Coral and Algae: A Remarkable Partnership

In my post, Corals: Nature’s Greatest Builders, I wrote about how corals build reefs, but I didn’t write about how corals themselves grow.  Here is an explanation (like the previous post, I am focusing on hard corals):

By Nbharakey (Own work) [Public domain], via Wikimedia Commons

Coral polyps with their tentacles extended. Photo by Nbharakey, via Wikimedia Commons


Most corals start as a single polyp.  The polyp divides in two**, then those divide into four and so on.  As they divide, hard corals create limestone beneath themselves, adding it to the the coral’s skeleton.  So a single hard coral actually consists of a colony of polyps that build the coral’s skeleton as they multiply.  Mature hard corals can have thousands of polyps covering their skeletons.

The polyps are usually small, with tentacles surrounding a mouth-like opening* and stomach.  The polyps extend their tentacles(usually at night) to catch food floating by in the water and pull it into their mouths.  They retract their tentacles for protection.  The polyps share the nutrients from their food with algae that live inside their bodies. Algae are like plants–they get their energy from the sun–and the algae inside coral polyps share nutrients with their hosts.  The algae help the coral grow, and the coral help the algae.  The algae are so important to the coral, that corals have a very difficult time building their skeletons without their algae partners.

Coral polyp

*It’s not exactly a mouth.  The polyps’ food enters through this opening, but it exits through it as well.

** Some corals whose polyps don’t divide and remain singular throughout their lives, but the majority of corals do form colonies.

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