Caribbean Coral Roundup: Star Coral

Star coral may not be the flashiest species, but this important reef-building coral dominates Caribbean reefs. The Caribbean is home to four species of star coral in two genera; here we'll discuss how to identify all four species. If you're just learning how to identify coral, read about coral biology here to learn important terms for identification.

Star coral is an encrusting or platting species that forms rather large colonies. Each coral averages a few feet in diameter, but leave it to grow uninterrupted and there's no stopping star coral. We found a giant colony of mountainous star coral while diving in Honduras, a whopping 23 feet wide and 13 feet high, estimated to be over 500 years old.

Orbicella and Montastrea

There are three species of star coral in the genus Orbicella. Orbicella annularis and Orbicella franski share the common name "boulder star coral." The third species is Orbicella faveolata, or mountainous star coral. The final species is Montastrea cavernosa, or great star coral, and it is the only coral left in the genus Montastrea.

Until recently, all three Orbicella species were classified as Montastrea. Although they share star-like corallites, the overall colony shapes, coloration and — most of all — corallite size differs noticeably. Using molecular data and general appearance, a study from 2012 reclassified three Montastrea star corals into the Orbicella genus.

You will find a handful of other Caribbean corals which use star in their common name. We'll focus on Orbicella and Montastrea star corals as they are the most common and easy to identify. Check out this coral identification guide if you'd like to identify other star corals.

Boulder star coral (annularis)

Orbicella annularis grows in clusters of long. thick columns with domed tops. The small corallites extend above the coral's surface and are usually 5 mm across. O. annularis is mostly yellow but can also be gray or blue.

Orbicella annularis is commonly called boulder star coral because the pillars that the colony creates look like small boulders. If you look between the boulders, tissue and coral skeleton connects each pillar. If sediments build up between the pillars, the tissue can die but the colony will survive at the surface. 

Boulder star coral (franski) 

Orbicella franksi grows into large plates or encrusting mounds, and irregularly shaped nodules cover the colony's surface. This is the most uncommon Orbicella, as well as the deepest of all three species.

The corallites are shaped like small cones. They are irregular in shape and size, ranging from 2.5 to 4 mm in diameter. The coral is orange-brown, greenish-brown or grayish-brown, but the extremities of the lumps can be pale or white.

O. Franski is also called boulder star coral, and it looks much like other Orbicella species. The biggest difference with this coral is the corallite structure, which forms uneven clumps as mentioned.

Mountainous star coral 

The common name for Orbicella faveolata is mountainous star coral because colonies can grow to an impressive size, and the surface often features characteristic peaks and ridges like a mountain. Polyps are small at around 5 mm across. When Orbicella retracts its polyps, corallites have tiny grooves, which gives it a star-like appearance.

It's difficult to tell the corallite structure of all three Orbicella species apart. Thusly, focus on the colony shape, surface texture, and size to identify each species. If this is your first try at identifying star coral, focus on identifying Orbicella corallites. Next, try identifying each species.

Great star coral

Montastrea cavernosa is easy to spot on almost every Caribbean reef. This coral has bulbous, puffy corallites and grows in bright, eye-catching colors. Montastrea forms massive colonies, which can be flat or domed.

Montastrea corals can be a solid color like pink, blue, red, and green, or they can have contrasting corallites and tissue color. We've found blue colonies with yellow corallites, white colonies with brown corallites, and — my all-time favorite — the sunset Montastrea coloration in the image above.

These are commonly called great star corals because of their large corallites. This hardy coral can grow in most reef habitats, and you may even spot a colony while snorkeling.

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