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Biology

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Like many corals, staghorn corals have a special symbiotic relationship with algae, called zooxanthellae. The zooxanthellae live inside the tissues of the coral and provide the coral with food, which it produces through photosynthesis and therefore requires sunlight. In return, the coral provides the algae with protection and access to sunlight. Staghorn corals are reef-building or hermatypic corals, and are incredibly successful at this task for two reasons. Firstly, they have light skeletons which allow them to grow quickly and out-compete their neighbouring corals. Secondly, the skeleton, or corallite, of a new polyp, is built by specialised 'axial' corallites. These axial corallites form the tips of branches, and as a result, all the corallites of a colony are closely interconnected and can grow in a coordinated manner (2). Staghorn corals reproduce sexually or asexually. Sexual reproduction occurs via the release of eggs and sperm into the water. Most staghorn corals on the Great Barrier Reef sexually reproduce simultaneously, an incredible event that occurs soon after the full moon, from October to December. Streams of pinkish eggs are released from corallites on the sides of branches, to be fertilized by sperm released from other polyps at the same time. The water turns milky from all the eggs and sperm released from thousands of colonies. Some of the resulting larvae settle quickly on the same reef, whilst others may drift around for months, finally settling on reefs hundreds of kilometers away (2). Asexual reproduction occurs via fragmentation, when a branch breaks off a colony, reattaches to the substrate and grows (3).
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Conservation

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Staghorn corals are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), and therefore trade in this coral should be carefully regulated, and a permit is required to bring the coral, or objects made from them, into the countries that have signed the CITES convention (1). Staghorn corals will also form part of the marine community in many marine protected areas, or in areas where management plans are in place to protect the coral community. In some areas, coral reefs restoration attempts are being undertaken; in Florida Keys National Marine Sanctuary, efforts have been made to reattach coral fragments, or culture and settle coral larvae. Both activities have had limited success, and new techniques are being pursued (4).
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Description

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Staghorn corals are among the fastest growing corals on reefs, and are excellent reef-builders (2). The name Acropora literally means a porous stem or branch (3), but Acropora species express a much greater variety of growth forms than the name suggests. Colonies can resemble antlers (staghorns) and be up to two meters tall, or can form delicately engineered plates and tables that may be up to three meters across. They can also form bush-like structures, some with short non-dividing branches like the fingers of a hand (2). Staghorn corals often out-compete all other corals in shallow tropical reefs, however, their speed of growth (which can be up to 10 to 20 centimetres a year (4)) is balanced by the fragility of some of the structures, as they are easily damaged in storms allowing other coral species a chance of growth. With 368 Acropora species currently known, and with such an amazing array of shapes, sizes and colours, identifying individual species can be a tricky task (2).
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Habitat

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Staghorn corals occur in tropical reef environments, down to a depth of 30 meters. The upper depth limit is defined by wave action, whilst the lower limit at which Acropora can inhabit is determined by light availability and the amount of suspended sediments. Staghorn corals require normal marine salinity (4).
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Range

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This is the most abundant coral of most reefs in the Indo-Pacific (2), and three species also occur in the western Atlantic and Caribbean region (3).
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Status

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Listed on Appendix II of CITES (1).
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Threats

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Staghorn corals face the many threats that are impacting coral reefs globally. At present, around one third of the world's reef-building corals are threatened with extinction. The principal threat to corals is the rise in sea temperature associated with global climate change. This leads to coral bleaching, where the symbiotic algae are expelled, leaving the corals weak and vulnerable to an increasing variety of harmful diseases. Climate change is also expected cause more extreme weather incidents and to increase ocean acidification, which impairs the coral's ability to form a skeleton. These global threats are compounded by localised threats from pollution, destructive fishing practices, invasive species and human development (5). Staghorn corals are considered to be environmentally sensitive corals that require clear, well-circulated water. Unlike other corals, which can obtain nourishment from zooplankton, staghorn corals are almost entirely dependent on the zooxanthellae for food. This means that sunlight is essential, and they are particularly sensitive to any human activities that increase water turbidity, reducing light availability (3). Two of the three Acropora species in the Atlantic were once very abundant, but in recent decades have remained at low levels of abundance, with no signs of recovery and in some areas, continued decline. These species are believed to be most greatly threatened by disease, temperature-induced bleaching, and physical damage from hurricanes. Threats from anthropogenic physical damage (e.g. vessel groundings, anchors, divers, snorkelers), coastal development, competition and predation are deemed to be moderate. The threat from collection or harvest was deemed abated by effective national and international regulations (3). Acropora species constituted 13 percent of the global coral trade between 1985 and 1997. Coral is harvested for building materials, curios, jewellery, and for aquariums. Staghorn corals are more common in the dead coral trade, rather than the live aquarium trade (6).
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One Species at a Time Podcast

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Coral reefs are bustling cities of marine life, until rising ocean temperatures turn them into ghost towns. Can reefs spring back from devastating bleaching events? Ari Daniel Shapiro and researcher Dr.Randi Rotjan of the New England Aquarium, journey to the remote Phoenix Islands to find out.

