Bright, multicolored cactus coral grows in every habitat from the shallowest to the deepest parts of the reef and make the perfect subjects for coral photography. Once scuba divers and snorkelers know what to look for, they should have no problem identifying these corals.
Cactus corals grow into thick, encrusting plates or large domes and are fairly common throughout the Caribbean. And they only grow in the Caribbean — nowhere else in the world.
There are five species: knobby cactus coral (Mycetophyllia aliciae); low-ridge cactus coral (Mycetophyllia danaana); rough cactus coral (Mycetophyllia ferox); ridged cactus coral (Mycetophyllia lamarckiana); and (Mycetophyllia reesi). The last coral, M. reesi, is a ridge-less deep-water species. Here we’ll focus on the first four.
Telling the species apart is less important than just being able to identify this type of coral. If you’re new to coral identification, here are a few introductory articles to learn the basics of coral identification and helpful terms.
Knobby cactus coral (Mycetophyllia aliciae)
This one gets its name from the short knobs that develop in the center of the coral. You can also distinguish this coral by looking at the white marking that resembles flowers surrounding each polyp.
Perhaps the most important feature, and what sets this coral apart from other Mycetophyllia species, is the number of corallites between each ridge. If you find two or more rows of polyps between ridges, the coral is most likely a knobby cactus coral. Other Mycetophyllia species typically have one row of polyps, or two sets of polyps near the edges in younger developing colonies.
Low-ridge cactus coral (Mycetophyllia danaana)
This coral forms round plates with low ridges. The coral is thicker in the center of the colony tapering to a narrow perimeter. The low-ridge cactus coral is the chunkiest of the Mycetophyllia species.
This coral’s ridges can connect in the center of the coral, forming long, meandering valleys. The ridges can also break apart, forming little nodes called monticules. It can be challenging to differentiate between species of Mycetophyllia. While some will be clearly distinctive, others straddle the line between two species, especially between the low-ridge and ridged cactus coral.
Rough cactus coral (Mycetophyllia ferox)
Spot rough cactus coral tucked into caves or on the side of rocks, particularly on dives deeper than 30 feet (10 m). This species forms thin plates with narrow valleys and short ridges that often connect around polyp mouths. This is the only Mycetophyllia species that has ridges that connect around polyps.
Ridged Cactus Coral (Mycetophyllia lamarckiana)
The ridged cactus corals have distinct scalloped edges and continuous ridges that run from the edge of the colony toward the center. The ridges, however, do not connect in the center, which is a distinguishing characteristic of this species.
Coral identification is not easy, and it can be tricky to tell these four species apart. It’s easy to mistake small colonies of ridged cactus coral for low-ridge cactus coral. Some scientists think ridged cactus coral could just be juvenile low-ridge cactus corals that haven’t developed their full skeletal ridges. It’s most important to recognize the coral as part of the right family by looking for the fleshy surface and bright, colorful patterns.
To learn more about Caribbean corals with a detailed description of each one check out this guide to Caribbean corals.