Staghorn coral  (Acropora horrida)

Classified as Vulnerable (VU) on the IUCN Red List (1) and listed on Appendix II of CITES (2).

With its delicately coloured branches twisting gently upwards, Acropora horrida is a beautiful feature of coral reefs. Acropora horrida displays an array of colours, most often pale blue (which often appears pink or purple in a photograph) but also dark blue, light yellow and brown (3). It occurs as irregular low patches; rounded, branching colonies up to two metres in diameter; or extensive thickets (4). The species also has colourful polyps extended during the day, which appear pale blue or white (5). Acropora is the largest living genus of reef-building corals and among the most widespread (4).

Acropora horrida is found in the Indian Ocean and western and central Pacific Ocean (1).

Acropora horrida occurs in turbid water around fringing reefs, on upper reef slopes, and in shallow lagoons, and can be found at depths between 5 and 20 metres (1). Branching colonies are more common in turbid water, while bushy colonies occur on upper reef slopes and in lagoons (5).

Like many corals, staghorn corals have a special symbiotic relationship with algae, called zooxanthellae, which provide the coral with energy through photosynthesis. The zooxanthellae live inside the tissues of the coral and, in return for providing the coral with energy, the coral provides the algae with protection and access to sunlight. This limits the depth at which Acropora horrida  can occur as sunlight does not pass down through water below around 20 metres. Zooxanthellae are very sensitive to changes in temperature and pH (a measure of how acidic or alkaline the water is); significant changes in either can result in the algae being expelled from the polyps and the coral dying.

Like all staghorn corals, Acropora horrida is a reef-building species and is incredibly successful at this task. One of the reasons for this is its light skeleton which allows it to grow quickly and out-compete neighbouring corals (5).

Staghorn corals reproduce sexually or asexually. Sexual reproduction occurs via the release of eggs and sperm into the water. Most staghorn corals on the Great Barrier Reef sexually reproduce simultaneously, an incredible event that occurs soon after the full moon, from October to December. Streams of pinkish eggs are released from polyps on the sides of branches, to be fertilized by sperm released from other polyps at the same time. The water turns milky from all the eggs and sperm released from thousands of colonies. Some of the resulting larvae settle quickly on the same reef, whilst others may drift around for months, finally settling on reefs hundreds of kilometres away (5).  Asexual reproduction occurs via fragmentation, when a branch breaks off a colony, reattaches to the substrate and grows (6). 

Acropora horrida is an uncommon coral which and has undergone dramatic population declines at locations in the Great Barrier Reef and Papua New Guinea (1). This, combined with its susceptibility to numerous threats, means that this species is now considered to be at risk of extinction (1).

Global warming is a major threat to Acropora horrida (and all other corals), as temperature extremes can lead to bleaching (when the zooxanthellae is expelled from the coral), leaving the coral weak and vulnerable to an increasing variety of harmful diseases. Coral diseases have become much more common over the last decade and are a major concern in the Indo-Pacific; this increase in diseases can be linked to an increase in the water temperature (1).

Corals also face numerous other threats, including an increase in industry, transport and tourism which frequently leads to increased pollution (1). In addition, Acropora horrida is collected for the aquarium trade (1). 

All corals, including Acropora horrida, have been placed in Appendix II of the Convention on International Trade in Endangered Species (CITES) which means that any international trade in this species should be carefully monitored (3). This is an important measure, as this species is targeted by collectors for the aquarium trade, but strict quotas and management of the species is also required. Other recommended conservation measures include conducting surveys of populations to monitor the effect of harvesting, especially in Indonesia, and establishing new Marine Protected Areas where coral species can be protected against some of the threats they face (1). 

For further information on the conservation of coral reefs see:

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