Despite rampant bleaching in the coral reefs of Lakshwadeep, certain species of butterflyfish have displayed remarkable survival skills. By adapting their foraging habits and dietary preferences, these resilient beauties have made the best of this rather grim situation.
The gentle blue light, the feeling of weightlessness, and the silence, broken only by the sound of my own breathing, were collectively having an effect on me. I was slowly drifting into a trance, feeling peaceful and more relaxed than ever before. Suddenly, there’s a flash of colour, and I am back to reality.
I was about 10 metres below the ocean’s surface over a gorgeous coral reef, off the island of Kadmat in Lakshadweep. I was here to study the reef fish ecology, part of my Master’s research project, and had a plan to spend five months in the area. Located about 350km off the western coast of India, the Lakshadweep archipelago comprises about 36 islands and sand banks. Part of an extensive mountain range called Chagos-Laccadive Ridge, the islands in Lakshadweep rose above the water surface when fine white sand from surrounding coral reefs started accumulating on top of the submerged peaks. The coral reefs that continue to flourish around the islands are amongst the best in the world, and they form protective atolls around the islands. These atolls attract a diversity of fauna that rival even the best rainforests around the globe. Species of all imaginable shapes, sizes and colours inhabit these reefs, making them one of the best sites for underwater diving as well.
Although remote and largely untouched, these reefs are not immune to the effects of global warming. Corals are very sensitive to increases in water temperature and are at the risk of getting bleached. Bleaching sets in when the symbiotic zooxanthellae (the photosynthetic algae associated with corals) is expelled from the coral tissue, subsequently resulting in coral death. When corals are exposed to warm water currents for prolonged periods, mass bleaching sets in. This can wipe out entire coral reefs and adversely affect the complete marine ecosystem. The reefs in Lakshadweep have experienced two such mass bleaching episodes over the last decade and have lost a significant proportion of their corals. This, in turn, has had a massive negative impact on the fish species associated with the reef.
Among the worst affected are the butterflyfish. These flamboyantly coloured fishes are highly territorial and feed almost exclusively on corals. Their high dependence on corals for nutrition and their limited ability to disperse makes them particularly vulnerable to bleaching. After mass bleaching, most species have shown drastic population declines; many like the Chevron Butterflyfish (Chaetodon trifascialis) and the Triangle Butterflyfish (C. triangulum) have even become locally extinct. However, some like the Melon Butterflyfish (C. trifasciatus) show little or no decline in population and continue to persist on reefs with very little live coral. What makes these species resilient to bleaching? What do they survive on, on such heavily bleached reefs? These were some of the questions I was hoping to find answers to during my time in Lakshadweep. I had a strong hunch that the answers might lie in their foraging behaviour. But to be sure, I had to get close to the fish, follow them and observe them foraging for long periods.
Back to my senses in the water, these unanswered questions once again filled my thoughts, until I came across a shoal of curious looking Hump-headed Tangs (Naso tonganus). As I continued my descent, the bright blue and pink colours of an open anemone caught my eyes. The long, tubular tentacles swaying gently in the current were quite a sight! Among the tentacles was a colony of Orange Maldives Anemonefish, a species which shares a strong bond with the anemone. The tentacles of the anemone have stinging cells which deter most fish. The anemonefish, however, is immune to the sting and seek refuge in the tentacles, thus avoiding predators. In return for the protection provided, the anemone gets food particles accidentally dropped by the fish. This mutually beneficial association has bound the two species so closely that neither can now survive without the other!
Once closer to the reef, it did not take long to spot a pair of Melon Butterflyfish, cruising effortlessly over the corals. They were swimming together; mirroring each other’s every move. Suddenly, they slowed down and started hovering over a section of the reef. As I caught up with them, the reason for the abrupt stop became obvious. They had spotted a patch of live coral! It was a massive Porites colony that had survived the bleaching. On a reef which had lost over 40% of its original coral, this was definitely a welcome sight for the corallivorous butterflyfish. The pair began a detailed examination of the colony, inspecting each and every crack and crevice in the coral. This was probably done to check for the presence of any predators around, but to me, it seemed like how a hungry person would react in front of a lavish buffet, confused over where to begin from. After a quick, but thorough inspection, the butterflyfish began their feast. It was quite a comical sight, like watching a kid slurping on noodles.
The feeding had only been going on for a couple of minutes when one of them frantically swam away. The other almost instantly followed the first. It was probably due to a Honeycomb Moray Eel, which had taken refuge in the numerous cracks in the coral, and had suddenly made itself visible. Moray eels are nocturnal predators, notorious for the painful bites inflicted by their sharp, conical teeth present in multiple rows. The eel flashed its weapons of choice at me, warning me to keep my distance.
I continued to follow the butterflyfish pair for the next 15 minutes. They swam far and wide, stopping only to check for remnant coral colonies to feed on. From their movement pattern, it was quite obvious that they were no longer territorial. They had become rovers, nomads, continually on the move in search of the few remaining live coral colonies on the reef.
I wanted to spend more time with the butterfly fish but was almost out of the air. As I slowly started ascending, I got a full view of the bleached reef. Dead coral skeletons remained, now covered with blue and green algae. Only a handful of the erstwhile diversity of the fish is still here.
Back on dry land, as I go through my observations, the secrets behind the Melon Butterflyfish’s survival are slowly starting to reveal themselves. It seems like this particular species has an amazing ability to change its foraging habits. On heavily bleached reefs with few live corals, the Melon Butterfly fish is not choosy about its diet. It will eat practically any live coral species it finds. In contrast, Melon Butterflyfishes on healthy reefs are extremely choosy, feeding exclusively on a few coral species and rejecting most others. This elasticity in their feeding preferences is likely to be one of the factors contributing to their continued persistence on bleached reefs.
There are, however, costs associated with living on bleached reefs. Individuals living in such habitats have to spend a greater amount of time, and therefore energy, travelling between coral patches which are spread far and wide on the reef. Whether their unique ability to change their dietary preferences can alone offset the cost associated with increased travelling remains unclear. But, at least for the time being, the Melon Butterflyfish continues to be resilient, surviving against all odds posed by rampant coral bleaching in their natural habitat.
Cover Photo: Bleached corals lose the symbiotic algae, which results in them losing their vibrant colours as well.
Currently a Project Assistant in Dr Maria Thaker’s Lab at the Centre for Ecological Sciences, Amod Zambre is studying lizard stress physiology and behaviour. He is an alumnus of the NCBS-WCS India Master’s course in Wildlife Biology and Conservation and has previously worked on coral reef fish, snakes and scorpions.