The evolution of complex life forms on our planet ultimately resulted from the fusion of simple, yet different, single-celled organisms during the early part of Earth’s natural history. Today all of life on our planet depends on the mutualism exhibited by many different kinds of both complex and simple organisms: plants, animals, fungi, bacteria etc.. Even as humans, we are highly dependent on the bacteria that live in our guts. Coral Reef ecosystems are a prime and vivid example of the importance of symbiotic interactions. Simple animals in the phylum Cnidaria have developed close relationships with single-celled, yet complex, species of dinoflagellate, an algal group important to the ocean’s plankton and marine food webs. These seemingly humble combinations are why coral reef ecosystems exist today; and why they are potentially endangered in the coming decades. As enthusiasts that grow, propagate, and trade in live coral come, learn more about these amazing mutualisms and how they function. Perhaps in doing so, raise your appreciation for them in ways previously unfathomable.