Ctenella coral
(Ctenella chagius)
VU
Overview
Ctenella chagius is a member of the small Meandrinidae family and found exclusively in the Chagos Archipelago, Mauritius and La Réunion. Ctenella chagius grows in hemi-spherical colonies which may be cream, green or light brown in colour. This species can be found on reef slopes and in lagoons at depths ranging between 3 and 45m. There is no population information for this species but it is thought to be in decline and is listed as endangered on the IUCN Red List of Threatened Species. Like other corals Ctenella chagius is threatened by coral bleaching and ocean acidification. The recently designated Chagos Archipelagos Marine Protected Area, an area of 210,000 miles2, now provides considerable protection from direct human.
Urgent Conservation Actions
No current conservation action is being completed on this species
Distribution
Ctenella chagius has a very restricted geographical range in the Western-Central Indian Ocean.

 
Fact
Ctenella chagius can withdraw its tentacles in less than 1.5 seconds.
Media from ARKive
ARKive image - <i>Ctenella chagius</i>
ARKive image - <i>Ctenella chagius</i> in coral reef habitat
ARKive image - <i>Ctenella chagius</i> small colony
ARKive image - <i>Ctenella chagius</i> elongated colony
ARKive image - <i>Ctenella chagius</i> colony
ARKive image - Damaged <i>Ctenella chagius</i>
Evolutionary Distinctiveness
Order: Scleractinia
Family: Meandrinidae
Ctenella chagius is mono-specific meaning that it does not share its genus with any other species. The family, Meandrinidae is poorly defined and this has led to previous debate regarding its members. In the Indo-Pacific there are just three representatives of the family, Ctenella chagius being one of these. This species is considered to have an above average evolutionarily distinctiveness.

 
Description
Size: 
Up to 1m across
Ctenella chagius colonies are mostly submassive, meaning that they have a rounded structure within which corallites, the external skeletons of each polyp, are joined in long rows that are separated by valleys. These qualities give the coral a characteristic brain-like appearance. Ctenella chagius is also an encrusting coral, which means that it can adhere strongly to a substrate and grow horizontally as a thin crust rather than developing a vertical structure. The colonies may be cream, green or light brown in colour with transparent tentacles.
Ecology
An overview of hard coral ecology can be viewed here.

Ctenella coral is a colonial, stony species meaning that as the individual animals (polyps) of this species grow they exude calcium carbonate to form exoskeletons (corallites) for protection. Specific oceanic conditions are required for polyps to synthesize and exude calcium carbonate.

As a zooxanthellate species this coral obtains a subset of its energy requirements from a symbiotic relationship with algae called zooxanthellae. Zooxanthellae live in the tissue of host corals and require sunlight for photosynthesis, a process that produces energy for the algae and its host coral.

Ctenella chagius has also shown an aggressive ability known as extracoelenteric digestion where a coral is able to extend its stomach in digestive filaments onto the living tissue of an adjacent coral and destroy it. This provides Ctenella with an important ecological advantage in a fiercely competitive struggle for light and space in reef ecosystems.

The reproductive habits of this species are currently unknown.
Habitat
Found on reef slopes and lagoons at depths of 3 - 45 m but is most abundant at 6-18 m on reef slopes and at around 3m in lagoons.
Distribution
Ctenella chagius has a very restricted geographical range. It is found only in the Indian Ocean around the Chagos Archipelago, Mauritius and Reunion.
Population Estimate
No specific population estimate has been made for this species however it is considered locally common.
Population Trend
The population of Ctenella chagius is thought to be in decline. It has been estimated that 58% of the habitat in which Ctenella chagius is found has been destroyed over the past three generations (30 years).
Status
Endangered (EN) 2012.2 IUCN Red List.
Threats
Corals worldwide are subject to a multitude of different threats, many of which vary spatially and/or temporarily as a result of climate or anthropogenic activity. Ctenella chagius is undoubtedly subject to at least some of these threats, most of which are detailed here.

Of the many threats faced by this species, all of which are increasing in severity, there are some to which Ctenella chagius is particularly predisposed, largely as a result of its geographical range.

Coral populations in the Chagos Archipelago have been particularly effected by coral bleaching as a result of rising sea temperatures over the past 60 years. Mass mortality across the archipelago occurred in 1998 (with 80% of reefs impacted) and again locally at Egmont Atoll in 2005 where 95% of corals died. However, recovery of coral reefs across the archipelago has been very promising in part due to the low level of human disturbance and low pollution levels. 

