CORAL 101: The Basics

Harriet Tyley

So we've all heard of coral reefs, but if I asked you what a coral was, would you honestly be able to answer? If yes, then nice! If no, then continue reading to find out...

Photo: Harriet Tyley @harriettyley

Photo: Harriet Tyley @harriettyley

Corals are (in my opinion) one of the world’s most simplistic yet complex animal. Corals are the only animals visible from space, yet they begin life as tiny little entities, called polyps, floating through the ocean. Corals are invertebrates belonging to the Phylum Cnidarian, making them similar to other marine organisms such as jelly fish and hydroids. All phylums are further ordered into classes, and the corals belong to the class Anthozoa, which is made up of multiple coral types, gorgonians and sea anemones.

Corals are the only animals visible from space

The corals we see from space are built by sclerectinain hermatypic species (or more simply, hard corals), and these corals are commonly nicknamed the ‘oceans architects’. This nickname stems from their amazing biological ability to build hard skeletons made up of calcium carbonate. The primary role of this calcium carbonate skeleton is to protect the delicate soft tissues of the coral polyp.

A polyp is made of an internal layer, the endoderm, and an external layer, the ectoderm, and sandwiched between these layers is a gloopy substance called the mesoglea. The polyp’s sac-like body has a base, which is attached to a substrate, and at the other end has a mouth surrounded by tentacles. These tentacles capture food in the night, which is then passed through to the mouth, and also act as defensive structures. Tentacles are often armed with specialised stinging cells called nematocysts, which can be used to capture and immobolise prey, or to defend territory. The polyp is the living coral animal, and these tiny polyps are the creators of coral reefs.

The polyp is the living coral animal, and these tiny polyps are the creators of coral reefs.

Hermatypic corals, or the architects, deposit calcium carbonate in the same way a 3D printer produces its masterpiece. Layer after layer build on top of each other, and after millions of years, immense and vast reef systems such as the GBR are created.

It’s undeniably impressive that such small organisms are capable of producing reefs of such grandeur and scale. However, it would be unfair to give all the credit to corals seeing as they get a lot (and I mean, a LOT) of help from their endosymbionts, the zooxanthellae. Inside the tissue of a coral polyp, thousands of microscopic dinoflagellates (algae) reside. Zooxanthellae are the energy factories of corals, and these tiny single-celled organisms fix as much as 90% of the energy used by corals.

Photo: Harriet Tyley @harriettyley

Photo: Harriet Tyley @harriettyley

The energy fixed by these zooxanthellae is used by corals for reproduction, growth and metabolic processes. In return, these microscopic zooxanthellae receive rent-free accommodation in the prime location of the euphotic zone, giving them premium access to the world’s largest producer of energy, the sun! Corals have a lot to thank zooxanthellae for. Without these symbionts corals would not be able to produce enough energy to produce calcium carbonate skeletons. Zooxanthellae also enable corals to grow in oligotrophic (low-nutrient) waters. Without zooxanthellae, corals would not be able to create the vast structures we know and love, so corals really owe them a lot!


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