Acropora Oken, 1815
Conservation and Management
The roughly 120 species of staghorn corals (Acropora) account for a large fraction of the world's coral reefs (IUCN 2009). These corals face several major threats:
Bleaching: Many reef-forming corals are very sensitive to high ocean temperatures, which may cause them to expel the symbiotic photosynthetic dinoflagellates on which they depend for survival. The loss of these pigmented symbionts may give the coral a bleached appearance. Mass coral bleaching is a recent phenomenon (dating back to the 1980s) and is now the main cause of coral mortality and reef deterioration globally (IUCN 2009). Coral vulnerability to bleaching varies among species, and staghorn corals are thought to be among the most vulnerable. Temperature-induced mass coral bleaching has caused widespread mortality of staghorns and other corals worldwide, including the well-protected Great Barrier Reef in Australia (IUCN 2009).
Another serious problem faced by these corals is acidification of the oceans as a consequence of the absorption of large amounts of atmospheric carbon dioxide. Because acidification affects the process of calcification, this directly impacts marine animals such as corals and molluscs that have calcareous skeletons or shells. The acidified marine environment results in weakened skeletons and slower growth rates. (IUCN 2009).
Disease is another worsening problem for many corals. Increasing water temperatures and acidification cause physiological stress, which increases susceptibility to disease. Warmer sea temperatures may also present more suitable conditions for the pathogens themselves. The rapid, large-scale loss of staghorn corals in the Caribbean is due to an unprecedented rise in coral diseases (IUCN 2009).
Climate change introduces a host of other impacts which may act synergistically with bleaching, acidification, and disease to threaten staghorns and other corals. These include sea level rise, changes to ocean circulation patterns, damage from increased storm intensity and frequency, and loss of light from increased river sediment loads. As of 2009, a third of coral species are currently listed as threatened on the IUCN Red List (IUCN 2009).
Undisturbed staghorn corals normally form a distinct “staghorn zone” in shallow waters between 5 to 15 m depth, though they also occur in shallower and deeper water (IUCN 2009).
Staghorn corals can be broadly divided into Atlantic and Indo-Pacific groups, and are generally found between 25˚N and 25˚S. The Atlantic group is by far the smaller of the two, being composed of only two extant species and a common hybrid, found along the Caribbean coasts of Central and South America, south-western Gulf of Mexico and the Bahamian archipelago. The Indo-Pacific
group is distributed across the tropics in suitable habitat all the way from the west coast of Central America to the Red Sea and East Africa, with the centre of diversity in the ‘Coral Triangle’ region of the Solomon Islands, Papua New Guinea, Indonesia, East Timor, Philippines and Malaysia (IUCN 2009)
Evolution and Systematics
Systematics and Taxonomy
Based on the most recent taxonomic work, there are approximately 120 known species of Acropora (Wallace 1999; S.D. Cairns, in litt. 2009).
Coral reefs are home to a third of all known marine species. About 8% of the world's human population lives within 100 km of a coral reef and tens of millions of these people depend on the productivity of coral reefs for their protein (IUCN 2009). Coral reefs shield thousands of kilometres of coastline from wave erosion, and protect lagoons and mangroves, which are vital habitats for diverse commercial and non-commercial species. A number of medically active compounds are derived from corals and associated reef species. The value of coral reefs for ecotourism is enormous. In total, the economic value of coral reefs probably amounts to several hundred billion dollars per year (IUCN 2009).