Bright blue-and-yellow melon butterflyfish (Chaetodon trifasciatus) that exclusively feed on corals tweak the way they eat to survive on bleached reefs of the Lakshadweep archipelago, in the Arabian Sea waters, reports a study published in Ethology – the International Journal on Behavioural Biology.
The reefs have been assailed by severe bleaching events caused by an abnormal rise in ocean temperatures.
The researchers said identifying and documenting species such as the melon butterflyfish that are able to buffer the negative effects of bleaching (at least temporarily) and shedding light on how they manage to do that, is “relevant” in the wake of future bleaching events related to climate change, that are likely to affect all species intimately associated with corals, like the butterflyfish.
Although coral reefs cover only a tiny fraction (less than one percent) of the earth’s surface and less than two percent of the ocean bottom, they host about 25% of all ocean species that depend on reefs for food and shelter.
Often referred to as “rainforests of the sea”, coral reefs are shallow water ecosystems composed of the skeletons of coral, which are marine animals. They are made up of hundreds to thousands of tiny soft-bodied creatures called polyps that secrete a hard outer skeleton of limestone (calcium carbonate) that attaches either to rock or the dead skeletons of other polyps.
Residing within the coral tissues are algae called zooxanthellae which live in a unique mutually beneficial partnership. The algae receive protection from the corals and use the coral’s metabolic waste products for photosynthesis. The corals thrive as the algae produce oxygen, remove wastes and pass on the organic products of photosynthesis to the corals to grow and build up the reef.
The algal pigments lend colour to the corals. Since they remain fixed to rocky bases in the reefs, food comes to them. The corals catch planktons with their tentacles as the microscopic animals drift along with water currents. The reef-building corals require warm temperatures for their survival.
Factors such as increased sea surface temperatures may shock corals to expel their symbiotic zooxanthellae, which leads to bleaching (turning white), and in many cases, death.
During the 1997-1998 El Nino season, extensive and severe coral reef bleaching occurred in the Indo-Pacific and Caribbean, including in Lakshadweep, India’s smallest union territory which is an archipelago consisting of 36 islands spanning an area of 32 square km.
The coral reefs in India thrive in temperature ranges from 24 to 29 degree Centigrade that prevail across the year. In the summer season, during the day the corals experience partial bleaching due to soaring temperatures but they recover when the mercury drops down frequently due to cloud cover and pre-monsoon showers among other factors. This is not lethal.
“When there is an anomalous rise in temperature that persists for more than a week or so, the heat levels cross the limit that the corals can withstand. This happened during the 1998 bleaching event brought on by an abnormal spike sea surface temperature due to El Nino. The temperature crossed 29 degree Centigrade and the increase persisted for more than two weeks and proved lethal for the corals. This was followed by another abnormal temperature spike in 2010,” Rajkumar Rajan of Zoological Survey of India told Mongabay-India.
In 1988, upto 80% of coral die-offs were reported in the Andaman and Nicobar reefs while between 40% to 60% of reefs were wiped out in Lakshadweep and 60% of corals in the Gulf of Mannar succumbed to bleaching prompted by warm surface waters.
The lesser-known Lakshadweep archipelago was also subjected to another major El Nino related bleaching event in 2010. These are India’s only atoll reefs, located a few hundred kilometres off the coast of Kerala.
In trying to understand how melon butterflyfish are coping with the changes to their coral shelters in the wake of the 2010 event, researchers Amod Zambre (from the National Centre for Biological Sciences) and Rohan Arthur (from the Nature Conservation Foundation) observed that the melon butterflyfish switch their foraging behaviour: they can be picky or generalist depending on how much and how good the food available to them is.
But while this flexibility allows the species to be resilient to bleaching, at least in the short term, it may not guarantee long-term persistence, they point out.
For the study, the biologists zeroed-in on three reefs in the Lakshadweep: Kadmat, Bitra and Kavaratti.
