A recent article in PLoS Biology about coral reef bleaching caused my thoughts to turn to tiny creatures that live inside corals, the zooxanthellae (zoo-ZAN-thell-ee)! These are the little beasties that give corals their fantastic array of colours, along with being one of the most fun biology terms to scream ("ZOOXANTHELLAE!").

Zooxanthellae are not actually beasties sensu stricto; they are algae, which are unicellular plants. So what the hell are algae doing living inside coral? Like lichens, coral have formed a symbiotic relationship with algae. By forming this association, the coral (an animal) receives photosynthetic products (sugars) and the zooxanthellic algae (a plant) receives shelter and the carbon dioxide it needs to survive. Also like lichens, the coral/zooxanthellae symbiotic relationship is "obligate". That is, neither the coral nor the zooxanthellae could survive on their own if they were separated.

Weird under-sea animals aside, obligate symbiosis is by no means an exotic phenomenon in the natural world. For example, the mitochondria and the cells of your body are in an obligate symbiotic relationship. That's right, mitochondria are in reality totally separate organisms from humans. They even have their own DNA. Like the zooxanthellae of plants, the mitochondria produce a resource (adenosine triphosphate) in exchange for a safe and happy environment in which to live their tiny lives in peace.