In Mauritius and Reunion direct human impacts on coral reefs are of greater concern than in the Chagos Archipelago where there is no resident population. Human impacts include pollution and coastal development, which can lead to sedimentation. Sediment may smother corals and cause them to starve or remain suspended in the water column and reduce light availability to underlying colonies. Over-fishing and destructive fishing techniques are also a threat to coral reefs. The loss of herbivorous fish as a result of over-fishing and can lead to phase shifts where algae become the dominant benthic species on the reef, out-competing the corals for space and light. 

Whilst the species has been subject to international trade, CITES recorded only 2 specimens exported since 2007 suggesting the threat from this activity is minimal.

 
Conservation Underway
Ctenella chagius is listed on CITES Appendix II which regulates the export of threatened species.

There are no species specific conservation measures for Ctenella chagius but in April 2010 the entire British Indian Ocean Territory was designated as the Chagos Marine Protected Area. At the time of creation, this was the largest marine reserve in the world. Much of the home range of Ctenella chagius lies within this reserve and is de-facto protected. Beneficially the Chagos MPA is a no-take area which means it is no longer subject to any fishing pressures and as the territory is mostly uninhabited the threat of pollution is greatly diminished. The challenge is now to enforce such a large and remote MPA.
Conservation Proposed
In the 2004 Convention on Biological Diversity, world leaders committed to protecting 10% of marine surface area by 2020. This target had seemed increasingly unlikely but in recent years there has been a real surge in MPAs. Since the creation of the Chagos Marine Protected Area in 2010, numerous other large scale marine reserves have been either proposed or implemented. The result is that 2.3% of the ocean’s surface is now protected, a small absolute percentage that nonetheless represents a ten-fold increase in MPAs within a decade.

In addition to expanding and creating new MPAs it is essential that these areas are properly managed, logically located and effectively enforced. 

To address the problem of ocean acidification and rising sea temperatures action must be taken to reduce the rise in atmospheric CO2 levels. Although governments need to address this issue directly, individuals can also contribute to this effort by changing their own behavior to reduce their carbon footprint.

In addition to the actions already proposed, further research into the ecology and life history of Ctenella chagius would help the development of species-specific conservation measures and may provide insight as to why this species is susceptible to bleaching. Moreover, more timely access to national trade data for this species would benefit monitoring of this species trade.
Links
References

Burke, L et al. 2011. Reefs at Risk Revisited. World Resources Institute. Washington DC.

Fukami, H., Chen, C.A., Budd, A.F., Collins, A. Wallace, C. Et al. 2008. Mitochondrial and Nuclear Genes Suggest that Stony Corals are Monophyletic but Most Families of Stony Corals Are Not (Order Scleractinia, Class Anthozoa, Phylum Cnidaria). PLoS ONE 3(9):e3222. Doi:10.1371/journal.pone.0003222.

Harrabin, R. Marine Protected Areas increase 10-fold in a decade. BBC website. Accessed 06 December 2012.

IUCN and UNEP. 2009. The World Database on Protected Areas (WDPA). UNEP-WCMC. Cambridge, UK.

Porter, J.W., Battey, J.F. and Smith, G.J. 1981. Perturbation and change in coral reed communities. Proc.Natl.Acad.Sci.USA. 79:1678-1681.

Secretariat of the Convention on Biological Diversity (2009). Scientific Synthesis of the Impacts of Ocean Acidification on Marine Biodiversity. Montreal, Technical Series No. 46, 61 pages.

Sheppard, C.R.C., Dinesen, Z.D. and Drew, E.A.. 1983. Taxonomy, Ecology and Physiology of the geographically restricted scleractinian species Ctenella chagius Matthai. B.Mar.Sci. 33(4):905-918.

Sheppard, C., Turak, E. & Wood, E. 2008. Ctenella chagius. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. . Downloaded on 06 December 2012.

Veron J.E.N. 2000. Corals of the World. Volume 2. Townsville. Australian Institute of Marine Science.

Veron, J.E.N. et al. 2009. The coral reef crises: The critical importance of <350 ppm CO2. Mar Pollut Bull58:1428-1436.

Wilkinson, C. 2008. Status of coral reefs of the world: 2008. Global Coral Reef Monitoring Network and Center, Townsville, Australia. p119-122, 105-111.

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