to the butterflyfish family are as bright or colourful. Some, like the phantom bannerfish (Heniochus pleurotaenia) and black pyramid butterflyfish (Hemitaurichthys zoster), are comparatively dull but just as important ecologically.

Factors like climate change, anthropogenic influences, and the resultant stress on corals, threaten butterflyfish and all marine life. While most butterflyfish species populations are stable and not at risk, some, like the chevron butterflyfish (Chaetodon trifascialis) are classified as “Near Threatened” on the IUCN Red List of Threatened Species.

Butterflyfish, like butterflies, are considered important indicators of climate and habitat changes due to their sensitivity to environmental changes. Given their live-coral-dependent diet, butterflyfish are considered good indicators of changes in coral cover or health of a reef. With a decline in healthy and live coral cover, these fish will move away from an affected reef to seek out healthier reefs for sustenance. Those that do not feed on corals exclusively (generalists) may adapt to changes in coral health and coral cover more easily than coral-dependent species. However, a healthy coral reef is likely to have a good mix of both groups of butterflyfish swimming around like a kaleidoscope of butterflies in a vast, thriving coral garden.