Staghorn coral (Acropora austera)
Acropora austera is classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (2).
A reef-building coral with tree-like branches, Acropora austera is amongst the fastest growing corals on reefs in the Indian and Pacific Oceans. Like other Acropora species, it often out-competes all other corals in reefs; however, its speed of growth is balanced by the fragility of the structure, as it is easily damaged in storms allowing other coral species a chance of growth. The branches typically curve away from each other and taper to a blunt point, but some may also be fused together (3). The skeleton surrounding the polyps, or corallite, is thick walled, forming a depressed cup, and is very fine, giving the coral a smooth appearance (4) (5). Colonies of Acropora austera may be one of many colours, but are most commonly blue or cream (3).
Acropora austera is found in the Indian and Pacific Oceans, ranging from the Red Sea, Gulf of Aden and southwest Indian Ocean, across the northern Indian Ocean to Southeast Asia, Japan and the East China Sea, and the West and Central Pacific Ocean (1).
Despite occurring in a wide range of reef environments, Acropora austera is typically uncommon except on upper reef slopes where it is exposed to water turbulence (3) (4). It is found between depths of 0.5 and 20 metres (1).
Like many corals, staghorn corals have a special symbiotic relationship with algae, called zooxanthellae. The algae gain a safe, stable environment within the coral's tissues, while the coral receives nutrients produced by the algae through photosynthesis. While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from zooxanthellae photosynthesis, the coral may also feed on zooplankton (3).
Staghorn corals are reef-building, or ‘hermatypic’,corals and are incredibly successful at this task for two main 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. By harnessing the sun's energy, staghorn corals are able to grow relatively rapidly and form vast reef structures, but are constrained to live near the water surface (3).
Acropora austera releases eggs and sperm into the water column at certain times of the year in mass spawning events. The sperm and eggs combine in the water column, with a peak in egg and sperm release occurring in November in the southern Pacific Ocean. Larvae subsequently develop, and float passively in the currents as part of the zooplankton community, before settling on the ocean floor (6).
With an estimated 20 percent of the world’s coral reefs already destroyed, Acropora austera faces many of the threats that are affecting coral reefs globally (7) (8). Worldwide there is increasing pressure on coastal resources resulting from human population growth and development. There has been a significant increase in domestic and agricultural waste in the oceans, poor land-use practices that result in an increase in sediment running on to the reefs, and over-fishing, which can have ‘knock-on’ effects on the reef (7). However, the major threat to corals is global climate change, with the expected rise in ocean temperatures increasing the risk of coral ‘bleaching’, in which the stressed coral expels its zooxanthellae, often resulting in the death of the coral (8). Corals in the genus Acropora are particularly vulnerable to such bleaching events and typically take a long time to recover from them (1). Climate change may also lead to more frequent, severe storms, which can damage reefs, and rising carbon dioxide levels may make the ocean increasingly acidic. Such stresses can also make corals more susceptible to disease, parasites and predators, such as the crown-of-thorns sea star (Acanthaster planci) (7) (8) (9).
In addition to being listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which makes it an offence to trade this species without a permit, Acropora austera also forms part of the reef community in numerous marine protected areas (1) (2). To specifically conserve this coral, recommendations have been made for a raft of studies into various aspects of its biology, population status, habitat and threats to its survival (1).
For further information on the conservation of coral reefs, see:
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- Algae: simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
- Colonies: a group of organisms living together. Individuals in the group are not physiologically connected and may not be related.
- Genus: a category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
- Larvae: stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Photosynthesis: metabolic process characteristic in which carbon dioxide is broken down, using energy from sunlight. Organic compounds are made and oxygen is given off as a by-product.
- Polyp: typically sedentary soft-bodied component of cnidaria, a group of simple aquatic animals including the sea anemones, corals and jellyfish. A polyp comprises a trunk that is fixed at the base, and a mouth that is placed at the opposite end of the trunk and is surrounded by tentacles.
- Spawning: the production or depositing of large quantities of eggs in water.
- Symbiotic relationship: relationship in which two organisms form a close association. The term is now usually used only for associations that benefit both organisms (a mutualism).
- Zooplankton: tiny aquatic animals that drift with currents or swim weakly in water.
IUCN Red List (October, 2010)
CITES (October, 2010)
- Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townville, Australia.
- Veron, J.E.N. (1986) Corals of Australia and the Indo-Pacific. Angus and Robertson Publishers, UK.
World Register of Marine Species - Acropora austera (October, 2010)
- Carroll, A., Harrison, P. and Adjeroud, M. (2006) Sexual reproduction of Acropora reef corals at Moorea, French Polynesia. Coral Reefs, 25: 93-97.
- Wilkinson, C. (2004) Status of Coral Reefs of the World: 2004. Volume 3. Australian Institute of Marine Science, Townsville, Australia.
- Carpenter, K.E. et al. (2008) One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science, 321: 560-563.
- Miththapala, S. (2008) Coral Reefs. Coastal Ecosystems Series (Volume 1). Ecosystems and Livelihoods Group Asia, IUCN, Colombo, Sri Lanka.