By Karan Deshpande and Nupur Kale
Use the island to orient yourself, stay close to your dive buddy, and see you on the other side!” instructed the dive leader as our five-member squad was about to plunge into the murky green waters that appeared to promise very little.
However, looks can be deceptive! Roughly three metres in, the visibility began to clear, revealing a bustling coral reef. As we descended into the aquatic world, we were welcomed into an underwater city shaped by corals and algae, jostling surgeon fish, and sneaky groupers and butterflyfish that hovered over the coral substrate. Parrotfish glided past us while sea urchins were serious about social distance-ing! Stonefish ensured that you maintained your buoyancy but hermit crabs made you want to take a closer look. The underwater denizens were in their rush hour while their coastal neighbours were yet to start their day.
Increasing intensity and frequency of natural and anthropogenic stressors are impacting coral reefs around the world, leaving scientists to document degraded habitats more often than ever, such as these massive coral mounds in the lagoon of Kadmat Island in Lakshadweep, India. Photo: Karan Deshpande.
In an era where rapidly degrading coral reefs are becoming the norm, the scientific community is identifying corals that are resistant to the many threats affecting the marine realm. These tenacious corals form large colonies that culminate into a resilient reef and protect the local marine biodiversity from changes in the environment. Such reefs have the potential to be climate refugia i.e., underwater sites that can tolerate the devastating effect of climate change in the future. In India, one reef that could serve as a climate refuge lies in the coastal waters of Goa, tucked away from tourist frenzy.
In early 2022, we visited Goa to identify an ecologically significant site that could be protected using area-based management strategies. As a Bengaluru-based team, Goa was within our reach considering COVID-related restrictions; but from an ecological perspective, our task was not straightforward as there were plenty of sites to choose from. We finally selected the coral reef of Grande Island, a lesser-known coastal feature not too far from the coastline. As it is a reef habitat near one of the most commercial coastlines, we were not expecting to witness any marvels. Besides, as ecologists who had experienced and fallen in love with the fabled reefs of the Andamans and the Lakshadweep archipelago, the prospect of diving at a mainland reef with potentially poor visibility did not excite us. A robust survey and over 10 dives later, our snobbish opinions were trashed for we had witnessed the ‘underdog’ of reefs in India!
While a conventional reef flourishes within sparkling blue waters that surround oceanic islands and atolls, we saw that this reef was thriving in greenish-brown murky waters and was perennially soaked in terrestrial run-off from its urban neighbourhood. Within the nutrient-rich waters, the metre-wide whorls of plate corals, swaying polyps of flowerpot corals and massive coral colonies nurtured a surprising diversity of reef fish, sponges, urchins, stingrays, and even welcomed the occasional sallies by migratory animals such as sea turtles and whales. Thriving so close to a heavily populated coastline, Grande Island’s reef’s survival was a mystery!
On our journey home, we considered different factors that could enable the Grande Island reef, situated in the turbid waters of Goa, to persist in the face of adversities such as coastal construction, marine pollution and sedimentation. How was the reef able to endure modern-day climatic disturbances and anthropogenic threats?
We began by tracing the history of Grande Island Reef through the experiences of dive operators and professionals. Common observations were that of a dip in the abundance of carnivorous fish (i.e., trevallies and barracudas) and the degradation of a shipwreck, a popular diving site. But surprisingly, most professionals agreed that the corals remained unchanged. Prompted by their responses, we examined our photos and observations from the surveys and realised that the answer perhaps lay within the coral families and their colonies at Grande Island.
Turbinaria sp. plates at Grande and Saint George Islands grow in psychedelic patterns, forming massive whorls. As slow-growing platy structures, these corals appear to be thriving for decades. Photo: Karan Deshpande.
Corals form the backbone of every reef – literally, with their calcified anatomy, and figuratively, owing to their function. As building blocks of underwater cities, different families of corals not only form the architecture of each habitat but also play an important role in the reef's response to alterations to its environment. Popularly, most coral families are distinguished by their dissimilar forms and resemblance to structures, such as the staghorn coral, brain coral, flowerpot coral, plate coral, and so on. While the physical form visually entices humans and underwater beings alike, it is the life-history strategies of the corals that hold the key to their survival, and strength against frequent stressors. At Grande Island, the reef’s solid foundation comprises hardy structures of ‘plate corals’ of the Dendrophyllidae family, colonies of carpet-like ‘encrusting’ Faviidae and Psammocoridae families, and massive mounds of Plesiastreidae, Merulinidae and Poritidae families. Studies that evaluated species traits of corals found these families (amongst others) to possess certain life-history strategies that make them ‘stress-tolerant’ i.e., the corals that adapt to conditions under constant stress or disturbance.
