Staghorn coral  (Acropora monticulosa)

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

The colonies of Acropora monticulosa form thick, tapering, finger-like branches, which are usually blue, cream or brown, with paler tips (3) (4) (5) (6). The branches may measure up to 11 centimetres in length and 5 centimetres in diameter (5), but those exposed to strong wave action are usually shorter and more conical (3). Some studies have identified a ‘branching’ form of this species, with green-tinged branches of variable lengths, and a ‘digitate’ (finger-like) form, with short, evenly-sized, pale to dark brown branches (7). Acropora monticulosa sometimes grows as large domes, which may measure over three metres across and are composed of many branching sub-colonies (3).

As in other corals, the colonies of Acropora monticulosa are made up of numerous tiny, anemone-like animals known as polyps, which secrete a hard coral skeleton. The skeleton of an individual polyp is known as a ‘corallite’ (3). Acropora species differ from most other corals in having two types of polyp: a central or ‘axial’ polyp, which grows at the tip of a branch, and ‘radial’ polyps, which bud from the sides of the axial polyp as it grows. Further branching may occur when a radial polyp turns into an axial polyp and begins to extend and bud (3) (4) (8). In Acropora monticulosa, the axial polyp is small, and the radial polyps are uniform in size and usually arranged in rows (3) (4) (5) (6).

Budding from the axial polyps allows Acropora species to grow quickly and out-compete other corals, often making them the dominant species on reefs. It also allows the colonies to grow in a great variety of shapes, including the typical branching pattern which gives the group the name of ‘staghorn corals’ (3) (4) (8). These corals are quite delicate, and are easily damaged by storms (8).

Acropora monticulosa is widely distributed in the Indian and Pacific Oceans. It occurs around the east coast of Africa and Madagascar, in the northern Indian Ocean around India, Sri Lanka, the Maldives and the British Indian Ocean Territory, around Southeast Asia and Australia, and into the western and central Pacific Ocean (1) (3) (4) (5).

This species occurs in shallow, tropical reefs to depths of about 12 metres (1). It is usually found on the upper slopes of reefs (1) (3) (4) (5), from around the low tide mark to just below the tide, on shallow reefs subject to wave action and strong tidal currents (1) (5).

Acropora monticulosa is a ‘zooxanthellate’ coral, meaning that it receives most of its nutrition from symbiotic algae, known as zooxanthellae, which live within its tissues. The zooxanthellae provide the coral with nutrients through photosynthesis, and in return receive a safe and stable environment in which to live. Although this restricts the coral to living in relatively warm, clear, shallow waters, it enables it to grow faster and form large reef structures. The polyps of Acropora monticulosa are also able to feed on tiny zooplankton, which are caught using stinging cells on the numerous tentacles which surround the polyp’s central ‘mouth’ (3). The tentacles of Acropora corals are usually only extended at night (3) (4).

Like other Acropora species, Acropora monticulosa is able to reproduce asexually by budding, a process in which polyps divide to form one or more new polyps (3). Acropora corals are also able to reproduce by ‘fragmentation’, in which branches that have been broken off by storms or other disturbances survive and grow into a new colony (9).

Sexual reproduction in Acropora monticulosa involves the simultaneous release of eggs and sperm into the water. The fertilised eggs develop into larvae, which eventually settle and grow into polyps (3) (4) (9). Acropora monticulosa has been recorded spawning in May in the Northern Hemisphere and in November in the Southern Hemisphere (9). On the Great Barrier Reef, off the coast of Australia, it may take part in mass spawning events, which involve many Acropora species and usually occur between October and December (4).

Although it is a widespread species and is sometimes common around Australia, Acropora monticulosa is generally uncommon elsewhere in its range (1) (3) (4) and is likely to be affected by the many threats facing corals worldwide (1). The major threat to corals is climate change, which may lead to increased water temperatures and more frequent, severe storms, which can damage reefs. Rising sea temperatures increase the risk of coral bleaching, in which the stressed coral expels its zooxanthellae, often leading to death. Increasing ocean acidity may also affect the ability of a coral to secrete its hard skeleton, and the combined effects of these stresses can make the coral more vulnerable to disease and parasites (1) (10) (11) (12). Like many Acropora species, Acropora monticulosa has a low resistance to bleaching and disease, and is slow to recover (1).

Coral reefs also face many localised threats, including human development, recreational activities, destructive fishing practices, invasive species, pollution and sedimentation (1) (10) (11) (12). Acropora monticulosa may also be collected for the aquarium trade, and is likely to be under threat from the crown of thorns starfish (Acanthaster planci), which shows a preference for feeding on Acropora species (1).

Acropora monticulosa is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning international trade in this species should be carefully regulated (2). Parts of its range fall within Marine Protected Areas (1), although enforcement within these can often be poor (10). It also occurs on the Great Barrier Reef, a World Heritage Site where a range of research and conservation projects are underway (13).

Recommended conservation measures for Acropora monticulosa include further research into its populations, abundance, ecology and resilience to various threats (1). It would also benefit from the expansion of protected areas, research into diseases and parasites, reef restoration projects, and efforts to combat climate change (1) (10) (12). Although Acropora corals are vulnerable to a wide range of threats, the ability to grow rapidly and survive fragmentation makes these species good candidates for reef restoration projects (8) (9).

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/