In any underwater documentary that you might have seen on channels like Discovery or National Geographic, amidst all the diverse flora and fauna, one particularly colourful and varying type of structure might have caught your attention. Yes, we are talking about coral reefs, the rainforests of the ocean. Coral reefs are underwater structures built of calcium carbonate that is secreted by corals. Corals are marine invertebrate animals that exist in colonies, and certain corals, who are designated reef builders, secrete the calcium carbonate that goes on to form the hardened exoskeleton that we now know as coral reefs. What makes this underwater structure unique is its diversity. While occupying merely 0.1% of the world’s ocean’s surface, they are home to more than 25% of its species and contribute in a multitude of ways to the environmental and geological stability of our planet. And at this point in time, they are in the worst condition they’ve ever been in. To understand why they’re important what led to their current predicament, we have to start from the basics of coral reefs.
The builders of the rainforests
One common misconception associated with coral reefs is that they’re alive. It would be akin to saying that hair, or nails, are alive by themselves. As stated earlier, coral reefs are formed by corals that are from the same family of creatures as sea anemones. That family is known as Cnidaria, and they have a common characteristic – each instance of an organism in this family is known as a polyp, whether it be a sea anemone or a coral. A polyp, in the case of corals, can be described as a tin can open at one end with tentacles coming out of the said end. Coral reefs aren’t built over the lifetime of a single coral – it takes hundreds of generations to produce the complex coral reef structures we see.
There are more than a hundred species of corals that can be found around the planet. Their exoskeleton colonies can form quite a visual display, from round, folded brain corals that resemble a human brain to tall, elegant sea whips and sea fans that look like intricate, vibrantly coloured trees or plants.
Corals, like almost all living things, survive by consuming other beings. They use their stinging tentacles to catch small sea life like plankton and small fish. Most corals, however, have a symbiotic relationship with algae called zooxanthellae (pronounced zo-zan-thel-ee). The algae that live inside the coral bodies are also a source of food. They make food for themselves and the coral, and in return get shelter and carbon dioxide. They’re also the reason for all the bright colours because most polyps are clear and colourless.
If all the coral reefs in the world were to be put together, they’d cover all of UK and Switzerland!
Although there’s debate between marine scientists as to exactly how reefs should be classified, there are mainly five types:
- Fringing reef: directly attached to a shore, or borders it with an intervening narrow, shallow channel or lagoon.
- Barrier reef: Separated from a mainland or island shore by a deep channel or lagoon.
- Platform reef: also called bank or table reefs, which form on the continental shelf, as well as in the open ocean, is the seabed rises close enough to the surface of the ocean.
- Atoll reef: more or less circular or continuous barrier reef extends all the way around a lagoon without a central island.
There’s a particular characteristic of coral reefs that is paradoxical – their very occurrence. First theorized by Charles Darwin in The Structure and Distribution of Coral Reefs in 1842, this paradox refers to the unlikely proliferation of coral reefs in tropical waters which contain few nutrients and conditions for their growth. In fact, the largest and strongest corals grew in parts of the reef exposed to the most violent surf and corals were weakened or absent where loose sediment accumulated.
So how do these oases of oceanic diversity come to be in the middle of aquatic deserts? Scientists explain it with something called the Island Mass Effect (IME) that describes a variety of factors that come together to create ideal conditions for coral reef formation.
Geology and bathymetry determine the structure of the sea floor and contribute in determining how much of it receives sunlight and is shallow water. Upwelling brings nutrient-rich water from the depths to the sunlight zone. Birds and fish contribute through their waste material. Even human activities like runoff from a nearby agricultural area can affect the IME.
Corals at risk
These beautiful aquatic ecosystems, as mentioned in the beginning, are at severe risk. And it’s not just the usual CO2 to blame. Yes, excessive CO2 emissions have led to acidification of the oceans, which in turn hinders the formation of the calcium carbonate essential for coral reefs… but that’s not all.
Fishing also affects corals. Methods like blast fishing (using explosives) or cyanide fishing (using cyanide to make fish easier to catch) can destroy a thousand-year-old complex reef ecosystem in minutes. The irony is, it is the same fishing industry, along with tourism and other areas, that comprise the $30bn+ global reef economy who are now tasked with reef conservation! Even if you don’t care about fish or the biodiversity of our oceans, the protection from storms that reefs provide coastal cities, once missing, can lead to devastating consequences!
How to protect coral reefs
Most coral reefs are dying because of rising temperature, which makes them eject the symbiotic algae, and eventually kills them. There are efforts to save them: in some regions, people are setting up sunshades to reduce the impact of solar heating. Cold water from deeper oceans is being pumped into the reefs, which also helps in nutrient cycling.
Scientists are leaving no stone unturned to save our reefs, even resorting to gene editing to identify and inhibit thermal vulnerability in corals. The CRISPR method is being trialled, in the hope that we’d be able to engineer coral that’s resistant to higher temperatures. Instead of it happening naturally, we’re trying to indulge in some artificial selection. Why? Because when the oceans start dying off, there’s a good chance we will go with them! Weather changes with climate change, and we really don’t want tsunamis wiping us out!