While most corals are colonial animals, many with hundreds to thousands of tiny polyps, one small group of corals lives alone. Solitary corals are a single-polyp species that lives freely on the ocean floor.
You can find most of these corals, which have developed ways to stay above the sand, living in muck habitats. Some solitary corals have large, fleshy tissue that allows them to float above the sand, while others have evolved the ability to pulse their tissue if they get buried. All of these corals have a mouth in the center of the polyp, which can be anywhere from 3 to 12 inches (7.6 to 30 cm) in diameter.
If you’re new to coral identification, check out our Coral Biology: Part I and Coral Biology: Part II articles. These will help you learn the basics of coral biology, as well as some helpful terms for identification. And if you’re new to Indo-Pacific Coral Identification make sure to check out our Introduction to Hard Corals of the Indo-Pacific.
Here are four solitary corals you may be able to spot on your next Indo-Pacific muck dive.
Heliofungia is commonly known as long-tentacle plate coral. You can often find little shrimp and, if you’re lucky, a pipefish living between the tentacles. These corals can be brown or green and, depending where you are, you may spot some extra -arge corals over 12 inches (30 cm) in diameter.
A diver with an untrained eye would likely think this is a sea anemone, as the long tentacles have a similar shape and size. But don’t be fooled because they are actually attached to a hard, solitary skeleton.
Acanthophyllia is the king of all solitary corals. The skeleton of an adult can be up to 6 inches (15 cm) in diameter but the large, fleshy tissue surrounding it can be double that in size. The fleshy texture and bright colors make this coral special — you won’t find any other single-polyp corals with such a bright appearance.
These corals have a large mouth in the center of the polyp. At night, the tissue will close up and feeding tentacles will come out. Using their sticky tentacles, acanthophyllia corals will try to catch zooplankton and even small fish.
Cycloseris are smaller disk corals that have a more skeletal, hard appearance. Interestingly, this coral boasts a wide range of colors and patterns. We’ve found this coral in bright red, green, yellow, and orange, and even some disks that have multiple colors. Look for a small ring around the edge of the coral to spot an extra-rare specimen.
At times, you can see little tentacles poking up from in between the skeleton ridges. The disks are usually no more than 3 inches (7.6 cm) in diameter, and sometimes you will find large quantities in muck habitats. Cycloseris can pulse their tentacles and the tissue beneath to unbury themselves from the sand, should they get covered by storms, waves, or sediment from divers’ fins.
We’ve truly saved the best for last. If you’ve been following along with our coral series, you will recognize this is the last coral on our list — but definitely not the last on our list of personal favorites. Trachyphyllia is my ultimate favorite coral and I truly have a soft spot for this solitary species.
And perhaps I’m cheating just a tad to include it on this list, because you can often find this coral with multiple mouths — which defeats the solitary definition. However, as I’ve mentioned, corals are like chameleons and sometimes don’t quite fit the mold. Juvenile colonies of this species with have a single mouth and live a solitary life for a decade or more before growing large enough to multiply into a second, connected polyp.
These corals are brilliantly colored, with striped patterns and psychedelic colors. For a real treat, bring a UV torch on your dive and watch these corals light up. The large tissue helps them float above the sand and is often two to three times the size of the skeleton beneath.
If you liked this series, take the knowledge you’ve gained on your next dive and head out coral spotting!