Staghorn coral  (Acropora selago)

Acropora selago is classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (3).

A branching coral with a delicate appearance, Acropora selago forms somewhat bushy colonies which vary from pale cream to brown, blue or pink (2) (4) (5). The branches of this species are short, slender and tapering, measuring around 0.3 to 0.8 centimetres in diameter and up to 4 centimetres in length (2).

The colonies of Acropora selago are composed of numerous tiny, anemone-like animals called polyps. The polyps secrete a hard skeleton, and the skeleton of each individual polyp is known as a ‘corallite’ (4). The Acropora corals are unusual in having two distinct types of polyp: ‘axial’ polyps, which are located at the tips of the branches, and ‘radial’ polyps, which bud from the side of the axial polyp as it grows. New branches may also form when a radial polyp turns into an axial polyp and begins to extend and bud (4) (5) (6). In Acropora selago, the axial polyps have long, tubular skeletons, while the radial polyps are scale-like, with a flaring outer rim to the skeleton. The radial polyps are crowded quite closely together on the branches (2) (4) (5).

Acropora corals have light skeletons, and growing by budding from the axial polyps enables them to grow quickly and out-compete other corals. As a result, Acropora species often become the dominant species on reefs. This pattern of growth also allows the colonies to develop into a variety of shapes, including the typical branching pattern that gives the group its name of ‘staghorn corals’ (4) (5) (6). However, Acropora corals are quite delicate, and are easily damaged by storms (6).

Acropora selago occurs in the Red Sea, Gulf of Aden and Arabian Gulf, in the south-western and northern Indian Ocean, around Southeast Asia and Australia, and into the western and central Pacific Ocean (1) (2) (4) (5).

Found in shallow, tropical reefs at depths of 5 to 25 metres (1), Acropora selago inhabits a wide range of reef environments, from upper reef slopes to lagoons (1) (2) (4) (5).

Acropora selago receives the majority of its nutrition from symbiotic algae, known as zooxanthellae, which live within its tissues. The algae provide the coral with nutrients through photosynthesis, and in return receive a stable environment in which to live. A dependence on photosynthesis restricts corals such as Acropora selago to living in relatively shallow, clear, warm waters, but enables them to grow quickly and form large reef structures (4). Acropora selago also feeds actively on tiny zooplankton, which it catches using stinging cells on the tentacles of the polyps (4). Unusually for an Acropora coral, Acropora selago frequently extends its tentacles during the day (4) (5).

Acropora selago is hermaphroditic, meaning that each polyp contains both male and female sex organs (7). This species releases eggs and sperm into the water for external fertilisation, and the resulting larvae drift in the water column before eventually settling onto the substrate and developing into polyps (2) (4) (5). Acropora selago has been recorded spawning in November in some parts of its range (2) (7), and is likely to take part in synchronised mass spawning events of Acropora corals on the Great Barrier Reef, off the coast of Australia (5) (7).

In addition to reproducing sexually, corals are able to reproduce asexually by budding, forming new polyps by the division of existing polyps. Acropora species also reproduce by ‘fragmentation’, in which branches broken from the colony by storms or other disturbances are able to survive and grow into new colonies (2).

Acropora selago is widespread but generally uncommon throughout its range, and is under threat from many of the factors affecting coral species worldwide (1). Of greatest concern is global climate change, with rising ocean temperatures likely to increase the rate of coral ‘bleaching’, in which the stressed coral expels its zooxanthellae, often leading to death. Bleaching can also increase the coral’s susceptibility to disease and parasites, particularly in combination with other stresses such as more severe, frequent storms, and increased water acidity due to rising carbon dioxide levels (1) (8) (9) (10). Acropora corals are reported to be particularly vulnerable to bleaching and disease (1).

Corals such as Acropora selago are also under threat from many localised impacts, including coastal development, pollution, sedimentation, destructive fishing practices, recreational activities and invasive species (1) (8) (9) (10). In addition, Acropora selago is collected for the aquarium trade, and Acropora corals are also a favourite food of the crown of thorns starfish (Acanthaster planci), which is becoming a serious threat to many reefs (1).

Parts of the range of Acropora selago fall within Marine Protected Areas (1), although enforcement within these can often be poor (8). It also occurs in the Great Barrier Reef, off the coast of Australia, where a range of conservation and research programmes are underway (11). International trade in Acropora selago should be carefully regulated under its listing on Appendix II of the Convention on International Trade in Endangered Species (CITES) (3).

Recommended conservation measures for Acropora selago include research into its populations, abundance, ecology and resilience to threats, as well as monitoring and regulation of its harvest for the aquarium trade (1). It would also benefit from the expansion of Marine Protected Areas, together with further research into coral diseases and efforts to combat climate change (1) (8) (10). Since Acropora corals are able to survive fragmentation and grow rapidly to colonise reefs, species such as Acropora selago make good candidates for reef restoration projects (2) (6).

ARKive is supported by OTEP, a joint programme of funding from the UK FCO and DFID which provides support to address priority environmental issues in the Overseas Territories, and Defra

Find out more about coral conservation:

• Reef Check:
  http://www.reefcheck.org/
• The Coral Reef Alliance:
  http://www.coral.org/