We often think of organisms and the environments in which they live as two very separate things. The environment is the space in which the organism lives, right? Well, in reality, it is never as straightforward as that. Environments shape organisms and organisms shape environments in subtle and in profound ways, and it is difficult to know where one ends and the other begins. In a new study led by NCF and NCBS, published in Scientific Reports, researchers found that in the coral reefs of Lakshadweep, wave conditions place strong limitations on which fish can access certain areas and on how much they can feed at these locations. Flat bodied species of fish that feed on algae have it the worst while more torpedo-shaped species seem to have much less trouble. These herbivores, in turn, can strongly modify the environment by mowing down algae that would otherwise limit new coral from growing.

Corals reefs are no longer what they were just a few decades ago. We know this, we’ve read multiple headlines about climate change and warming waters turning once-healthy reefs, teeming with life, to ghostly-white, dying, crumbling coral graveyards over the past few years. As the world in our oceans shifts to a new normal, it’s hard not to feel like the proliferation of these stories about dead and dying reefs are making us somewhat indifferent to them.

Far from being lifeless deadscapes, these reefs are still dynamic ecosystems, just functioning very differently from the healthy reefs they once were. This new normal is much more complex than first meets the eye and we are only just beginning to understand how different species are coping with the catastrophic changes of recent times. It turns out that for reefs that are in a state of perpetual recovery, what the herbivores do is critical.

Why is it so important to know what herbivorous fish are up to on reefs? Well, herbivorous fish play a key role in laying the foundations of recovery on reefs that have been battered by climate change disturbances. They graze on the algae that live on these reefs, making sure that it does not overgrow and take over every vacant spot. This ensures that there’s plenty of space available for new coral to settle and grow.

The study finds that how exposed a reef is to waves has a strong effect on the herbivorous fish that live and feed on that particular reef. Researchers found that the activity of herbivores on reefs that were exposed to stronger waves was different from those that lived on reefs sheltered from waves.

Let’s try and paint an imaginary satellite image of the three Lakshadweep islands where the study was conducted. The Lakshadweep is what the researchers of the study call an ‘ideal natural laboratory’ to study something like wave action on reefs because some parts of the islands are exposed to stronger waves than others. Says Pooja Rathod, one of the lead authors of the study,

The study consisted of a combination of detailed field observations (which basically consisted of hours of stalking different kinds of herbivorous fish as they gorged themselves on an algal buffet recorded on GoPro cameras), careful experiments to keep herbivores out (mesh cages constructed on top of reef substrate to keep out herbivorous fish and study how algae grows in the absence of these fish), and a small dose of sheer luck which allowed them to make rare observations on herbivorous fish larvae settling on these reefs. All of these collectively helped researchers gain insights into how wave exposure influences the kinds and numbers of herbivores on the reef and their behaviour as well.

What they found was that reefs exposed to strong waves (on the western sides of the three islands), had fewer species of herbivorous fish, the numbers of the fish were fewer too. And the ones that did live there seemed to be eating less algae than those that lived on the eastern sheltered reefs. Basically, these western coral reef kingdoms were less controlled by herbivores. The fish, however, had a stronger reign on the calmer eastern reef sites.

The study found that the shapes and swimming styles of fish played an important role in determining how they performed on wave-battered reefs. Laterally compressed fish (narrow from side to side, having a flattened body shape) were unable to do well here, likely because they find it harder to manoeuvre through rough waters. Fusiform or torpedo-shaped fish on the other hand, did better on these wave-battered reefs.

Researchers of this study were also extremely lucky to encounter a rare recruitment event of two species of surgeonfish on these reefs on an unusually calm February day in 2015. Says Rucha, who co-led the study together with Pooja Rathod,

This was a golden opportunity to better-understand recruitment patterns in reefs exposed to different intensities of waves. They documented this event at both a wave-battered reef site and a sheltered one in the island of Kadmat.

Larger numbers of both species of surgeonfish (striated surgeonfish and spot-cheeked surgeonfish) larvae settled on the sheltered reef than on the wave-battered reef in Kadmat. These settlers seemed to prefer the stable and complex structures of reefs in calmer waters.

Herbivorous fish are the gardeners of reefscapes. How their activity is distributed across the reefscape can therefore determine how these reefs function. As this study shows, their distribution is lumpy, limited by wave action and by the ability of differently-shaped fish to access turbulent areas. In turn, the herbivores can determine how algae can grow and therefore are critical to the survival of coral settlers. When coral are able to successfully settle, healthy reefs can bloom, recovering from calamities we often cannot imagine reefs ever recouping from. Says Rohan Arthur, one of the authors on the study,

There has never been a time more important than this to take a deeper look into the ways in which fish behave and drive recovery in post-disturbed reefs if we are to imagine a future where reefs are healthy and bustling with life again.

A video put together by the authors of the study

Read the full paper here.