Corals belonging to families such as Poritidae, Faviidae, Merulinidae, Plesiastreidae, Psammocoridae and Dendrophyllidae have traits that include slow to moderate growth rates, large corallites (to maximise surface area for photosynthesis), long generational time and high fecundity, i.e., the ability to produce many offspring during spawning (or reproductive) events. These traits are particularly advantageous in habitats such as the Grande Island since it is plagued by low sunlight, continuous sediment load and stressful environments. Another ecological feature of the Grande Island reef that contributes to its ability to withstand extreme conditions is its structural design owing to the presence of massive, plate and encrusting coral forms. These corals have a higher tolerance to low-light conditions as they cover larger surface areas and maximise light absorption. The presence of larger corallites also facilitates efficient shedding of sediment as the corals frequently experience disturbances on the seabed.
Commonly referred to as flower-pot corals, Goniopora sp. initially grow as encrusting corals and eventually takes the shape of massive life forms. Their polyps extend outwards, giving it a bigger appearance as they sway in the direction of the water currents. Photo: Karan Deshpande.
Does it then mean that the coral colonies at Grande can win the battle against climate change and anthropogenic threats? The habitat has withstood the burgeoning pressures from natural and anthropogenic disturbances, but new enemies and old foes continue to haunt this reef. Nutrient-rich waters and heavy metal pollution around the island have introduced coral diseases on the reef, while ambient conditions favour the opportunistic growth of Clionaid sponges that compete with coral, suffocate corallites, and leave entire massive colonies to perish. Old foes in changing climatic conditions, increased storm surges in the Arabian Sea, and the predicted mass-bleaching event linked to the El Niño Southern Oscillation event question the future of the biological fortress encircling Goa’s coastal island.
A single coral consists of a colony of corallites, each unit acting independently as well as in a group, carrying out important life functions such as feeding (both by photosynthesis and through polyp tentacles) and reproduction (budding and broadcast spawning). Corallites are also used to differentiate between coral families based on their skeletal structures and size. Corallite size, represented by measuring the diameters, ranges from 0-6 mm. (top left; Porites sp.), 8-15 mm. (top right and bottom left; Goniastrea sp. and Plesiastrea sp.), or more than 15 mm. (bottom right; Favites sp.) in specific coral families. Photo: Karan Deshpande.
The Grande Island reef that we see today is not entirely ‘immune’, but rather a product of modern-day climatic disturbances and anthropogenic threats, making it a seasoned fighter but perhaps not necessarily safe. The ‘stress-tolerant’ reef is a result of decades-long processes of building and gradual recoveries from environmental pressures encountered by the Goan coast. While the corals belonging to ‘resilient’ families could still emerge winners in certain conditions of reduced light exposure in turbid waters, studies suggest that prolonged exposure to sedimentation, pollution and coral pathogens could push colonies (irrespective of their structure) beyond their thresholds and cause tissue death, bacterial infections, and periods of starvation. The colonies may perish or weaken, making the reef vulnerable to degradation and substrate takeover by algae or sponge, followed by the gradual disappearance of reef-associated fish and other faunal communities.
All is not lost though! Whether these unique and hardy life history strategies of corals indeed enhance the habitat’s ability to survive future adversaries remains to be seen. Our community of scientists, SCUBA divers and tourists certainly need to collaborate for this exercise. To ascertain the reef’s long-term survival and ability to serve as a climate refuge, it is imperative to examine not only the biological pillars that sustain this reef but also the social and economic aspects that contribute towards its ecosystem functioning. This could be accomplished through sustained efforts at studying the reef and documenting changes over the next few years. Simultaneously, the corals also require the help of their urban neighbours in managing stressors that include simple steps such as sewage management and adopting responsible dive tourism and fishing practices.
At several sites around the island, corals and sponges (top) are observed competing for space. The unchecked growth of sponges such as the Cliona sp. (bottom) can take over live coral and grow over entire colonies that eventually starve and perish. Photo: Karan Deshpande.
While the coral families at Grande Island continue to serve as the first line of defence against modern-day climatic changes, a joint effort to protect the reef will go a long way in constructing a sanctuary for safeguarding its inhabitants and boosting its ecosystem functioning.
Karan Deshpande and Nupur Kale A marine biologist interested in coral reefs and participatory management models, Karan works with the Wildlife Conservation Society – India. Nupur is interested in all things marine and is currently exploring the beaches of Wales and the world of movement ecology.
