Ctenella coral (Ctenella chagius)
- Ctenella chagius is a solid green, cream or light brown coral shaped like a half-globe with highly convoluted, wavy valleys across its surface.
- Endemic to the Chagos Archipelago, Ctenella chagius is the only member of its family found in the Indian Ocean.
- Ctenella chagius is known as a reef-building coral, as the soft-bodied polyps secrete a hard skeleton which contributes to reef formation.
- Like many other coral species, Ctenella chagius forms a symbiotic relationship with algae, which provide the coral with nutrients through photosynthesis.
Ctenella chagius is classified as Endangered (EN) on the IUCN Red List (1) and is listed on Appendix II of CITES (2).
An ancient 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 being 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 which create characteristic wavy patterns across their surface (4).
Like those of 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 the 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 living 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 cause 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).
Localised 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, and 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 damage coral (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 the 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:
Find out more about the conservation of coral reefs:
The Coral Reef Alliance:
Coral Cay Conservation:
For more information on the British Indian Ocean Territory and the UK overseas territories:
The UK Overseas Territories Conservation Forum:
This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
- Algae: simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
- Colony/Colonies: a group of organisms living together. Individuals in the group are not physiologically connected and may not be related, such as a colony of birds. Another meaning refers to organisms, such as bryozoans, which are composed of numerous genetically identical modules (also referred to as zooids or ‘individuals’), which are produced by budding and remain physiologically connected.
- El Niño: a natural phenomenon that happens every 4 to 12 years, and lasts for several months, when upwelling of cold, nutrient-rich water does not occur. This causes the warming of ocean surface water off the western coast of South America and causes die-offs of plankton and fish. It also affects Pacific jet stream winds, altering storm tracks and creating unusual weather patterns in various parts of the world.
- Endemic: a species or taxonomic group that is only found in one particular country or geographic area.
- Photosynthesis: metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.
- Polyp: typically sedentary soft-bodied component of cnidaria, a group of simple aquatic animals including the sea anemones, corals and jellyfish. A polyp comprises a trunk that is fixed at the base, and a mouth that is placed at the opposite end of the trunk and is surrounded by tentacles.
- Symbiotic: describing a relationship in which two organisms form a close association, the term is now usually used only for associations that benefit both organisms (a mutualism).
- Zooplankton: tiny aquatic animals that drift with currents or swim weakly in water.
IUCN Red List (February, 2013)
CITES (February, 2013)
- Spalding, M., Ravilious, C. and Green, E.G. (2001) World Atlas of Coral Reefs. University of California Press, California, USA.
- Veron, J.E.N (2000) Corals of the World. Volume 2. The Australian Institute of Marine Science, Australia.
Procter, D. and Fleming, L.V. (1999) Biodiversity: The UK Overseas Territories. Joint Nature Conservation Committee, Peterborough, UK. Available at:
- Kitahara, M.V., Cairns, S.D., Stolarski, J., Blair, D. and Miller, D.J. (2010) A comprehensive phylogenetic analysis of the Scleractinia (Cnidaria, Anthozoa) based on mitochondrial CO1 sequence data. PLoS One, published online 8 July 2010.
- Barnes, R.S.K., Calow, P., Olive, P.J.W., Golding, D.W. and Spicer, J.I. (2001) The Invertebrates: A Synthesis, 3rd Edition. Blackwell Science, Oxford.
- Carpenter, K.E et al. (2008) One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science, 321: 560-563.