Starry cup coral (Acanthastrea echinata)
|Size||Length: hemispherical colonies may be 1 metre across (2)|
Classified as Least Concern (LC) on the IUCN Red List (1) and listed on Appendix II of CITES (3).
The most common species of Acanthastrea (1), this coral forms large encrusting colonies over the surface of rocks, or takes the form of a massive, hemispherical mound (2). Like other colony-forming corals, Acanthastrea echinata colonies are composed of numerous small polyps, which are soft-bodied animals, related to anemones. Each polyp bears numerous tentacles which direct food into a central mouth, where it is digested in a sac-like body cavity. One of the most remarkable and ecologically important features of corals is that the polyps secrete a hard skeleton, called a “corallite”, which over successive generations contributes to the formation of a coral reef. The coral skeleton forms the bulk of the colony, with the living polyp tissue comprising a thin, often colourful, veneer. In Acanthastrea echinata, the polyps are interconnected, sharing corallite walls, while the living polyp tissue is thick and forms concentric rings. Polyps of this species are generally uniform or mottled brown, grey or green, but may occasionally appear more brightly coloured exhibiting, for example, vibrant shades of purple (2).
Acanthastrea echinata is widespread throughout much of the Indo-West Pacific, with colonies occurring as far west as the Red Sea, the Gulf of Aden, and the Arabian Gulf, and east as far as French Polynesia (1).
Acanthastrea echinata tolerates a relatively broad range of temperatures, depths and light availability, and is therefore found in most reef environments to depths of 50 metres (1).
Like many coral species, Acanthastrea echinata is zooxanthellate, which means that its tissues contain large numbers of single-celled algae called zooxanthellae. The coral and the algae have a symbiotic relationship, in which the algae gain a stable environment within the coral’s tissues, while the coral receives nutrients produced by the algae through photosynthesis. By harnessing the sun’s energy in this way, corals are able to grow rapidly and form vast reef structures, but are constrained to live near the water surface (2).
While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from zooxanthellae photosynthesis, the coral may also feed on zooplankton (4). The polyps’ tentacles, which contain stinging cells called “nematocysts”, trap the drifting zooplankton, directing it into the central mouth, which also acts as an anus to excrete waste products after digestion (2) (4). Neighbouring polyps within an Acanthastrea echinata colony are linked by small tubes that distribute water and nutrients evenly, creating a similar rate of growth, and preventing competition for space (2).
Acanthastrea echinata is capable of both asexual reproduction, whereby the polyps proliferate through the production of clones, and by sexual reproduction, in which the polyps spawn large numbers of sperm and eggs. The fertilised eggs develop into planktonic larvae, which travel through the water column, before settling and metamorphosing into a sessile polyp (5).
Around one third of the world's reef-building corals are threatened with extinction (6). 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 (1) (6).
Acanthastrea echinata is particularly targeted for the aquarium trade, with 1,000 live pieces exported from Indonesia each year. While this species is currently abundant and widespread, loss of coral reef habitat indicates that it is declining, and with so many potential threats to its survival, could rapidly become threatened (1).
Like all coral species, Acanthastrea echinata is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which means that all international trade is strictly controlled by the use of permits and annual quotas. In addition, Acanthastrea echinata falls within several Marine Protected Areas across its range (1).
Specific conservation measures recommended for this species include carrying out extensive studies of its population, biology, and ecology, along with more research into existing and potential threats. In addition, surveys to monitor the effects of collecting for the aquarium trade would be useful to ensure that Acanthastrea echinata is not being overexploited. Finally, in response to the threat of climate change, artificial propagation and preservation of sperm and eggs may be necessary to safeguard this species against complete extinction (1).
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- Photosynthesis: 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).
- Sessile: permanently attached; not freely moving.
- Symbiotic: describing a 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 (December, 2008)
- Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townsville, Australia.
CITES (December, 2008)
- 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.
- Richmond, R.H. and Hunter, C.L. (1990) Reproduction and recruitment of corals: comparisons among the Caribbean, the Tropical Pacific, and the Red Sea. Marine Ecology Progress Series, 60: 185 - 203.
- 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.