Ctenella coral  (Ctenella chagius)

Ctenella chagius is classified as Endangered (EN) on the IUCN Red List (1) and listed on Appendix II of CITES (2).

A long time survivor from the Cretaceous period (3), Ctenella chagius is the only coral species of the family Meandrinidae found within the Indian Ocean, with all other species found in the Caribbean Sea (4). Ctenella chagius forms green, cream or light brown solid, reef-building colonies that are shaped like half-globes and have highly convoluted valleys, in characteristic wavy patterns (4). 

Like other colony-forming corals, colonies of Ctenella chagius are composed of numerous small polyps, which are soft-bodied animals, related to anemones. Each polyp bears numerous tentacles that direct food into a central mouth, where it is digested in a sac-like body cavity. One of the most remarkable and ecologically important features of these corals is that the polyps secrete a hard skeleton, called a ‘corallite’, which over successive generations contributes to the formation of a coral reef. The coral skeleton forms the bulk of the colony, with the living polyp tissue comprising only a thin veneer (4).

Ctenella chagius is endemic to the Chagos Archipelago, where it is found around numerous islands including British Indian Ocean Territory, Mauritius and Réunion Island (1). The coral reefs of the British Indian Ocean Territory cover roughly 4,000 square kilometres, which is approximately one and a half percent of the total global area of coral reefs (5), and are among the most diverse reefs known in the Indian Ocean (3). 

This locally common species is found on reef slopes (1) (4) and lagoons, between depths of 3 and 45 metres (1). 

Like many coral species, Ctenella chagius is zooxanthellate (6), meaning its tissues contain large numbers of single-celled algae called zooxanthellae. The coral and the algae have a symbiotic relationship, in which the algae gain a 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 (4). While, on average, a zooxanthellate coral can obtain around 70 percent of its nutrient requirements from zooxanthellae photosynthesis, the coral may also feed on zooplankton (7). 

The major threat to one third of the world’s reef building corals is global climate change (8), with temperature extremes leading to coral bleaching. During bleaching, the symbiotic algae are expelled, resulting in weak and vulnerable corals that are susceptible to disease. Climate change may also increase the severity of El Niño events and storms with strong waves and currents which destroy coral reefs. Greater levels of carbon dioxide in the atmosphere also causes increased ocean acidification, which can reduce a coral’s ability to create its hard skeleton. In addition, rising sea levels may cause less sun to reach the coral, thereby decreasing photosynthesis (1). 

Localized threats to Ctenella chagius include human development, such as industry, settlements, and agricultural and industrial pollution. Sediment which runs into the sea from soil erosion disrupts photosynthesis, tourism and its related activities release pollutants into the water, whilst invasive species can disrupt the natural balance of the reef. Also, several methods of fishing, including dynamite, chemical and bottom trawling can all damage coral, which has a very slow growth cycle (1). 

In addition to being listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which makes it an offence to trade Ctenella chagius without a permit, this coral falls within several Marine Protected Areas across its range. The recommended measures for preserving this species include more research into suitable habitats, population numbers and the threats to that species, as well as identifying, establishing and managing new protected areas, along with expanding existing protected areas. Disease also needs to be managed and controlled, and it is further recommended that colonies should be artificially grown as an extra safety measure (1).  

Learn about efforts to conserve Ctenella chagius:

• EDGE of Existence:
  http://www.edgeofexistence.org/coral_reef/species_info.php?id=1852

For more information on the conservation of coral reefs:

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

For more information on the British Indian Ocean Territory and the UK overseas territories:

• The UK Overseas Territories Conservation Forum:
  http://www.ukotcf.org/territories/biot.htm