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Life of Corals: The Reef Assembly

Within the magical world of coral reefs is an unimaginable marine diversity dense with life forms, and interwoven with unique symbiotic relationships and partnerships between critters

By Chetana Babburjung Purushotham

Very often we are told that coral reefs are the ‘tropical forests of the sea’ — beautiful, diverse, and important ecosystems. This comparison, however, falls short. It doesn’t describe how tremendously flabbergasting marine diversity truly is, and just how unimaginably concentrated with every kind of living creature coral reefs are.

This was the realisation I had during one of my first research ‘fieldwork’ dives when I was working on a project about corals in the Andaman Islands. I had to take calibrated photographs of coral colonies to identify them and measure their size. Sounded easy enough as a plan, but in reality it was very difficult to adhere to. I had not factored in the several different animals that would be hiding between the corals, under, or even inside them. There were fishes swimming past my frame, at times stopping to eat the subject of my research, or others that let me catch but a glimpse of their tails before disappearing. I was a fairly new diver at the time and I wanted to observe everything that caught my eye. I was eager to know who they were, what they were doing, and where they were going. My air supply and depth limits allowed me only an hour at a time, which was never enough. I was constantly overwhelmed and urged myself to focus on the task at hand.

My coral photographs ended up becoming windows into the magical world of reefs, and I could get lost in them for hours without having to worry about running out of air, although there were plenty of pictures that nearly took my breath away. Corals were the subject of my research, but I was also learning about the all the other critters that shared the same space, those that relied on corals, and others on which corals most certainly depended too. They made up what I referred to as the coral reef assembly.

Who goes there?

Of all the natural elements, the one that determines who survives and who thrives in the ocean is the water itself. The properties of water that influence us as we dive into the depths of the ocean affects its inhabitants too. Deeper the water, more the pressure, greater is the need for buoyancy and more pronounced is the loss of light. To survive in the ocean is to respect and embrace its currents. And of course, salt is the way of life. A combination of many of these physical and environmental factors has resulted in the evolution of some pretty bizarre life forms in the ocean.

The reef as home

Coral reefs are the product of an epic symbiosis between corals and zooxanthellae (Life of Corals: The Origin Story) and this is where 25 per cent of all marine diversity has a home.

Cute and feisty would be a good way to describe coral crabs. A peculiar and wonderful partnership forged by corals with survival benefits for everyone involved. Photo: Umeed Mistry  Coral reefs are home to 25 per cent of all marine life despite occupying less than one per cent of the ocean floor. Quite literally, they are bursting at the seams with diversity. Cover Photo: Umeed Mistry

Cute and feisty would be a good way to describe coral crabs. A peculiar and wonderful partnership forged by corals with survival benefits for everyone involved. Photo: Umeed Mistry
Coral reefs are home to 25 per cent of all marine life despite occupying less than one per cent of the ocean floor. Quite literally, they are bursting at the seams with diversity. Cover Photo: Umeed Mistry

Coral crabs are tiny armoured denizens of the reef that typically live in corals whose colonies grow into branches or ‘staghorns’. Coral crabs have evolved to fit snugly in the intricate crevices of branching corals, feeding on the mucous secreted by the coral polyps. In return, the polyps can depend on these crabs for their life, when predators with big appetites like the crown-of-thorns sea stars descend upon them. The crabs will show no fear when these enormous sea stars begin to unleash their stomachs to digest the seemingly helpless sedentary corals. Instead they will bravely step outside the confines of their colony and use their pincers to snip off as many tube feet of the predator as needed to protect their host.

Coral crunchers

Eating coral as part of your diet is not the most common trait if you are a marine animal. Corals being the foundation on which reefs are built can only be eroded and eaten into so much before the balance is tipped in the favour of competitors, such as macro-algae, waiting to take over. Around 3 per cent of all fish species feed on corals. This includes several species of butterflyfishes which even have mouths like forceps built for pinching and slurping out coral polyps. When not scraping off algae from the reef, parrotfish quite literally use the cutting-edge technology in their teeth to eat coral polyps along with their hard calcium walls, which are then pooped out as fine ocean sand.

Crown-of-thorns sea stars are aptly named for the numerous rows of venomous spines that dot their arms; a defence strategy that protects them from predators as they devour coral polyps. Photos: Chetana Babburjung Purushotham

Crown-of-thorns sea stars are aptly named for the numerous rows of venomous spines that dot their arms; a defence strategy that protects them from predators as they devour coral polyps. Photos: Chetana Babburjung Purushotham

The most extraordinary of all corallivores is the crown-of-thorns sea star (COT). These many-armed sea stars feed on coral by exuding their stomachs onto their colonies and consuming the polyps whole. In the 21st century, COTs are considered villains on the reef after widespread outbreaks have wiped out swathes of coral overnight. What we tend to forget is how humans have overharvested its primary predator — the triton snail (Charonia), while also making the ocean conducive for COTs to reproduce in unusually large blooms by increasing ocean temperatures and pollution.

