Orbicella

Orbicella is a genus of stony corals in the Merulinidae family. The Orbicella species complex comprises three sister species, namely Orbicella faveolata, Orbicella annularis and Orbicella franksi, all of which are shallow-water, zooxanthellate species and are native to the tropical western Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico.

Orbicella
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Order: Scleractinia
Family: Merulinidae
Genus: Orbicella
Dana, 1846 [1]
Species
See text

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 morphotypes “bumpy”, “columnar” and “massive”. These growth forms were believed to arise as a response to abiotic 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).[2][3][4]

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).[5] O. annularis and O. faveolata are commonly called the boulder star coral [6] and the mountainous star coral,[7] 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.[8]

Skeleton properties

Orbicella skeleton is made from CaCO3 in the crystal form of aragonite. The growth rate has been correlated with depth.[9] In addition, the skeleton also contains Brucite [Mg(OH)2] in the interseptal spaces (microbialites). These brucite particles encrusts microbes growing inside the coral skeleton.[10]

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 corals and release gametes 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,[11] but most reports support that the Orbicella species are temporally isolated by a few hours.[12][13] Sperm and eggs 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.[12] Hence, Orbicella species are reproductively isolated by at least these two ways, allowing for species distinction and evolution.

Species

The following species are currently recognized by the World Register of Marine Species :[1]

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References
  1. Hoeksema, Bert (2015). "Orbicella Dana, 1846". WoRMS. World Register of Marine Species. Retrieved 2015-06-14.
  2. Weil, Ernesto; Knowlton, Nancy (1994). "A Multi-Character Analysis of the Caribbean Coral Montastraea Annularis (Ellis and Solander, 1786) and its Two Sibling Species, M. Faveolata (Ellis and Solander, 1786) and M. Franksi (Gregory, 1895)". Bulletin of Marine Science.
  3. Knowlton, Nancy; Weil, Ernesto; Weigt, Lee A.; Guzmán, Héctor M. (1992-01-17). "Sibling Species in Montastraea annularis, Coral Bleaching, and the Coral Climate Record". Science. 255 (5042): 330–333. CiteSeerX 10.1.1.467.9390. doi:10.1126/science.255.5042.330. ISSN 0036-8075. PMID 17779583.
  4. Knowlton, Nancy (1993-01-01). "Sibling Species in the Sea". Annual Review of Ecology and Systematics. 24 (1): 189–216. doi:10.1146/annurev.es.24.110193.001201.
  5. Budd, Ann F.; Fukami, Hironobu; Smith, Nathan D.; Knowlton, Nancy (2012-11-01). "Taxonomic classification of the reef coral family Mussidae (Cnidaria: Anthozoa: Scleractinia)". Zoological Journal of the Linnean Society. 166 (3): 465–529. doi:10.1111/j.1096-3642.2012.00855.x. ISSN 1096-3642.
  6. "WoRMS - World Register of Marine Species - Orbicella annularis (Ellis & Solander, 1786)". www.marinespecies.org. Retrieved 2016-05-26.
  7. "WoRMS - World Register of Marine Species - Orbicella faveolata (Ellis & Solander, 1786)". www.marinespecies.org. Retrieved 2016-05-26.
  8. "Montastraea faveolata (Ellis 1786)". CoralPedia. University of Warwick. Retrieved 2015-06-14.
  9. Dustan, P. (1975-11-01). "Growth and form in the reef-building coral Montastrea annularis". Marine Biology. 33 (2): 101–107. doi:10.1007/BF00390714. ISSN 0025-3162.
  10. Nothdurft, Luke D.; Webb, Gregory E.; Buster, Noreen A.; Holmes, Charles W.; Sorauf, James E.; Kloprogge, J.T. (2005). "Brucite microbialites in living coral skeletons: Indicators of extreme microenvironments in shallow-marine settings". Geology. 33 (3): 169. doi:10.1130/g20932.1.
  11. Sánchez, Juan; Alvarado, Elvira M; Gil, Maria; Charry, Henry; Arenas, Olga; Chasqui, Luis H; García, Rocio (1999). "Synchronous mass spawning of Montastraea annularis (Ellis & Solander) and Montastraea faveolata (Ellis & Solander) (Faviidae: Scleractinia) at Rosario Islands, Caribbean coast of Colombia". Bulletin of Marine Science. 65 (3).
  12. Levitan, Don R.; Fukami, Hironobu; Jara, Javier; Kline, David; McGovern, Tamara M.; McGhee, Katie E.; Swanson, Cheryl A.; Knowlton, Nancy (2004-02-01). "Mechanisms of Reproductive Isolation Among Sympatric Broadcast-Spawning Corals of the Montastraea Annularis Species Complex". Evolution. 58 (2): 308–323. doi:10.1111/j.0014-3820.2004.tb01647.x. ISSN 1558-5646.
  13. Levitan, Don R.; Fogarty, Nicole D.; Jara, Javier; Lotterhos, Katie E.; Knowlton, Nancy (2011-05-01). "Genetic, Spatial, and Temporal Components of Precise Spawning Synchrony in Reef Building Corals of the Montastraea Annularis Species Complex". Evolution. 65 (5): 1254–1270. doi:10.1111/j.1558-5646.2011.01235.x. ISSN 1558-5646. PMID 21521188.
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