Re: what are the animals that live inside corals cells and what do they do?

Hi Abigail:

Most tropical corals (as well as some anemones and jellyfish and all the giant clams) are full of brown, photosynthetic cells called algae (not bacteria); any such algae living in coral are called "zoothanthellae". To be more specific, these zooxanthellae (which appear as tiny spheres, having diameters only 8 times larger than a bacterium)are a type of dinoflagellate. They live inside the inner of the two cell layers of coral in enormous numbers. So, when you see a healthy, brown coral, it is the dinoflagellates that give it its characteristic color. (Blue, purple or pink corals have their own colorful pigments that mask the brown algal color).

Like all plant-like cells, zooxanthellae contain chlorophyll and produce oxygen and sugar. A sugar by-product, glycerol, is released to the coral cells. This provides most of the energy corals need to produce their hard, calcium carbonate skeleton. (Corals that live in caves or in deep, dark water grow much more slowly, since they lack zooxanthellae; they get all their food from plankton.)

Sometimes the zooxanthellae are expelled, and the living coral becomes transparent. This is called "bleaching" (the white of the underlying coral skeleton becomes visible) and is typically caused when water temperatures are too high (i.e., above 30 or 31 degrees C). Bleaching can even be caused by too-cold water or others types of stress. Coral bleaching is a great concern to many biologists. It has gotten much worse in the past few decades, with the super-El Nino of 1998 killing huge amounts of coral in Australia, Palau and elsewhere. Many scientist blame global warming. If the bleaching is not too severe or long-lasting, the zooxanthellae (which become swimming cells outside the coral) can go back into the coral, and it slowly re-establishes to its brown coloration. Otherwise the coral dies.

This partnership between coral and zooxanthellae has been called the most perfect example of symbiosis, two different organisms living together for their mutual benefit. The coral gets its food and some of its oxygen from the algae, the algae gets its nutrients (nitrogen and phosphorus)and CO2 from the coral.