Great Star Coral, Montastraea cavernosa
Great Star Coral, Montastraea cavernosa. Underwater photograph taken in the coastal waters off Bonaire, Caribbean Netherlands, December, 2019. Photograph and identification courtesy of Bob Hillis, Ivins, Utah.
Phylogeny: The Great Star Coral, Montastraea cavernosa (Linnaeus, 1767), is a hexacoral in the Montastraeidae Family of Star Corals. The genus Montastraea is the only genus in this family, and the Great Star Coral is the only species in this genus.
Morphology: Great Star Coral polyps have radial symmetry, a hollow digestive cavity, and specialized stinging structures in tentacles surrounding the mouth. Their polyps have a flower-like appearance and produce a hard, calcareous skeleton. They live in large colonies, forming boulders, domes, and in deeper water, flat plates. It is thought that the flat plates gather more sunlight. The calcium carbonate skeletons produced by individual polyps (corallites) are usually conical. The corallites have 36 to 48 septa radiating from their center. The long and short septa strongly alternate, with alternate septa joined to the columella. The corallites measure 5.5 mm (0.2 inches) to 7.5 mm (0.3 inches) in diameter and are separated from neighboring corallites by 5 mm (O.2 inches) to 9 mm (0.4 inches). Great Star Coral skeletons show sex-related differences, with females having a less dense skeleton compared to males, which is presumably due to reallocating energy to egg production. There appear to be two morphs of this species, with one being active night and day, and the other being nocturnal. Great Star Coral can be brown, gray, green, or orange in color. The boulder and dome-shaped colonies can reach 1.5 m (4.9 feet) in diameter.
Habitat and Distribution: Great Star Corals are found on hard substrates such as rock or reef and can be found to depths up to 95 m (312 feet), though most are found between 10 m (33 feet) and 30 m (98 feet). This species is able to dominate other corals in turbid and silty environments due to the sediment rejection capabilities of their shape and their sweeper tentacles, which are able to remove sediments from the polyp surface. Great Star Corals are a tropical Atlantic species that are found in the Gulf of Mexico and the Caribbean, from La Pesca, Tamaulipas to Belize.
Diet: Great Star Corals are zooxanthellate. They have a symbiotic relationship with single-cell dinoflagellates, known as zooxanthellae. Zooxanthellae live within certain coral polyps, sea anemones, jellyfish, and nudibranchs. The zooxanthellae produce energy, during daylight, by means of photosynthesis. This energy is passed along to their hosts, sometimes providing up to 90% of the host’s total energy needs. In return, the host provides nutrients, carbon dioxide, and a secure, sunlit, platform for the zooxanthellae. Great Star Corals supplement the energy provided by zooxanthellae by using their tentacles to capture amphipods and other plankton from the surrounding water. The tentacles then pass the food to the mouth.
Predators: There is limited documentation of predation related to Great Star Corals. Some of their known predators include butterflyfish, damselfish and parrotfish.
Reproduction: Great Star Coral colonies are gonochoric (male or female). Reproduction can be asexual, through budding, or sexual. Sexual reproduction is accomplished through broadcast spawning, with external fertilization. The gametes are shed into the coelenteron and spawned through the mouth. The zygote develops into a planktonic planula larva. Metamorphosis begins with early morphogenesis of tentacles, septa and pharynx before larval settlement on the aboral end. The single new polyp forms a new colony by budding. The polyps deposit calcium carbonate below them thus building on the current structure and forming the large boulder shape the species is known for. Spawning takes place one week after the full moon in late August.
Ecosystem Interactions: Great Star Coral colonies defend their living space from other coral species by use of their sweeper tentacles, which are extensions of the digestive system. This allows them to basically digest the encroaching corals. Great Star Corals are parasitized by ciliates, and the pathogens that cause Black Band Disease and White Plague. Great Star Corals have a symbiotic relationship with zooxanthellae.
Human Interactions: Great Star Corals have no direct impact on human activities. From a conservation perspective they are fairly common with a relatively wide distribution and are considered to be of Least Concern.
Synonyms: Astraea (Orbicella) argus, Astraea (Orbicella) radiata, Astraea astroites, Astraea astroites var. porosa, Astraea astroites var. stellaris, Astraea endothecata, Astraea radiata, Astrea argus, Astrea cavernosa, Astrea conferta, Astrea radiata, Explanaria argus, Explanaria argus var. columnaris, Explanaria argus var. complanata, Explanaria argus var. impressa, Explanaria radiata, Favia cavernosa, Favia cavernosa astroites, Heliastraea cavernosa, Heliastraea conferta, Heliastraea cylindrica, Heliastraea endothecata, Heliastraea radiata, Madrepora astroides, Madrepora cavernosa, Madrepora radiata, Matrepora radiata, Montastraea brasiliana, Montastrea braziliana, Montastrea cavernata, Montastrea cavernosa, Montastrea cavernosa guineensis, Orbicella argus, Orbicella braziliana, Orbicella cavernosa, Orbicella cavernosa hirta, Orbicella cavernosa var. compacta, Orbicella cavernosa var. hirta, and Orbicella radiata.