If you’ve ever had the opportunity to dive or snorkel at a coral reef, you might’ve seen impressive coral structures. In Jamaica, I saw massive Boulder Coral, Acrapora species (like the Elkhorn Coral) that looked like alien trees, and Pillar Coral that appeared to be the main feature of the reef, from the perspective of a photographer.
In fact, if you look at pictures of reefs online, a lot of them have huge corals that draw the eye. Now, how do coral get that big? They only eat plankton and there’s only so much one can eat in a day, but they require a lot of energy for everything they do. Lucky for the coral, at least the reef building varieties, they don’t have to acquire all that necessary energy by themselves.
Let me introduce you to what is, in my opinion, one of the best examples of a mutualistic relationship in the animal kingdom. High on the to-do list of most coral polyps is to acquire the best of best friends in the ocean, zooxanthellae (zo-zan-THEL-ee).
This organism, or group of organisms, is a type of dinoflagellate (a special kind of algae) that forms a positive symbiotic relationship with creatures like coral and jellyfish. A symbiotic relationship is defined as any relationship between two or more individuals of the same or different species that lasts over an extended period of time. You and a pet have a symbiotic relationship.
For coral and zooxanthellae, they’re like best friends; not only do they live together, but they help each other, making it a mutualistic relationship. In exchange for a safe haven from predators like zooplankton, and necessary ingredients for health and photosynthesis like carbon dioxide and nitrogenous waste products (they eat coral poop), the zooxanthellae give coral their excess food.
That’s right, zooxanthellae are like those friends that bring huge amounts of food to your party or potluck and leave the leftovers with you, giving you meals for days. In fact, zooxanthellae provide up to 90% of the energy needed for coral growth and reproduction; they are the reason those Pillar Coral can grow as tall as you are and why coral reefs exist.
In summation, zooxanthellae are friends, not food! No, wait; that’s not quite right. Zooxanthellae are totally the friends you want to have in order to build a successful community, or if you want to mooch off their leftovers because you can’t make enough food on your own.

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