Star corals are the quiet workhorses of Caribbean reef ecology. They don't have the headline-grabbing branching drama of the Acropora species, and they don't have the visual distinctiveness of brain corals. But for at least the last several thousand years, three species of Orbicella star corals have quietly built more Caribbean reef structure than probably any other coral group — slowly, patiently, one calcium carbonate layer at a time, colonies growing for centuries. This post covers the three Orbicella species (lobed, mountainous, and boulder star coral), the recent taxonomic story that split them from the closely related Montastraea genus, their outsized role in Caribbean reef building, their now-threatened conservation status, and what to look for on a Punta Cana dive.
The Taxonomic Story: Montastraea to Orbicella
Until relatively recently, the three Caribbean star coral species were all considered varieties of a single species called Montastraea annularis. Researchers had noticed morphological differences in colony shape, but the differences were subtle enough that scientists treated them as within-species variation for decades. In the early 1990s, careful multi-character analysis by Weil and Knowlton demonstrated they were actually three genetically distinct species: Montastraea annularis, M. faveolata, and M. franksi. Reproductive isolation between them was confirmed by direct genetic evidence in 1997.
In 2012, additional taxonomic revision moved all three species out of Montastraea into a new genus, Orbicella. The genus Montastraea now contains only one Caribbean species — the great star coral (Montastraea cavernosa) — which is a distinct but related species we'll cover briefly below. If you're reading older reef guidebooks or scientific literature from before 2012, you'll see all three star coral species classified as Montastraea. The current names in scientific and conservation literature are Orbicella annularis, Orbicella faveolata, and Orbicella franksi.
Lobed Star Coral (Orbicella annularis)
Orbicella annularis (also called lobed star coral or, confusingly, sometimes boulder star coral in older guides) is the most visually distinctive of the three species. It grows in clusters of thick, columnar pillars with domed tops, each pillar tapering slightly from base to top. Individual pillars are typically 20-50 centimeters across at the base with rounded tops. A single colony can contain many such pillars connected by shared tissue at their bases, giving the whole structure a distinctive lobed appearance from above. Colors are mostly yellow, occasionally gray or blue-gray.
Corallites (the individual polyp openings) are small — about 5 millimeters across — and extend slightly above the coral surface, giving it a fine texture up close. If sediment builds up between the pillars, the tissue in the crevices can die but the colony survives by continuing to grow at the pillar tops. Historically one of the dominant species of lagoons and upper reef slopes throughout the Caribbean. Now listed as Endangered on the IUCN Red List and Threatened under the US Endangered Species Act.
Mountainous Star Coral (Orbicella faveolata)
Orbicella faveolata grows as very large, solid colonies shaped like mounds with skirt-like plate extensions around the base. The surface is smooth and undulating, featuring small lumps, bulges, and lobes that give the coral its "mountainous" nickname. Corallites are uniform in size (about 5 millimeters diameter), with thinner walls than the other Orbicella species, giving the colony surface a smoother appearance overall. Colors range from pale brown through yellowish-green and gray, with deep brown specimens sometimes showing distinctive fluorescent green highlights.
Mountainous star coral occurs throughout the Caribbean and Gulf of Mexico in most reef environments, often forming very large colonies that dominate specific reef sections. Historically one of the most important reef-building corals in the region. Currently listed as Endangered on the IUCN Red List and Threatened under the US Endangered Species Act. The 2023 marine heat wave and ongoing SCTLD outbreak have both hit Orbicella faveolata particularly hard.
Boulder Star Coral (Orbicella franksi)
Orbicella franksi grows as massive clumps with distinctly uneven, lumpy surfaces — irregular nodules and bumps cover the colony giving it a rougher texture than its two sibling species. In shady positions it can grow as encrusting plates instead of massive clumps. Corallites are larger and more conical than the other two Orbicella species, of unequal sizes, and the coral tends to have more prominent costae (the ridges running down the sides of each corallite). Colors are orange-brown, greenish-brown, or grayish-brown, with the tops of lumps often paler or white.
O. franksi is typically found on fore-reef habitats between 15 and 30 meters, though its full depth range spans 5 to 50 meters. It's the most consistently deep-water of the three Orbicella species. Listed as Near Threatened by IUCN and Threatened under the US Endangered Species Act. Interestingly, O. franksi has shown slightly better resistance to bleaching than the other two Orbicella species — possibly because its deeper distribution puts it into cooler water during peak thermal stress events.