Listen to the podcast, meet the featured scientist, see images of coral reef research in the Phoenix Islands and find relevant educational resources on the Learning + Education section of EOL.

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Comprehensive Description

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"Branched Acropora colonies are abundant and show infinite variety, even within species. Massive or encrusting colonies are rarely seen. Among the branched forms it is possible to recognize staghorns, clusters, plates and tables, and between them lie many intermediates. The branches of staghorn Acropora are usually 10 cm in length and may be 1.5 cm or more in width. They arise from a single main stem and rarely fuse. Clusters consist of profusely divided branches that form a thicket or branchlets. The latter are usually less than 5 mm in diameter and may interlock. Plates and tables have a short, stout stem attached by a spreading base. Branches arise from the top of the stem and spread in a horizontal rather than vertical direction, often fusing together. The closely set and interlocking branches form a roughly circular plate sometimes two or three meters in diameter. Young colonies do not fall into these categories because virtually all are encrusting or knobby. Acropora species are among the most brightly colored corals on the reef. Often the are blue, green, purple or pink, sometimes cream, yellow, brown or red. The branch tops are usually paler. Each polyp has six or 12 slender tentacles that when extended may be 3 or 4 mm long. They are often white. Acropora is easily recognized by the shape and general characteristics of the corallites. In branched colonies the corallites are of two types. At the branch tip is a symmetrical and usually larger axial corallite, and down the elngth of the branches are many smaller asymmetrical. These radial corallites have been budded off from the axial corallite, and any of the radial poylps have the capacity to take up this reproductive role. Lobed or semi-massive colonies have scattered axial corallites, but they are scarcely more prominent than the other. Corallites are round in cross section and protrude several millimeters from the surface. Radial corallites often protrude only on one side because they lie at an angle to the branch. The size of corallites varies even in a single specimen, but their width is around 1.5 to 2.5 mm. It is common for the corallite wall to be fairly thick and the fossa relatively small. Calices are often crowded with the walls touching." (Dr. Elizabeth M. Wood, 1984).

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Acropora

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Acropora is a genus of small polyp stony coral in the phylum Cnidaria.[3] Some of its species are known as table coral, elkhorn coral, and staghorn coral. Over 149 species are described.[4] Acropora species are some of the major reef corals responsible for building the immense calcium carbonate substructure that supports the thin living skin of a reef.

Anatomy and distribution

Depending on the species and location, Acropora species may grow as plates or slender or broad branches. Like other corals, Acropora corals are colonies of individual polyps, which are about 2 mm across and share tissue and a nerve net. The polyps can withdraw back into the coral in response to movement or disturbance by potential predators, but when undisturbed, they protrude slightly. The polyps typically extend further at night to help capture plankton and organic matter from the water.

The species are distributed in the Indo-Pacific (over 100 species) and Caribbean (3 species). However, the true number of species is unknown: firstly, the validity of many of these species is questioned as some have been shown to represent hybrids, for example Acropora prolifera;[5] and secondly, some species have been shown to represent cryptic species complexes.[6]

Threats

Symbiodinium, symbiotic algae, live in the corals' cells and produce energy for the animals through photosynthesis. Environmental destruction has led to a dwindling of populations of Acropora, along with other coral species. Acropora is especially susceptible to bleaching when stressed. Bleaching is due to the loss of the coral's zooxanthellae, which are a golden-brown color. Bleached corals are stark white and may die if new Symbiodinium cells cannot be assimilated. Common causes of bleaching and coral death include pollution, abnormally warm water temperatures, increased ocean acidification, sedimentation, and eutrophication.

