Coral Reefs: the Underwater Metropolis

Have you ever wondered why we nearly always snorkel near coral reefs? It is because these intricate biological formations attract a lot of interesting marine life. Coral reefs are analogous to bustling metropolises. They attract and condense diverse marine life into a small area, with interdependent systems for food production, waste disposal, coastal protection, and building infrastructure. The biological systems powering this marine urban dwelling are fascinating.

Other high density ecosystems in the ocean are found in nutrient-rich zones like kelp or mangroves forests, hydrothermal vents, cold seeps etc. What is fascinating about coral reefs specifically is that they are incredibly sturdy, durable rock-like structures created by tiny invertebrates that can last for thousands to millions of years.

On my last snorkeling trip, I couldn’t help but wonder how exactly coral reefs form, why they are shaped in the way that they do, and what makes them so long-lasting.

who is creating all this underwater infrastructure?

Coral polyps are the tiny invertebrates responsible for building coral reefs. They absorb calcium from seawater, and secrete it back in the form of calcium carbonate (or calcite), creating a protective skeleton around their soft and vulnerable bodies. This protects them from predators and environmental damage. Moreover, having the ability to build marine infrastructure allows them to effectively compete for marine real estate, creating a stable environment to grow their colonies. Imagine what we, as humans, could achieve if our bodily secretions served as viable construction material?

These polyps wouldn’t stand a chance without their symbiotic soulmate - the microscopic algae called zooxanthellae. They live inside the coral’s tissue. Since algae can photosynthesize and coral polyps cannot, the algae are able to capture and convert sunlight into food for their host. In return, coral polyps provide algae with shelter and nutrients.

It’s a nice little barter system. Food in exchange for shelter.

Algae also help the coral polyps in calcification by regulating the coral's internal pH, while also helping them with waste removal by, uh, absorbing their waste.

why do coral reefs form where the waves constantly hit them?

Did you know that beaches are formed as a result of continuous wave action eroding rocks and depositing sediment? Over the course of millions of years, waves are able to eat away at rocks and cliffs pretty significantly, creating fine-grained sand. Then, why have coral polyps decided to put themselves in the line of fire? Why are reefs almost always where the waves are breaking, and how are they able to withstand wave action over hundreds to millions of years?

Reason 1: Sunlight

Coral reefs depend on the algae for food production, waste disposal, and a few other things like managing the water pH, which aids calcification. The sunlight is easily accessible near shallow waters, where waves tend to break, sadly.

Reason 2: Sediment Removal

Closely connected to the previous point, waves tend to remove sediment that would otherwise choke coral reefs. If waves didn’t remove sediment from reefs, the algae that power the reefs won’t be able to access sunlight and produce food for the polyps. Overtime, this would lead to reef death.

Reason 3: Temperature regulation

They want to be near sunlight, but they don’t want to get too hot. Waves regulate water temperature by mixing water layers, which keeps everything nice and cool.

Reason 4: Nutrient transport

Waves tend to circulate oxygen, nutrients, and larvae around, aiding in food production, reproduction, and waste removal.

To be continued...