Staghorn coral (Astreopora expansa)
Classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (2).
One of the most distinctive species in its genus, the variably brown, pink or dark green colonies of Astreopora expansa take the form of flat, bifacial plates arranged in tiers or whorls. In common with other colony-forming corals, the colonies are comprised of a vast assemblage of tiny, soft polyps, each of which is equipped with numerous tentacles that extend at night to direct food into a central mouth. The polyps secrete a hard skeleton known as a ‘corrallite’, which over time contribute to the formation of coral reefs (3).
Astreopora expansa has an Indo-West Pacific distribution ranging from Mozambique up to the Red Sea and the Arabian Gulf, and east to Japan, Southeast Asia, Australia, and the islands of the western Pacific (1).
Occurs in shallow reef environments from depths of 5 to 15 metres (1) (3).
In common with other corals in the Acroporidae family, Astreopora expansa is zooxanthellate, living in symbiosis with unicellular algae known as zooxanthellae, which are essential to the coral’s growth and survival. Protected within the coral tissue, the algae provide the host with nutrients and energy, whilst also helping to remove metabolic wastes. The cost of this symbiosis is that zooxanthellate corals are constrained to live in relatively shallow waters, where the algae are able to photosynthesise (3).
Although a zooxanthellate coral can obtain on average around 70 percent of its nutrient requirements through the photosynthesis of zooxanthellae, the coral may also feed on zooplankton, dissolved organic matter, and planktonic bacteria (4).
Around one third of the world’s reef building corals are threatened with extinction (5). 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 to increase ocean acidification and result in a greater frequency of extreme weather events such as destructive storms. The global impact of climate change on coral reefs is compounded by localised threats from pollution, destructive fishing practices, invasive species, human development, harvesting for the aquarium trade and other activities (1) (5).
Although Astreopora expansa is still relatively widespread and common in parts of its range, evidence of an overall global decline in coral habitat is an indication that this species is almost certainly declining (1).
In addition to being listed on Appendix II of CITES (2), which makes it an offence to trade Astreopora expansa without a permit, this coral falls within several Marine Protected Areas across its range. To conserve Astreopora expansa, recommendations have been made for a raft of studies into various aspects of its taxonomy, biology and ecology, including an assessment of threats and potential recovery techniques (1).
For further information on the conservation of coral reefs see:
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- Bifacial plates: in corals, plates which have corallites on both sides.
- Colony: Relating to corals: corals composed of numerous genetically identical individuals (also referred to as zooids or polyps), which are produced by budding and remain physiologically connected.
- Photosynthesise: Metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.
- Planktonic: Aquatic organisms that drift with water movements; may be either phytoplankton (plants), or zooplankton (animals).
- Polyps: Typically sedentary soft-bodied component of Cnidaria (corals, sea pens etc), which comprise of a trunk that is fixed at the base; the mouth is placed at the opposite end of the trunk, and is surrounded by tentacles.
- Symbiosis: 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 (April, 2009)
CITES (April, 2009)
- Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townsville, Australia.
- Barnes, R.S.K., Calow, P., Olive, P.J.W., Golding, D.W. and Spicer, J.I. (2001) The Invertebrates: A Synthesis, 3rd Edition. Blackwell Science, Oxford.
- Carpenter, KE et al. (2008) One-Third of Reef-Building Corals Face Elevated Extinction Risk from Climate Change and Local Impacts. Science, 321: 560 - 563.