Flexible Fish: new study from the Lakshadweep

In their bright yellow-and-blue finery, melon butterflyfish are hard to miss—you must have seen them on one of your dives. And if you’re like me, who hasn’t ever been on one, you’ve probably seen them swimming across your screen in some underwater video.

These fish—the melon butterflyfish—really love being around coral, and for good reason. Coral are their food and coral provide them with structures for homes—they depend on coral to survive. And so, the more damaged coral get on their reefs, the more trouble these fish face. Quite naturally then, the recent global bleaching events haven’t spelt good news for them.

Researchers Amod Zambre (from National Centre for Biological Sciences) and Rohan Arthur (from Nature Conservation Foundation) were interested to know more about how these coral-loving melon butterflyfish are coping with massive changes taking place to coral in their homes.

For their study, they chose sites across three different reefs in the Lakshadweep—Kadmat, Bitra and Kavaratti. Coral in these three reefs suffered from a varying degree of damage after 2010’s devastating El Nino.  So, while Kadmat suffered most from bleaching, Bitra was not as bad, and at Kavaratti coral were relatively unaffected.

A decline in coral would lead to a decline in numbers for these coral-eating butterfly fish too, right? But not so for these melon butterflyfish—they’ve maintained their numbers across all three reefs! The obvious question that Amod and Rohan asked themselves was, “So how do they do this?

And off they set on a journey to better-understand the secret to the tenacity of the melon butterflyfish. They followed fish around in all three sites, looking closely at how they swam, what they liked to eat and their eating habits.

After months of following these fish around”, says Amod, “it became quite clear that melon butterflyfish alter their foraging behaviour depending on how much and how good the food available to them is.

In areas where
coral is increasingly scarce, these butterflyfish spend more time travelling and less time feeding. This, we expected.

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.

And this flexibility was clearly critical to allow them to persist.

And what of their food choices? Amod continues, “The real secret to the melon’s ability to survive is that it can switch from being extremely fussy to very flexible about what it eats too. In reefs like Kavaratti, where it has a buffet of choices, it becomes very picky, carefully pecking at only the choicest coral. In contrast, in coral-poor reefs like Kadmat, it eats voraciously whatever it can find—including coral genera that it would not so much as glance at in better reefs.

Picture this: I’m a melon butterflyfish living in a healthy reef with plenty of pretty live coral all around me. And you, you’re another melon butterflyfish just like me, except you live in a unhealthy reef with mostly dead coral everywhere with a few bits of live coral scattered within your territory.

I live in a reef where there’s a massive buffet of different kinds of coral laid out before me. There are some kinds of coral that are more nutritious than others, and I can easily pick and feed on those—I can afford to be fussy and spoilt. I have plenty of coral and can eat at leisure, so I don’t need to gobble it all up in one go. I don’t even need to travel far away from my home in search of food because everything is right here beside me! And so, I spend less time traveling between coral patches.

You, on the other hand, don’t have it as easy. You have fewer options to choose from for food. Many of the coral that are in your home are either dead or low in nutritional value, and what’s worse—they’re all spread out, unlike the ones in my home. And so, to meet your daily nutritional requirements, you need to travel between coral patches in search of more coral.  And when you do find coral worth eating, you put all your energy into gobbling them up because you’re clever, you know it’s going to be hard work for you to find more food. And you’re way less picky than I am about which types of coral you eat—you eat ones that are lower in nutritional value too as you don’t have much of a choice.

Although I have it easier, you work hard, way harder than I do to survive. You’re displaying a special quality—something scientists like to call ‘behavioural plasticity’. And that’s what’s cool about you, fellow melon butterflyfish!

 

coral-richcoral-poor


Illustrations: Rohan Arthur
GIF: Janhavi Rajan

 

Alright, well… now time to snap out of our little melon butterflyfish world.

Enter reality. Climate-induced bleaching events are on the rise, and it’s heartbreaking to know that we’re losing more and more of healthy coral with each passing day—coral that’s important to so many lifeforms in our oceans, coral that many cannot live without.

And in these troubled times, the kind of behavioural flexibility displayed by melon butterflyfish—their supercool ability to alter their behaviour according to the condition of coral available to them—this represents something marvellous and critical.

This flexibility allows them to persist even in unhealthy reefs—it all gives them a distinct advantage over other species that aren’t able to cope well with these catastrophic changes.

Says Rohan, “Since I first started working on coral reefs, I have seen them decline—from the once-vibrant ecosystems of boggling diversity, to sad deadscapes struggling to cope with the frequent mortalities that climate change causes. Yet, even in these declining reefs, there are signs of tenacity that we could not have imagined before. 

Species like the melon butterflyfish show that reef creatures have a larger suite of behavioural plasticity than we ever thought possible. They are tougher and more flexible than we think. While many species are destined to die out because of climate change, these plastic species will emerge as clear winners. 

Whether this plasticity will also be sufficient to maintain all the critical functions  reefs require to thrive in the face of continued climate change, that’s anyone’s guess. But for me, species like the melon butterflyfish are reasons for some small optimism.

 

The full paper is up for early view here!

 

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