Giant/Boulder Brain Coral

A boulder brain coral (Colpophyllia natans) growing on a coral reef. Photo taken by D. Alex Mustard, more can be found at

Domain: Eukarya
Kingdom: Animalia
Phylum: Cnidarian
Class: Anthozoa
Order: Scleractinia
Family: Mussidae
Genus: Colpophyllia
Species: C. natans

First let me state that I have a love/hate relationship with common names. Most people call the Colpophyllia natans the Boulder Brain Coral; however, there are some texts and articles that call it the Giant Brain Coral. Why is this frustrating and worth mentioning? I spent a good chunk of time trying to confirm that these guys are the same species with different common names—I really didn’t want to make a fool of myself!

C. natans are named Boulder Brain Coral because they typically form large rounded structures that look like, drum roll please, boulders. They can also form large rounded plate-like structures, encrusting over rocks and existing coral colonies.

Like other brain coral, C. natans look like someone gave a chisel to a child and told them to go wild on the coral, creating a random pattern of valleys and ridges on the surface. A thin groove runs along the very top of the ridges, though you usually can’t see it when diving because you’re too far away (and if you’re not, you should be).

A second line is found halfway down the ridge where the angle decreases and slopes to create the valleys. The valleys are usually long and wandering, almost path-like, but sometimes they’re closed up and look more like individual cells squished together. The coral polyps are found within the valleys; the long ones containing multiple individuals, while the closed ones hold one or two polyps.

The ridges and valleys are normally different colors from each other. Ridges are either brown or gray while the valleys can be green, tan, or whitish.

You can commonly find them on reef tops or seaward reef slopes in the tropical waters of the Gulf of Mexico and the Caribbean. They can grow to have a diameter of up to 16 feet and can live as long as 100 years! C. natans are extremely popular tourist attractions, especially in the Florida Keys. But divers aren’t the only things these guys attract; they also attract all kinds of fish, including some gobies that live permanently on the coral.

I’ve only ever seen these guys during day dives—which, trust me, is still really cool to find them because of their size and coloring. However, they’re even better to see on a night dive because that’s when the polyps let out their tentacles to fish for zooplankton. I’ve been told that the coral can look completely different at night, and I can’t wait to see it for myself in person!

Sources and cool links:
Coral Reef Identification: Florida, Caribbean, Bahamas 3rd Edition by Paul Humann and Ned DeLoach
Ocean The Definitive Visual Guide made by the American Museum of Natural History

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