Spine coral  (Hydnophora exesa)

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

As with other species in the genus Hydnophora, the creamy-brown and green spine coral Hydnophora exesa has characteristic cone-shaped humps, or ‘hydnophores’, covering its surface (3) (4). Each hump is 3 to 5 millimetres tall with a diameter of 5 to 8 millimetres (3) (4), although they can sometimes be elongated and up to 20 millimetres long (4).

Colonies of Hydnophora exesa are irregularly shaped, forming plates or short, thick branches (3) (4).

A widespread but sporadically occurring coral (5), Hydnophora exesa can be found in the Indian Ocean and the western and central Pacific Ocean, as well as the Red Sea, Gulf of Aden, Arabian Gulf and east China Sea (1).

Hydnophora exesa prefers low turbidity areas in protected reefs and lagoons, or exposed locations on reef slopes up to 30 metres deep (1) (3) (4) (5).

Like many coral species, the tissue of Hydnophora exesa contains large numbers of single-celled algae called zooxanthellae. The coral and the algae have a symbiotic relationship in which the algae gain a safe, stable environment within the coral's tissues, while the coral receives nutrients produced by the algae through photosynthesis. By harnessing the sun's energy in this way, corals are able to grow rapidly and form vast reef structures, but are constrained to live near the water surface (6).

While, on average, zooxanthellate coral can obtain around 70 percent of its nutrient requirements from photosynthesis, Hydnophora exesa also feeds on zooplankton. The polyps' tentacles are extended during the day (4) and contain stinging cells called ‘nematocysts’. These trap drifting zooplankton, directing it into the central mouth, which also acts as an anus to excrete waste products after digestion (6).

Hydnophora exesa is widespread and relatively common throughout its range (1) (4), but is under threat from many of the factors affecting coral species worldwide (1). It is estimated that 20 percent of the world’s coral reefs have already been effectively destroyed and show no immediate prospect of recovery, and 24 percent of the world’s reefs are under imminent risk of collapse due to human pressures. These human impacts include poor land management practices that are releasing more sediment, nutrients and pollutants into the oceans and stressing the fragile reef ecosystem. Over fishing has ‘knock-on’ effects that results in the increase of macro-algae that can out-compete and smother corals, and fishing using destructive methods physically devastates the reef (7).

The major threat to corals is global climate change, with the expected rise in ocean temperatures increasing the risk of coral ‘bleaching’, in which the stressed coral expels its zooxanthellae, often resulting in the death of the coral. Climate change may also lead to more frequent, severe storms, which can damage reefs, and rising carbon dioxide levels may make the ocean increasingly acidic, reducing the ability of coral to create its hard skeleton. Such stresses can also make corals more susceptible to disease, parasites and predators, such as the crown of thorns sea star (Acanthaster planci) (1) (7) (8) (9) (10).

These global threats are compounded by localised human impacts, such as coral harvesting, disturbance by fisheries, irresponsible tourism, invasive species, pollution and sedimentation (1) (7) (8) (9) (11). Hydnophora exesa is popular in the live aquarium trade. In 2005 Fiji had a quota of 662 live pieces, but Indonesia is the main exporter, with a quota of 10,900 pieces exported in 2005 (1).

In addition to being listed on Appendix II of CITES (2), which makes it an offence to trade Hydnophora exesa without a permit, some of this coral’s range falls within Marine Protected Areas. To specifically conserve Hydnophora exesa, further research is recommended into various aspects of its taxonomy, biology and ecology, including an assessment on threats such as disease and potential recovery techniques. Fisheries management is especially important in Indonesia, to monitor the effects of harvesting Hydnophora exesa for the aquarium trade (1).
 

View information on this species at the UNEP World Conservation Monitoring Centre.

Environment Agency - Abu Dhabi is a principal sponsor of ARKive. EAD is working to protect and conserve the environment as well as promoting sustainable development in the Emirate of Abu Dhabi.

For further information on the conservation of coral reefs see:

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