Acropora (Acropora digitifera)
Acropora digitifera is classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (2).
A branching coral with small, finger-like colonies that are cream with pink tips, Acropora digitifera is a member of a group of fast-growing corals known as the ‘staghorns’ due to their branching structure. Their speed of growth, however, is balanced by the fragility of some of the structures they form, as they are easily damaged in storms (3).
As in other colony-forming corals, colonies of Acropora digitifera are composed of numerous small polyps, soft-bodied animals related to anemones. Each polyp bears numerous tentacles that 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 these 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 only a thin veneer (3).
Acropora digitifera 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 into the West Pacific Ocean (1).
Occurring on shallow tropical reefs, Acropora digitifera is typically found on intertidal reef flats and on the back margins of reefs where the waves wash across the reef structure. It has been recorded to depths of 12 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, a 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 forming reefs for two main reasons. Firstly, they have light skeletons which allow them to grow quickly and out-compete 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 through the zooxanthellae, staghorn corals are able to grow relatively rapidly and form vast reef structures, but are constrained to live near the water surface (3).
Staghorn corals reproduce sexually or asexually. Sexual reproduction occurs via the release of eggs and sperm into the water. Streams of pinkish eggs are released from corallites on the sides of branches, to be fertilised 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. Asexual reproduction occurs via fragmentation, when a branch breaks off a colony, reattaches to the substrate and grows into a new colony (3).
With an estimated 20 percent of the world’s coral reefs already destroyed, Acropora digitifera faces many of the threats that are affecting coral reefs globally (4) (5). 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 (4).
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 (5). Corals in the genus Acropora are particularly vulnerable to such bleaching events and typically take a long time to recover (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) (4) (5) (6).
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 digitifera 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 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.
- Asexual reproduction: reproduction that does not involve the formation of sex cells (‘gametes’). In many species, asexual reproduction can occur by fission (or in plants ‘vegetative reproduction’); part of the organism breaks away and develops into a separate individual. Some animals, including vertebrates, can develop from unfertilised eggs; this process, known as parthenogenesis, gives rise to offspring that are genetically identical to the parent.
- 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.
- Intertidal: pertaining to the intertidal zone, the region between the high tide mark and low tide mark.
- 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.
- Zooplankton: tiny aquatic animals that drift with currents or swim weakly in water.
IUCN Red List (February, 2011)
CITES (February, 2011)
- Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townville, Australia.
- 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.