Acropora (Acropora granulosa )

GenusAcropora (1)

Acropora granulosa is classified as Near Threatened (NT) on the IUCN Red List (1), and is listed on Appendix II of CITES (2).

Acropora granulosa belongs to the ‘staghorn’ group of corals, which have characteristic branching growth forms and are among the fastest growing coral species. They are important and often dominant reef-builders (3). Like other colony-forming corals, colonies of Acropora granulosa are composed of numerous small polyps. 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) (4). 

The name Acropora literally means a porous stem or branch (5), but Acropora species express a much greater variety of growth forms than the name suggests. Colonies of Acropora granulosa typically form semicircular horizontal plates, with thin, flattened, regularly spaced horizontal branches. The branches have a granular texture on the underside, and are rounded on the upper surface, with a number of shorter, upright branchlets (3) (4) (6) (7). The branchlets have elongate, tapering, smoothly rounded corallites at the tips (6). Acropora granulosa also has a number of small, pocket-like corallites which are found lower down the branchlets (3) (4) (6).

Acropora granulosa is usually uniform cream, grey or pale blue (which appears purple in photographs), although it is also known to occur less commonly in other colours (3) (6).

Acropora granulosa is found from the Red Sea and south-west and northern Indian Ocean, to the central Indo-Pacific, and the west and central Pacific (1) (7).

Colonies of Acropora granulosa are found in most subtidal reef environments, particularly on reef slopes that are protected from wave action (1) (3). It is seldom found in shallow water, but is common in clear water on reef walls and lower reef slopes, usually below depths of 15 metres (4) (6) (7). It may be found to depths of 40 metres (1) (6).

Like many corals, staghorn corals such as Acropora granulosa 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 photosynthesis by zooxanthellae, the coral may also feed on zooplankton (3).

Acropora granulosa and its zooxanthellae are very sensitive to changes in water temperature and acidity. Any increase in water temperature greater than one or two degrees Celsius above the average can stress the coral and cause ‘bleaching’, a phenomenon in which the coral expels it zooxanthellae and turns white (3) (7).

Staghorn corals are reef-building, or ‘hermatypic’ corals, and are incredibly successful at building reefs for two main reasons. Firstly, they have light skeletons which allow them to grow quickly and out-compete their neighbouring corals. Secondly the 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. This means that 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). 

Very little is known about the specific reproductive biology of Acropora granulosa, although it is likely to be able to reproduce both sexually and asexually. Asexual reproduction occurs via fragmentation, when a branch breaks off a colony, reattaches to the substrate and grows (3). Sexual reproduction occurs via the release of eggs and sperm into the water. On the Great Barrier Reef, most staghorn corals appear to sexually reproduce simultaneously (8), an incredible event that usually occurs on just a few nights soon after the full moon, during one or two specific months of the year (8) (9). 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. The eggs and sperm form slicks on the surface that can be up to a few kilometres in length, making them visible from the air (10). Some of the resulting larvae from these mass spawnings settle quickly on the same reef, whilst others may drift around for months, finally settling on reefs that may be hundreds of kilometres away (3).

With an estimated 20 percent of the world’s coral reefs already destroyed, Acropora granulosa faces many of the threats that are affecting coral reefs globally (11) (12).

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 (11).

However, the major threat to corals is global climate change, with the expected rise in ocean temperatures increasing the risk of coral bleaching, often resulting in the death of the coral (12). 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) (11) (12) (13).

Acropora species are also in the top three genera collected for the aquarium trade, and harvesting of wild colonies of Acropora granulosa may pose some threat to this species (1).

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 internationally without a permit, Acropora granulosa also forms part of the reef community in several Marine Protected Areas (MPAs) (1) (2). To specifically conserve this coral, recommendations have been made for studies on its biology, population status, habitat and threats to its survival (1).

For further information on the conservation of coral reefs:

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  1. IUCN Red List (February, 2011)
  2. CITES (February, 2011)
  3. Veron, J.E.N. (2000) Corals of the World. Australian Institute of Marine Science, Townville, Australia.
  4. Wallace, C.C. (1999) Staghorn Corals of the World: A Revision of the Coral Genus Acropora. CSIRO, Collingwood, Australia.
  5. Acropora Biological Review Team. (2005) Atlantic Acropora Status Review Document. Report to National Marine Fisheries Service, Southeast Regional Office.
  6. World Register of Marine Species - Acropora granulosa (February, 2011)
  7. Veron, J.E.N. (1986) Corals of Australia and the Indo-Pacific. Angus and Robertson Publishers, UK.
  8. Guest, J.R., Baird, A.H., Goh, B.P.L. and Chou, L.M. (2005) Reproductive seasonality in an equatorial assemblage of scleractinian corals.Coral Reefs, 24: 112-116.
  9. Baird, A.H., Marshall, P.A. and Wolstenholme, J. (2000) Latitudinal Variation in the Reproduction of Acropora in the Coral Sea. Proceedings 9th International Coral Reef Symposium, Bali, Indonesia.
  10. Guest, J.R., Baird, A.H., Goh, B.P.L. and Chou, L.M. (2005) Seasonal reproduction in equatorial reef corals. Invertebrate Reproduction and Development, 48(1–3): 207-218.
  11. Wilkinson, C. (2004) Status of Coral Reefs of the World: 2004. Volume 3. Australian Institute of Marine Science, Townsville, Australia.
  12. 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.
  13. Miththapala, S. (2008) Coral Reefs. Coastal Ecosystems Series (Volume 1). Ecosystems and Livelihoods Group Asia, IUCN, Colombo, Sri Lanka.