Staghorn coral  (Acropora nana)

Acropora nana is classified as Near Threatened (NT) on the IUCN Red List (1) and listed on Appendix II of CITES (4).

Acropora nana is a small, branching coral which forms compact clumps of long, straight, evenly-spaced branches that radiate from a solid base or short stalk (2) (3) (5) (6) (7). The branches are slender, measuring around 0.3 to 1.4 centimetres in diameter and up to 6 centimetres in length (2) (7), and taper slightly towards the tip (2) (6) (7). Colonies of Acropora nana may be cream, blue, brown or purple, usually with purple or bright blue tips to the branches (2) (3) (5) (6). Although Acropora nana colonies are quite delicate, readily breaking apart if disturbed, this species is able to withstand quite strong wave action (5) (6).

Colonies of Acropora nana are made of up numerous tiny, anemone-like animals known as polyps. These secrete a hard coral skeleton, with the skeleton of an individual polyp being known as a ‘corallite’ (5). As in other Acropora species, the polyps occur in two types, known as ‘axial’ and ‘radial’ polyps. Axial polyps grow at the tip of a branch, while radial polyps bud from the sides of the axial polyp as it grows. Further branching may occur if a radial polyp turns into an axial polyp and begins to extend and bud (5) (6) (8). In Acropora nana, the axial polyps have tubular skeletons with rounded margins, while the radial polyps have long, tubular skeletons with round or oval openings and an outer wall that extends upwards. The radial polyps are regularly arranged on the branches, just touching each other (2) (3) (5) (7).

Acropora corals have light skeletons, and by budding from the axial polyps they are able to grow quickly and out-compete other corals. These corals often become the dominant species on reefs, and are also able to grow in a great variety of shapes, although it is the typical branching pattern that gives the group the name of ‘staghorn corals’ (5) (6) (8).

Acropora nana is found in the south-west and eastern Indian Ocean, as well as around Southeast Asia and Australia, and into the western and central Pacific Ocean (1) (3) (5) (6).

Occurring in shallow, tropical reef environments, Acropora nana is commonly found on outer reef flats with strong currents or wave action, or just below the level of the tide at the reef edge or the top of reef slopes (1) (2) (3) (5). It has been recorded at depths of up to 10 metres (1).

Like other corals, Acropora nana feeds on tiny zooplankton, which the polyps catch using stinging cells on their tentacles (5). In Acropora species, the tentacles are usually only extended at night (5) (6). However, Acropora nana receives most of its nutrition from single-celled algae, known as ‘zooxanthellae’, which live within its tissues. The coral provides the zooxanthellae with a stable environment in which to live, and in return it receives nutrients produced by the zooxanthellae during photosynthesis. This restricts the coral to living in relatively shallow, clear, warm waters where photosynthesis is possible, but allows it to grow quickly and form large reef structures (5).

Acropora nana is able to reproduce asexually by budding, in which polyps divide to form one or more new polyps (5). Like other Acropora species, it is also able to reproduce by ‘fragmentation’, in which branches broken off the colony by storms or other disturbances are able to survive and grow into a new colony (2).

Most Acropora corals reproduce sexually by releasing eggs and sperm into the water for external fertilisation. The resulting larvae drift with water currents before settling and developing into polyps (2) (5) (6). Acropora nana produces pink eggs (9), and has been recorded spawning in April in the Northern Hemisphere (2) and in February off the coast of Kenya (9). In the Great Barrier Reef, Australia, most Acropora species spawn at the same time, usually between October and December (6).

Acropora nana is a widespread but uncommon coral, and is likely to be affected by many of the threats faced by corals worldwide (1). The main threat to corals is climate change, which may lead to more severe, frequent storms and an increase in ocean temperatures. Rising temperatures increase the risk of coral bleaching, in which a corals expels its zooxanthellae, often leading to death (1) (10) (11) (12). Acropora species are particularly vulnerable to bleaching, as well as to disease, and are slow to recover from such events (1). Corals are also likely to be impacted by increased ocean acidity as a result of rising carbon dioxide levels, and the combined effects of these stresses make them more vulnerable to disease and parasites (1) (10) (11) (12).

Further threats to coral reefs include destructive fishing practices, increasing coastal development, pollution, invasive species and sedimentation (1) (10) (11) (12). Acropora nana is also collected for the aquarium trade, and is likely to be preyed upon by the crown of thorns starfish (Acanthaster planci), which appears to prefer feeding on branching corals such as Acropora species (1). In addition, Acropora nana is reported to be heavily preyed upon by fish (1).

This species is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning international trade in Acropora nana should be carefully controlled (4). It also occurs in Marine Protected Areas in parts of its range (1), although enforcement of these areas is often poor (10). Acropora nana also occurs on the Great Barrier Reef in Australia, where a range of conservation and research efforts are underway (13).

Recommended conservation measures for corals such as Acropora nana include further research into their populations, ecology and diseases, as well as reef restoration projects, the expansion of protected areas, and efforts to combat climate change (1) (10) (12). The ability of Acropora species to grow relatively rapidly and to survive fragmentation makes these corals particularly good candidates for reef restoration (2) (8). Acropora nana would also benefit from population surveys to monitor the effects of collection for the aquarium trade, as well as management measures, such as quotas, to control harvest levels (1).

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/