What are corals? Animals, vegetables, or rocks?

Why can an animal be as hard as a rock, grow in so many different shapes, and have so many different colors? The corals, what are they?

What are corals? Animals, vegetables, or rocks?
The conservation and management of coral reefs will allow us to continue enjoying their beauty. Photo by Matteo Vella / Unsplash

You have seen corals, in photos, videos, or images on the internet. They are those structures of various sizes, shapes, and colors; that form large extensions of reefs (the submerged structure that provides food and shelter) where many fish and other marine animals live, so they have been called "marine rainforests".

How can an animal be as hard as a rock, grow in so many different shapes, and have so many different colors? Starting from the basics, we will define a coral as an invertebrate organism, i.e. without a skeleton, that lives in tropical and sub-tropical waters, depends on a rocky substrate to adhere to, clear water so that it can have light, as well as nutrients for food.

Are corals animals?

Yes, they are animals, but they are not just one animal, since a coral is a collection of millions of polyps that live together, working simultaneously to grow as a colony. To this colony can be added (by cloning) more polyps and in some cases join with another colony. But although these polyps live all together and are united in the same colony, each one individually grows, eats, and produces.

Although polyps form large structures, they are simple organisms and do not have defined organs, but they have important structures for their activities such as i) tentacles, to catch and eat microscopic organisms that are suspended in the seawater (zooplankton), ii) a gastric cavity that is like their stomach and also stores their gametes when they are in reproductive season and ii) nematocysts, cells like harpoons that shoot when they feel threatened or help to catch their prey.

If they are animals, why do they look like rocks?

They can generate a hard, white calcium carbonate structure that functions as an outer shell and a base for further growth; resulting in the appearance of a 'big living rock', known as hard corals or hermatypic corals. However, some corals do not have these calcium carbonate structures and are called soft or ahermatypic corals, which are easily moved by currents.

Why is it said that they are also vegetables?

Polyps live in a close relationship (symbiosis) with a very small alga, called zooxanthellae, which has pigments that give color to the coral and are responsible for photosynthesis to generate energy or food for itself and the polyp. Therefore, they need to live in places where there is enough light and nutrients, otherwise, the coral would lose its color.

What are corals used for?

Corals have several functions and benefits for humans: 1) they are feeding and sheltering sites for other organisms (such as fish, crustaceans, snails, and many other species in early stages of development), and 2) their large structures can serve as natural barriers to protect beaches when there are storms or hurricanes, 3) they are important for fisheries (mainly artisanal), being a source of income or food for local people and 4) they are visited by tourists, so they support the local economy.

Similar to other coastal ecosystems, coral reefs are affected by natural factors, but mainly by humans, such as the emission and dumping of pollutants, settlements, and constructions in the coastal zone (beaches), and irresponsible tourism. And given that not all reef communities can recover, actions have been generated to raise awareness, as well as restoration or rehabilitation for their correct (or sustainable) use, management, and conservation.

By Alma Paola Rodríguez-Troncoso, Marine Ecology Laboratory, University Center of the Coast, University of Guadalajara, Mexico.