Staghorn coral  (Acropora nasuta)

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

Acropora nasuta forms cream, pale brown or greenish colonies, with irregular, tapering branches that usually arise from a short, thick stalk (2) (4) (5) (6) (7). The branches of Acropora nasuta usually have blue tips, which sometimes appear pink in photographs (2) (4) (5) (6), and measure up to 7 centimetres in length and 0.7 to 1.6 centimetres in diameter (2) (7). Acropora nasuta may grow in a variety of different colour morphs (8), and can sometimes be difficult to distinguish from related species with similar branching patterns (2).

As in other corals, the colonies of Acropora nasuta are composed of numerous tiny, anemone-like animals known as polyps. The polyps secrete a hard coral skeleton, with the skeleton of an individual polyp being known as a ‘corallite’ (4). Acropora species, or ‘staghorn corals’, are unusual in having two distinct types of polyp: ‘axial’ polyps, which are found at the tips of the branches, and ‘radial’ polyps, which bud from the side of the growing axial polyp. A radial polyp may also turn into an axial polyp and begin to extend and bud, forming new branches (4) (5) (9). In Acropora nasuta, the axial polyps have tubular skeletons, while the skeletons of the radial polyps have the appearance of upside-down noses, giving this species its scientific name, ‘nasuta’. The radial polyps are arranged in neat rows on the branches (2) (4) (5) (6) (7).

Growing by budding from the axial polyps enables Acropora corals to grow quickly and out-compete other species, often allowing them to become the dominant species on reefs (4) (5) (9). However, they are quite delicate and easily damaged by storms (9).

Acropora nasuta is widely distributed in the Indo-Pacific, occurring in the Red Sea, Gulf of Aden and Arabian Gulf, through the Indian Ocean to Southeast Asia and Australia, and into the western and central Pacific Ocean (1) (2) (4) (5) (6).

An inhabitant of shallow, tropical reefs, Acropora nasuta is found in most reef habitats, but is particularly common on upper reef slopes (1) (2) (4) (5) (6). It has been recorded at depths of 3 to 15 metres (1).

Acropora nasuta is a ‘zooxanthellate’ coral, meaning it has symbiotic algae, known as zooxanthellae, which live within its tissues and provide it with nutrients through photosynthesis. In return, the coral provides the algae with a safe, stable environment and access to sunlight. Although this restricts zooxanthellate corals to living in relatively warm, clear, shallow waters, it enables them to grow more quickly and form large reef structures (4). Acropora nasuta also feeds on tiny zooplankton, which it catches using stinging cells on its tentacles. In Acropora species, the polyp tentacles are usually only extended at night (4) (5).

The polyps of Acropora nasuta are hermaphroditic, containing both male and female sex organs (10). Like most Acropora corals, this species releases eggs and sperm into the water for external fertilisation, with the resulting larvae eventually settling onto the substrate and developing into polyps (2) (4) (5). Acropora nasuta produces orange or white eggs (11). This species has been recorded spawning around November and December on the Great Barrier Reef, Australia, where it is likely to take part in mass coral spawning events (5) (10). It has also been recorded spawning in May and June in parts of the Northern Hemisphere (2), in July and August in the Central Pacific (10), and from October to November and February to March in reefs off Kenya (11).

Corals are also able to reproduce asexually by budding, with polyps dividing to form one or more new, ‘daughter’ polyps (4). Like other Acropora species, Acropora nasuta is also likely to be able to reproduce by ‘fragmentation’, in which new colonies grow from branches that have broken off the main colony as a result of a storm or other disturbance (2).

Although it is a common and widespread species, Acropora nasuta is under threat from a range of factors affecting corals worldwide (1). The most significant of these is climate change, which may lead to more severe, frequent storms and increased ocean temperatures. Rising temperatures in turn increase the rate of coral ‘bleaching’, in which the stressed coral expels its zooxanthellae and turns white, often leading to the death of the coral or making it more vulnerable to disease and parasites (1) (12) (13) (14). Bleaching has also been shown to decrease fertilisation rates in Acropora nasuta, so leading to a decline in its reproduction (15).

Increased ocean acidity as a result of rising carbon dioxide levels is likely to negatively affect corals by making it harder for them to secrete the hard coral skeleton. In addition, coral reefs face many localised threats, including coastal development, pollution, sedimentation, destructive fishing practices and invasive species (1) (12) (13) (14).

Acropora species are particularly vulnerable to attack by predatory crown of thorns starfish (Acanthaster planci), and many, including Acropora nasuta, are collected for the aquarium trade (1). Members of this group have a low resistance to bleaching and disease outbreaks, and are slow to recover from these events (1).

Acropora nasuta is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning that international trade in this species should be carefully regulated (3). It also occurs in Marine Protected Areas in parts of its range (1), including the Great Barrier Reef in Australia, where a range of conservation and research activities are underway (16). Unfortunately, enforcement within many Marine Protected Areas is often poor (12).

Recommended conservation measures for Acropora nasuta include further research into its populations, ecology and resilience to threats, as well as surveys to monitor the effects of collection for the aquarium trade (1). Coral reefs would also benefit from the expansion of protected areas, disease research and management, and efforts to combat global climate change (1) (12) (14). Acropora corals are considered particularly good candidates for reef restoration projects, as they are able to survive fragmentation and grow rapidly to colonise reef space (2) (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/