Coral Reefs

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.

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.

No specific population estimate has been made for this species however it is considered locally common.

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).

Endangered (EN) 2012.2 IUCN Red List.

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.

 

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.

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.