Also: Great Star Coral (Montastraea cavernosa)
Great star coral (Montastraea cavernosa) is not in the Orbicella genus, but it's often confused with them and worth knowing about. It's now the only Caribbean species left in the genus Montastraea after the 2012 reclassification. Distinguishing features: much larger corallites (about 10 millimeters across, compared to 5 millimeters for Orbicella species), a distinctive appearance where each polyp mouth is clearly visible as a large opening surrounded by a slightly raised rim. Colonies form massive rounded boulders or sheets. Very common throughout the Caribbean, and less severely threatened than the Orbicella species — currently listed as Least Concern by IUCN, though local declines have been documented.
Why Star Corals Matter So Much
Orbicella species have been quietly doing more of the actual Caribbean reef-building work than most people realize. As we cover in our post on how coral reefs are built, the massive dome-shaped and boulder-shaped corals contribute the bulk of the persistent reef limestone framework, while faster-growing branching species (like the historically-dominant Acropora) built out the shallow-reef complexity. In deeper reef zones between 10 and 30 meters, Orbicella species have been the dominant limestone contributors for millennia. Their extreme slow growth — about 0.5 to 1 centimeter per year of radial expansion — combined with individual colony ages of centuries to over a thousand years makes them among the most consequential coral species in Caribbean reef history.
Their ecological function extends beyond structure. Orbicella colonies are spawning grounds for many reef fish species that use large massive corals as reference points and shelter for reproductive aggregations. They provide microhabitat structure through their corallites and lobed surfaces. And their slow, steady limestone deposition has been the primary driver of vertical reef building over the last several thousand years since sea levels stabilized.
Threats and Current Status
All three Orbicella species are listed as Threatened under the US Endangered Species Act (2014 listing by NOAA Fisheries), and their IUCN Red List statuses range from Near Threatened (O. franksi) to Endangered (O. annularis and O. faveolata). The threats are the standard modern coral suite: warming water causing bleaching events (2005, 2023 particularly severe), disease outbreaks including yellow band disease (which specifically affects the Orbicella complex), Stony Coral Tissue Loss Disease (SCTLD) since 2014, hurricanes, sedimentation from coastal development, and predation by parrotfish that scrape algae from coral surfaces.
The slow growth that makes Orbicella species such important long-term reef-builders also makes them uniquely vulnerable — losses that would be recovered within decades for a fast-growing branching coral take centuries or millennia for Orbicella. A hurricane or bleaching event that kills a 500-year-old star coral colony sets the local reef structure back 500 years. Restoration is possible but the timescales are dramatically longer than for Acropora species like elkhorn and staghorn.
What Punta Cana Divers See
Star corals are among the most reliably visible massive coral species at our Cabeza de Toro dive sites. On a typical two-tank dive, you'll pass over multiple Orbicella and Montastraea colonies of varying sizes. The largest colonies you see (over a meter across) are almost certainly hundreds of years old, and some may be older. Distinguishing the three Orbicella species takes practice — look for the columnar pillars of O. annularis, the smooth mounded shape of O. faveolata, and the lumpy irregular surface of O. franksi. If in doubt, look at corallite size and spacing under close observation.
Because these are among the slowest-growing corals on the reef, sustainable diving practices matter especially. A meter-wide star coral you fin into may have been growing since before your great-great-grandparents were born, and its damaged tissue could take decades or centuries to fully recover — if it recovers at all under compounding disease and warming stress. Maintain excellent buoyancy, don't rest hands on massive corals for stability, and never touch.
The Bottom Line
The three Orbicella star coral species — lobed (O. annularis, distinctive columnar clusters), mountainous (O. faveolata, large smooth mounds), and boulder (O. franksi, irregular lumpy surfaces) — have quietly been the most important slow-growing reef-builders in the Caribbean for millennia. Formerly classified as Montastraea before genetic analysis split them in the early 1990s and taxonomically renamed them in 2012. All three are now threatened by disease, bleaching, and other stressors, with US ESA protection since 2014. Great star coral (Montastraea cavernosa) is a related but distinct species also common on Caribbean reefs. All are highly visible on typical Punta Cana dives if you know what to look for. To see them in person with someone who can point out the differences, message us through our contact form with your dates and we'll set up a coral-focused dive trip.
