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Adapting to bleached reefs: The story of a corallivore

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Reefscape showing bleached corals along with bleaching resilient corals species- Pocillopora and Porites. Melon butterflyfish usually avoid the latter in unbleached reefs but are forced to feed on them when bleaching kills their preferred corals
Reefscape showing bleached corals along with bleaching resilient corals species- Pocillopora and Porites. Melon butterflyfish usually avoid the latter in unbleached reefs but are forced to feed on them when bleaching kills their preferred corals   (Photo: Amod Zambre)

“Ice ages have come and gone. Coral reefs have persisted” - Sylvia Earle

An underwater dive in a coral reef is a mesmerizing experience. Amongst the many vibrant life forms present, one that is crucial for maintaining reef health is the butterflyfish. Now, with coral reefs under threat worldwide, a new study shows that certain butterflyfish species can show remarkable pliancy in their behaviour following reef bleaching. This study highlights how marine species deal with stressful events and habitat loss.

Over the last three decades, human-induced changes in global sea temperatures have given rise to a phenomenon called bleaching. Corals live in an interdependent relationship with an alga which is crucial for reef ecosystem health. When sea temperatures rise, the corals expel this algae causing the corals to lose their colour and appear white (hence ‘bleached’), and eventually die. Several species of butterflyfish are dependent on corals for food and are often referred to as “obligate corallivores”. In an event of bleaching, what happens to these fish? Do they survive or perish?

Reefscape and butterflyfish
Clockwise from left: Reefscape showing bleached and unbleached corals, Melon Butterflyfish, Triangle Butterflyfish and Red-tailed Butterflyfish (Photo: Melon Butterflyfish by Bernard E. Picton [GFDL or CCBY-SA 4.0], from Wikimedia Commons, all others by Amod Zambre)

To investigate this, scientists led by Amod Zambre from the National Centre for Biological Sciences and Wildlife Conservation Society-India programme and Rohan Arthur from Nature Conservation Foundation examined the densities and feeding behaviour of Melon butterflyfish across three reefs in the Lakshadweep Islands that varied in terms of their bleaching mortality and thereby the coral cover post the 2010 El Niño event.

While densities of two butterflyfish species (Chaetodon meyeri and Chaetodon collare) were poor in the island that had the highest reef mortality due to bleaching, one of the species Chaetodon trifasciatus was able to persist at similar densities across bleached and unbleached reefs. This provoked the authors to examine how these species behaviourally adapt to bleached reefs.

The authors used high resolution video recording of several individuals of Chaetodon trifasciatus to estimate the time spent feeding, diet choice and number of morsels eaten by the fish while feeding across the reefs. They found that in the bleached reefs, Chaetodon trifasciatus spent less time feeding and more time travelling between coral patches, when compared to the reefs which had more live coral cover. In the bleached reefs, the fishes ate more morsels of coral and were not particular about their choice of food.

Melon butterflyfish on reefs
Photo: Bernard DUPONT from FRANCE (Melon Butterflyfishes (Chaetodon trifasciatus)) [CCBY-SA 2.0], via Wikimedia Commons

While such behavioural patterns provide important cues as to how species respond to environmental changes, the authors fear that these effects could be temporary. “Flexibility in foraging can buffer effects of bleaching only for a limited time. Though this behavioural change may be useful, it does not mean that they are immune to bleaching and populations may eventually crash,” says Zambre.

Nonetheless, the ability to forage in resource poor reefs demonstrates the adaptability of Chaetodon trifasciatus which allows it to live in sub-optimal conditions. “Understanding behavioural mechanisms that allow animals to cope with environmental stressors provides valuable insights on how populations are likely to respond to environmental change,” says Kavita Isvaran, a scientist at the Centre for Ecological Sciences, Indian Institute of Science, who was not associated with the study.

Though there are costs and limits to the flexibility in feeding strategies of Melon butterflyfish, this does allow them to persist on bleached reefs, longer than most other species. On healthy reefs, these fishes spent most of their time (~90%) feeding on corals. According to Zambre, though their numbers may eventually decline in bleached reefs, the extended persistence of these fish in these reefs may have important consequences for post-bleaching reef recovery.

Coral-corallivore relationships in marine environments can be compared to terrestrial plant-herbivore relationships. Ensuring the long term survival of both corals and corallivores is therefore important for the health and resilience of these reef systems.