Orbicella
A genus of major reef-building stony corals in the Caribbean.
Orbicella is a genus of stony corals belonging to the family Merulinidae. It includes three closely related species: *Orbicella faveolata*, *Orbicella annularis*, and *Orbicella franksi*. These are all shallow-water corals that host symbiotic zooxanthellae and are found naturally in the tropical western Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. They are widespread and play a key role as major reef-builders throughout the Caribbean.
For a long time, scientists mistakenly grouped these corals into a single species, *Montastraea annularis*, based on their similar colony shapes. Three growth forms were recognized—"bumpy," "columnar," and "massive"—which were thought to result from environmental factors like depth and light. Later research on their ecology, reproduction, genetics, and physical features challenged this classification, leading to the recognition of three separate species: *Montastraea faveolata* (massive), *M. annularis* (columnar), and *M. franksi* (bumpy). In 2012, a taxonomic revision placed this group into a new genus, *Orbicella*, with the three species now named *O. faveolata*, *O. annularis*, and *O. franksi*. Common names include boulder star coral for *O. annularis* and mountainous star coral for *O. faveolata*.
The colonies are massive, forming dome-shaped mounds with uneven surfaces and bulging projections. Their corallites are small and tightly packed. The corals are usually light brown, though some have green oral discs.
The skeleton is made of calcium carbonate in the form of aragonite. Growth rate is linked to depth. The skeleton also contains brucite, a magnesium hydroxide mineral, found in the spaces between the septa, where it encrusts microbes living inside the coral.
Speciation in *Orbicella* involves reproductive isolation. These corals are broadcast spawners, releasing gametes into the water on the same evening each year, typically in August or September—the warmest months—five to eight nights after the full moon. Some researchers claim that *O. annularis* and *O. faveolata* spawn at the same time, but most evidence suggests the species are separated by a few hours. This timing is thought to be controlled by light levels and perceived sunset or moonrise times. Sperm and eggs are released in small bundles that break open at the water’s surface due to surface tension. Timing is crucial for gametes to meet.
Quick Facts
- Taxon
- Orbicella
Facts from the source article.
Lore & Background
The Orbicella species complex was historically lumped into a single species, Montastraea annularis, based on similar colony morphologies. Three morphotypes—'bumpy', 'columnar', and 'massive'—were believed to arise as responses to abiotic factors such as depth and light availability. This classification was later challenged by ecological, reproductive, genetic, and morphologic evidence, leading to the re-description of three separate species: Montastraea faveolata (massive), M. annularis (columnar), and M. franksi (bumpy). A taxonomic revision published in 2012 established that this complex formed a separate clade now in the genus Orbicella.
Characteristics of Orbicella colonies include massive, dome-shaped mounds with uneven surfaces and bulging projections. The corallites are small and closely packed. Colonies are mostly some shade of light brown, sometimes with green oral discs. The skeleton is made from CaCO3 in the crystal form of aragonite, and growth rate has been correlated with depth. The skeleton also contains brucite [Mg(OH)2] in the interseptal spaces, which encrusts microbes growing inside the coral skeleton.
Speciation in Orbicella involves reproductive isolation. These species are broadcast spawners, releasing gametes annually in the same evening into the water column, normally in August and September, five to eight nights after the full moon. Most reports support that the species are temporally isolated by a few hours, with timing dictated by light availability and perceived sunset/moonrise times. Hybridization studies reported some success crossing O. annularis and O. franksi in laboratory conditions, but crosses with O. faveolata were consistently unsuccessful. These laboratory crosses are unlikely to occur in nature, and gametes in nature dilute and age quickly, reinforcing reproductive isolation.
Reader's Guide
The significance of Orbicella lies in its role as a ubiquitous and major reef-builder in the Caribbean, contributing substantially to the structure of shallow-water coral reefs. The genus is notable for its complex taxonomic history, having been originally misclassified as a single species (Montastraea annularis) due to similar colony morphologies. The reclassification into three distinct species—Orbicella faveolata, O. annularis, and O. franksi—was driven by multiple lines of evidence including ecological, reproductive, genetic, and morphologic data, and was formalized in a 2012 taxonomic revision. This revision highlights the importance of detailed biological study in understanding coral diversity. The legacy of Orbicella includes its use as a model for studying speciation mechanisms in broadcast-spawning corals. The species are reproductively isolated primarily by temporal differences in spawning timing (a few hours apart) and by gamete incompatibility, as shown by unsuccessful crosses with O. faveolata. These barriers allow for species distinction and ongoing evolution. The presence of brucite in the skeleton, associated with microbialites, adds a unique mineralogical aspect to these corals. The common names—boulder star coral for O. annularis and mountainous star coral for O. faveolata—reflect their distinctive growth forms.
Did You Know?
- Orbicella species spawn annually in August and September, five to eight nights after the full moon, and are temporally isolated by a few hours.
- Crosses between Orbicella faveolata and other Orbicella species are consistently unsuccessful, even in laboratory conditions.
Frequently Asked Questions
What is Orbicella?
Orbicella is a genus of shallow-water stony corals in the family Merulinidae, native to the tropical western Atlantic, Caribbean Sea, and Gulf of Mexico. It comprises three closely related species that all host symbiotic zooxanthellae and serve as major reef-builders throughout the Caribbean.
How many species belong to the Orbicella genus?
There are three recognized species: Orbicella annularis (boulder star coral), Orbicella faveolata (mountainous star coral), and Orbicella franksi. All three share similar colony morphology and ecological niches.
Where do Orbicella corals naturally live?
You'll encounter them across the tropical western Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. They are widespread shallow-water corals throughout that entire region.
Why were Orbicella corals once lumped under Montastraea annularis?
For a long stretch, scientists treated all three species as a single taxon, Montastraea annularis, because their colony shapes look nearly identical at a glance. Later taxonomic work recognized enough distinct traits to split them into the separate genus Orbicella with three species.
Why is Orbicella important for Caribbean reefs?
As a major reef-building genus, Orbicella helps construct and maintain the physical framework of shallow-water reefs across the Caribbean. Its three widespread species contribute heavily to the structural complexity that supports the broader reef ecosystem.
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