In 2014 the U.S. Fish and Wildlife Service listed ten Acropora species as 'threatened'.[7]

Reef-keeping

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Close-up of a network of Acropora polyps

Most Acropora species are brown or green, but a few are brightly colored, and those rare corals are prized by aquarists. Captive propagation of Acropora is widespread in the reef-keeping community. Given the right conditions, many Acropora species grow quickly, and individual colonies can exceed a meter across in the wild. In a well-maintained reef aquarium, finger-sized fragments can grow into medicine ball-sized colonies in one to two years. Captive specimens are steadily undergoing changes due to selection which enable them to thrive in the home aquarium. In some cases, fragments of captive specimens are used to repopulate barren reefs in the wild.[8]

Acropora species are challenging to keep in a home aquarium. They require bright light, stable temperatures, regular addition of calcium and alkalinity supplements, and clean, turbulent water.

Common parasites of colonies in reef aquariums are "Acropora-eating flatworms" Amakusaplana acroporae,[9] and "red bugs" (Tegastes acroporanus).

Species

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Acropora (Acroporidae) at French Frigate Shoals, northwestern Hawaiian Islands
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A. tenuis cells of the IVB5 line and symbiosis with photosynthetic dino­flagellate Breviolum minutum (Suessiales) — in vitro: Symbiotic inter­actions of coral cells (b and c) and dino­flagellates (x and y). Coral cell b inter­acted with symbiont x, but did not in­cor­porate it, whereas coral cell c endo­cytosed both x and y.

The following species are recognised in the genus Acropora:[10]

References

  1. ^ Wallace, C. C; Rosen, B. R (2006-04-22). "Diverse staghorn corals (Acropora) in high-latitude Eocene assemblages: implications for the evolution of modern diversity patterns of reef corals". Proceedings of the Royal Society B: Biological Sciences. 273 (1589): 975–982. doi:10.1098/rspb.2005.3307. ISSN 0962-8452. PMC 1560246. PMID 16627283.
  2. ^ WoRMS (2010). "Acropora Oken, 1815". WoRMS. World Register of Marine Species. Retrieved 2012-02-10.
  3. ^ "Acropora". Integrated Taxonomic Information System.
  4. ^ [https://web.archive.org/web/20110812213705/http://www.eol.org/pages/11130041 Archived 2011-08-12 at the Wayback Machine Acropora] at Encyclopedia of Life
  5. ^ Vollmer, S.; Palumbi, S. (2002). "Hybridization and the Evolution of Reef Coral Diversity". Science. 296 (5575): 2023–2025. doi:10.1126/science.1069524. PMID 12065836.
  6. ^ Ladner, Jason T.; Palumbi, Stephen R. (2012). "Extensive sympatry, cryptic diversity and introgression throughout the geographic distribution of two coral species complexes". Molecular Ecology. 21 (9): 2224–2238. doi:10.1111/j.1365-294X.2012.05528.x. PMID 22439812.
  7. ^ U.S. Fish and Wildlife Service. "Endangered and Threatened Wildlife and Plants; Adding 20 Coral Species to the List of Endangered and Threatened Wildlife" (PDF). Federal Register. 79 (219): 67356–67359.
  8. ^ "Restoration". The Global Coral Repository. 2011. Archived from the original on 2011-12-19. Retrieved 2011-12-08.
  9. ^ Rawlinson, K. A.; Gillis, J. A.; Billings, R. E.; Borneman, E. H. (2011). "Taxonomy and life history of the Acropora-eating flatworm Amakusaplana acroporae nov. sp. (Polycladida: Prosthiostomidae)". Coral Reefs. 30 (3): 693–705. doi:10.1007/s00338-011-0745-3.
  10. ^ "WoRMS - World Register of Marine Species - Acropora Oken, 1815". www.marinespecies.org. Retrieved 2018-05-28.
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Acropora: Brief Summary

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Acropora is a genus of small polyp stony coral in the phylum Cnidaria. Some of its species are known as table coral, elkhorn coral, and staghorn coral. Over 149 species are described. Acropora species are some of the major reef corals responsible for building the immense calcium carbonate substructure that supports the thin living skin of a reef.

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Description

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Colonies are usually ramose or arborescent, bushy or plate-like, rarely encrusting or submassive. Corallites are of two types, radial and axial; septa are in two cycles; columellae are absent; corallite walls and coenosteum are porous. Polyps are usually only extended at night (Veron, 1986). Occur as plate, table and branching colonies. Most have light skeletons and are fast growing. Corallites are characteristically densely-packed and cup-shaped, 2-3 mm across, often protruding 2-3 mm from the branch surface. In most species, terminal corallites at the tips of branches are enlarged and obvious. Colour: terminal corallites are often bright pink, pale blue or yellow (Richmond, 1997).
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bibliographic citation
Veron JEN. (1986). Corals of Australia and the Indo-Pacific. <em>Angus & Robertson Publishers.</em>
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