''Orbicella'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of stony corals in the

Merulinidae Merulinidae is a family of reef-building stony corals. Characteristics All the genera in this family are colonial, reef-building corals. Skeletal structures are similar to those of Faviidae but are highly fused, without paliform lobes. The valle ...

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

. The Orbicella species complex comprises three sister species, namely ''

Orbicella faveolata ''Orbicella faveolata'', commonly known as mountainous star coral, is a colonial stony coral in the family Merulinidae. ''Orbicella faveolata'' is native to the coral coast of the Caribbean Sea and the Gulf of Mexico and is listed as "endangered" ...

'', ''

Orbicella annularis ''Orbicella annularis'', commonly known as the boulder star coral, is a species of coral that lives in the western Atlantic Ocean and is the most thoroughly studied and most abundant species of reef-building coral Corals are marine inverteb ...

'' and ''

Orbicella franksi ''Orbicella franksi'', commonly known as boulder star coral, is a colonial stony coral in the family Merulinidae. It is native to shallow waters in the Caribbean Sea, the Gulf of Mexico, the Bahamas, Bermuda and Florida, and is listed as a "vulne ...

'', all of which are shallow-water,

zooxanthellate Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''Sy ...

species and are native to the tropical western Atlantic Ocean, the

Caribbean Sea The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...

and the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

. These corals are ubiquitous and major reef-builders in the Caribbean. Their similar colony morphologies misled scientists to historically lump them into a single species, ''Montastraea annularis'', which included three

morphotype In biology, polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative ''phenotypes'', in the population of a species. To be classified as such, morphs must occupy the same habitat at the s ...

s “bumpy”, “columnar” and “massive”. These growth forms were believed to arise as a response to

abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...

factors (e.g., depth, light availability). This taxonomic classification was challenged by further ecological, reproductive, genetic, and morphologic evidence, which led 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 the “''Montastraea annularis'' species complex” formed a separate clade now in the genus ''Orbicella'' with three species names (''O. faveolata, O. annularis, O. franksi''). ''O. annularis'' and ''O. faveolata'' are commonly called the boulder star coral and the mountainous star coral, respectively.

Characteristics

The colonies of these corals are massive and form dome-shaped mounds, with uneven surfaces and bulging projections. The corallites are small and closely packed. These corals are mostly some shade of light brown but sometimes have green oral discs.

Skeleton properties

''Orbicella'' skeleton is made from CaCO₃ in the crystal form of

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

. The growth rate has been correlated with depth. In addition, the skeleton also contains

Brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and ...

g(OH)₂in the interseptal spaces ( microbialites). These brucite particles encrusts microbes growing inside the coral skeleton.

Speciation and reproductive barriers

Speciation is the gradual process by which species originate. Speciation can be studied from many perspectives, but regardless of the point of view it often requires reproductive isolation between species. In the case of the ''Orbicella,'' these species are

broadcast spawner Spawn is the eggs and sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of releasing the eggs and sperm, and the act of both sexes is called spawning. Most aquatic animals, except for aquat ...

corals and release

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

s annually in the same evening into the water column. The night of spawning is normally within the warmest month of the year, and five to eight nights after the full moon. Some authors argue these species spawn simultaneously, but most reports support that the ''Orbicella'' species are temporally isolated by a few hours. Sperm and

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

s are released packed in small bundles that break open when they reach the water surface due to the surface tension. Timing is important to improve the chances of gametes finding each other in the water column. In addition, gametes must be incompatible between species to prevent the formation of hybrid zygotes. Hybridization studies reported some success in crossing ''O. annularis'' and ''O. franksi''. Yet crosses with ''O. faveolata'' are consistently unsuccessful. However, these crosses were done in laboratory conditions that are not natural (covering ''O. annularis'' in advance to trick it into spawning earlier) so these crosses are unlikely to occur in nature. Furthermore, in nature gametes get diluted and age quickly. Hence, ''Orbicella'' species are reproductively isolated by at least these two ways, allowing for species distinction and evolution.

Species

The following

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

are currently recognized by the

World Register of Marine Species The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive list of names of marine organisms. Content The content of the registry is edited and maintained by scientific specialis ...

: *''

'' (Ellis & Solander, 1786) *''

'' (Gregory, 1895)

References

{{Taxonbar, from=Q18748030 Merulinidae Scleractinia genera