Overview

Brief Summary

Biology

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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Description

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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Comprehensive Description

"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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Distribution

Staghorn corals can be broadly divided into Atlantic and Indo-Pacific groups, and are generally found between 25˚N and 25˚S. The Atlantic group is by far the smaller of the two, being composed of only two extant species and a common hybrid, found along the Caribbean coasts of Central and South America, south-western Gulf of Mexico and the Bahamian archipelago. The Indo-Pacific
group is distributed across the tropics in suitable habitat all the way from the west coast of Central America to the Red Sea and East Africa, with the centre of diversity in the ‘Coral Triangle’ region of the Solomon Islands, Papua New Guinea, Indonesia, East Timor, Philippines and Malaysia (IUCN 2009)

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Range

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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Physical Description

Diagnostic Description

Description

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).
  • Veron, J.E.N. (1986). Corals of Australia and the Indo-Pacific. Angus & Robertson Publishers, London.
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Ecology

Habitat

Depth range based on 15349 specimens in 223 taxa.
Water temperature and chemistry ranges based on 9050 samples.

Environmental ranges
  Depth range (m): 0 - 4800
  Temperature range (°C): 1.157 - 29.241
  Nitrate (umol/L): 0.000 - 38.858
  Salinity (PPS): 30.220 - 40.360
  Oxygen (ml/l): 2.938 - 5.013
  Phosphate (umol/l): 0.020 - 2.680
  Silicate (umol/l): 0.523 - 165.990

Graphical representation

Depth range (m): 0 - 4800

Temperature range (°C): 1.157 - 29.241

Nitrate (umol/L): 0.000 - 38.858

Salinity (PPS): 30.220 - 40.360

Oxygen (ml/l): 2.938 - 5.013

Phosphate (umol/l): 0.020 - 2.680

Silicate (umol/l): 0.523 - 165.990
 
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Undisturbed staghorn corals normally form a distinct “staghorn zone” in shallow waters between 5 to 15 m depth, though they also occur in shallower and deeper water (IUCN 2009).

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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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Evolution and Systematics

Evolution

Systematics and Taxonomy

Based on the most recent taxonomic work, there are approximately 120 known species of Acropora (Wallace 1999; S.D. Cairns, in litt. 2009).

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:90Public Records:10
Specimens with Sequences:52Public Species:4
Specimens with Barcodes:44Public BINs:1
Species:26         
Species With Barcodes:16         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Acropora

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Genomic DNA is available from 1 specimen with morphological vouchers housed at Queensland Museum
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Genomic DNA is available from 7 specimens with morphological vouchers housed at Queensland Museum
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Genomic DNA is available from 1 specimen with morphological vouchers housed at Museum of Tropical Queensland
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Genomic DNA is available from 2 specimens with morphological vouchers housed at Museum of Tropical Queensland
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Conservation

Conservation Status

Status

Listed on Appendix II of CITES (1).
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Threats

The roughly 120 species of staghorn corals (Acropora) account for a large fraction of the world's coral reefs (IUCN 2009). These corals face several major threats:

Bleaching: Many reef-forming corals are very sensitive to high ocean temperatures, which may cause them to expel the symbiotic photosynthetic dinoflagellates on which they depend for survival. The loss of these pigmented symbionts may give the coral a bleached appearance. Mass coral bleaching is a recent phenomenon (dating back to the 1980s) and is now the main cause of coral mortality and reef deterioration globally (IUCN 2009). Coral vulnerability to bleaching varies among species, and staghorn corals are thought to be among the most vulnerable. Temperature-induced mass coral bleaching has caused widespread mortality of staghorns and other corals worldwide, including the well-protected Great Barrier Reef in Australia (IUCN 2009).

Another serious problem faced by these corals is acidification of the oceans as a consequence of the absorption of large amounts of atmospheric carbon dioxide. Because acidification affects the process of calcification, this directly impacts marine animals such as corals and molluscs that have calcareous skeletons or shells. The acidified marine environment results in weakened skeletons and slower growth rates. (IUCN 2009).

Disease is another worsening problem for many corals. Increasing water temperatures and acidification cause physiological stress, which increases susceptibility to disease. Warmer sea temperatures may also present more suitable conditions for the pathogens themselves. The rapid, large-scale loss of staghorn corals in the Caribbean is due to an unprecedented rise in coral diseases (IUCN 2009).

Climate change introduces a host of other impacts which may act synergistically with bleaching, acidification, and disease to threaten staghorns and other corals. These include sea level rise, changes to ocean circulation patterns, damage from increased storm intensity and frequency, and loss of light from increased river sediment loads. As of 2009, a third of coral species are currently listed as threatened on the IUCN Red List (IUCN 2009).

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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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Management

Conservation

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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Relevance to Humans and Ecosystems

Risks

Risk Statement

Coral reefs are home to a third of all known marine species. About 8% of the world's human population lives within 100 km of a coral reef and tens of millions of these people depend on the productivity of coral reefs for their protein (IUCN 2009). Coral reefs shield thousands of kilometres of coastline from wave erosion, and protect lagoons and mangroves, which are vital habitats for diverse commercial and non-commercial species. A number of medically active compounds are derived from corals and associated reef species. The value of coral reefs for ecotourism is enormous. In total, the economic value of coral reefs probably amounts to several hundred billion dollars per year (IUCN 2009).

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Wikipedia

Acropora

Acropora is a genus of small polyp stony coral in the phylum Cnidaria.[2] Some of its species are known as table coral, elkhorn coral, and staghorn coral. Over 149 species are described.[3] Acropora is one 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[edit]

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.

Acropora is most common in shallow reef environments with bright light and moderate to high water motion. Many small reef fishes live near their colonies and retreat into the thicket of branches if threatened.

Threats[edit]

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.

Reef-keeping[edit]

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.[4]

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 the Acropora-eating flatworm, and "red bugs" (Tegastes acroporanus).

Species[edit]

Acropora (Acroporidae) at French Frigate Shoals, northwestern Hawaiian Islands

References[edit]

  1. ^ WoRMS (2010). "Acropora Oken, 1815". World Register of Marine Species. Retrieved 20120210. 
  2. ^ "Acropora". Integrated Taxonomic Information System. 
  3. ^ Acropora at Encyclopedia of Life
  4. ^ "Restoration". The Global Coral Repository. 2011. 

Further reading[edit]

  • Shinzato, C., Shoguchi, E., Kawashima, T., Hamada, M., Hisata, K., Tanaka, M., Fujie, M., et al. 2011. Using the Acropora digitifera genome to understand coral responses to environmental change. Nature, advance online publication. doi:10.1038/nature10249
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