These reefs suffered differential bleaching mortality after the 2010 El Nino Southern Oscillation, so the sites represented a variety in terms of both bleaching losses and coral cover.
“Our fish surveys showed that densities of this species were pretty much the same across reefs with very different coral cover indicating it is more resilient than most other species. This trend has been observed by several researchers working in other tropical reefs and we wanted to know how foraging behaviour might contribute to this resilience,” Zambre told Mongabay-India.
So how did the fish maintain their numbers across the reefs despite the contrasts in bleaching damages to the study sites? The clincher came with the discovery that they were able to bounce back due to their ability to adapt their feeding behaviour in response to the changes they perceived in their surroundings.
“As coral cover declines, fish spend more time travelling between patches and less time feeding. In addition, though, when butterflyfish from coral-poor areas did find a coral patch, they fed longer, eating as much as they could before leaving to search for another coral,” Zambre explained.
Secondly, they become more generalist as coral cover declines. They eat species that they would usually avoid in coral rich reefs.
For example, in reefs like Kavaratti, where they have a buffet of choices, the fish become very picky, carefully pecking at only the choicest coral. In contrast, in coral-poor reefs like Kadmat, they voraciously eat whatever they can find – including coral genera that they would not so much as glance at in better reefs, the researchers stated.
Analysis of high-resolution video recordings of multiple focal fish provided clues to this behavioural plasticity. The data revealed that time budgets, bite rates and diet selectivity differed significantly.
Bite rates or the number of bites per unit time indicates how “compulsively” the fish are feeding and on which species. Bite rate data allows one to infer things about diet preferences.
“Bites are very, very clear and hence easy to observe. After you acclimatise the fish, they allow you to come very close to them. I took five videos and then used BORIS open source software to slow down the video and take screen shots to quantify what species of corals the fish were feeding on and how many bites they took,” Zambre elaborated.
The researchers argue in the study that this flexibility in behaviour, especially the ability to become generalist and increase bite rates, allows this species to survive on bleached reefs, something that most species of obligate corallivores cannot.
There is a catch though. According to Zambre, this flexibility may help the species survive longer. However, given that they aren’t able to devote longer periods for feeding, over time this adaptability might result in reduced body condition and reproduction and eventually, decline in population.
“While this might allow short-term persistence it may not guarantee long-term persistence. This has in fact been shown in other studies, where populations of melon butterflyfish decline after a three to five year lag period,” Zambre noted.
Species that are intimately linked to corals, such as the butterflyfish, are likely to be negatively affected, said Zambre, as the frequency and intensity of bleaching events is predicted to go up due to global warming.
“There is now also mounting evidence in support of these predictions. Lakshadweep has, since 2010, suffered at least one other mass bleaching and several smaller lower intensity bleaching events. In this context, our studies, which identified some species are able to buffer the negative effects of bleaching (at least temporarily) and elucidated how they manage to do that is relevant,” the researcher said.
“Additionally, since these resilient butterflyfish are only ones surviving on bleached reefs and the fact that they feed on those left-over corals, they are also the ones which could significantly impact post-bleaching recovery,” Zambre stressed.
According to marine biologist TT Ajith Kumar, principal scientist with the ICAR – National Bureau of Fish Genetic Resources, the findings of the research point to possible clues to the survival and recovery of these vitally important aquatic habitats.
Mass coral bleaching was observed in early 1980s due to the steady rise in sea temperature subsequently pushing the reef-building corals closer to their upper limits of temperature tolerance. More frequent recurrence, as predicted, is likely to further reduce both coral cover and diversity on reefs over the next few decades, he said.
“Many of the damaged reef habitats show resilience to bounce back. This calls for continuous monitoring programs in the Lakshadweep coral reefs to quantify changes and enhance public awareness. An effective management strategy is indispensable to minimise further damage to these already ailing reefs,” Kumar told Mongabay-India.
This article first appeared on Mongabay.