Boring through

Coral reefs are not only extremely diverse, but also dynamic; while some animals like coral add to its foundation, others like the giant clam will gradually erode it as it builds its own shell. It is fascinating to think that corals are not the only ones to have figured out the benefits of partnering with the photosynthetic zooxanthellae. While giant clams filter feed on drifting plankton for nutrition, they depend heavily on food made for them by these microscopic plants that they host within their tissues. Giant clams are the largest clams known to man, capable of growing up to 4 feet in size and 250 kilograms in weight, during a lifespan that could easily be over a century.

Giant clams can be so large that they become mini-ecosystems themselves, providing space for critters on its shell or between its folds. Photo: Umeed Mistry Giant clams can be so large that they become mini-ecosystems themselves, providing space for critters on its shell or between its folds. Photo: Umeed Mistry

Giant clams can be so large that they become mini-ecosystems themselves, providing space for critters on its shell or between its folds. Photo: Umeed Mistry

The visitor

Coral reefs being bustling marine metropolises act as crucial pit-stops for nomadic animals that need to make food, or as cleaning stops during their migration across the seas. We also tend to forget that while reefs lie on the sea floor, as a habitat they extend all the way into the blue of the water column. This is where numerous top-of-the-food-chain predators such as barracudas, jacks, and sharks ominously hover.

Silvery and streamline barracudas patrolling the blue in large schools can make any smaller fish weak in the fins. Photo: Umeed Mistry

Silvery and streamline barracudas patrolling the blue in large schools can make any smaller fish weak in the fins. Photo: Umeed Mistry

The service provider

One of the most extraordinary interactions that encompass survival, cooperation, and adorableness among seemingly unrelated animals on a reef is that between cleaning agent and client. Deep tissue cleaning, mucous and parasite removal, scaling and dental work are just some of the numerous services offered by cleaners to the residents of the reef. The role of cleaning is most often taken up by shrimps and small wrasses that specifically feed on material that most fish want removed from their bodies. Trying to decode how they communicate with one other  — who needs a clean, where it needs a clean, who is satisfied, who is displeased — is something that continues to take hours of my time underwater even today.

A sabre squirrelfish getting a wholesome cleaning, as a cleaner shrimp works in its mouth and a cleaner wrasse tends to its scales.
Photo: Umeed Mistry A sabre squirrelfish getting a wholesome cleaning, as a cleaner shrimp works in its mouth and a cleaner wrasse tends to its scales.
Photo: Umeed Mistry

A sabre squirrelfish getting a wholesome cleaning, as a cleaner shrimp works in its mouth and a cleaner wrasse tends to its scales. Photo: Umeed Mistry

Completing the cycle

A marbled sea cucumber standing erect is signalling to others of its kind in the vicinity that it is ready to spawn. Photo: Umeed Mistry A marbled sea cucumber standing erect is signalling to others of its kind in the vicinity that it is ready to spawn. Photo: Umeed Mistry

A marbled sea cucumber standing erect is signalling to others of its kind in the vicinity that it is ready to spawn. Photo: Umeed Mistry

Sometimes dependencies and relationships between living beings on a coral reef are more subtle. It is not uncommon to be asked “What do sea cucumbers do exactly” following someone’s first encounter with seemingly lifeless cylindrical lumps lying lazily on sandy patches abutting a reef. Relatives of the more popular sea stars, sea cucumbers are some of the most crucial players in the coral reef assembly. Working quietly and moving slow, sea cucumbers use a row of tentacles around their mouths to feed on decomposing material in the ocean sand. By systematically sifting the sediment, sea cucumbers inadvertently supply oxygen to critters in deeper layers of the sand. Their poop has been found to reduce ocean acidification in its surroundings while also allowing several tiny invertebrates to live on its body.

We need to make sure coral reefs remain vibrant and healthy. Not just because they contribute to our climate, the air we breathe, our food, and livelihoods, but also because they are some of the most exquisite places on planet Earth. Photo: Umeed Mistry We need to make sure coral reefs remain vibrant and healthy. Not just because they contribute to our climate, the air we breathe, our food, and livelihoods, but also because they are some of the most exquisite places on planet Earth. Photo: Umeed Mistry

We need to make sure coral reefs remain vibrant and healthy. Not just because they contribute to our climate, the air we breathe, our food, and livelihoods, but also because they are some of the most exquisite places on planet Earth. Photo: Umeed Mistry

Understanding the life story of corals is not complete until we talk about our relationship with them as human beings. It would not be an exaggeration to say that coral reefs are like vibrant, living tapestries that are quite literally losing their colour. Despite having existed on this planet for hundreds of millions of years, never have they transformed so rapidly and significantly as they have in the past couple of decades. The key to protecting reefs lies in understanding how much of the life we lead so comfortably on land depends on them staying alive.

Chetana Babburjung Purushotham
Chetana Babburjung Purushotham

is a wildlife biologist and PADI divemaster based in the Andaman Islands. She is also an avid writer, keen on sharing her experiences with wildlife to help more people reconnect with nature.

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