Blister Coral
(Horastrea indica)
Horastrea indica is a massive colonial coral species that is only found in the Western Indian Ocean in places such as Mozambique, Tanzania, Madagascar and some other smaller islands in the region. It is a rare species and as such, there is little information about Horastrea indica beyond that it is a zooxanthellate coral. Pale brown with blue-grey oral discs this species is found in sandy reef areas. Current threats to the population include destructive fishing, over fishing, pollution from sewage and coastal run-off, Crown-of-Thorn starfish, cyclones and coral bleaching. The frequency of cyclones and coral bleaching is exacerbated by the current change in global climate. There are currently no specific conservation measures underway for this species.
Urgent Conservation Actions
No current conservation action is being completed on this species.
African coastlines in the West Indian Ocean.

Evolutionary Distinctiveness
Order: Scleractinia
Family: Siderastreidae
The family Siderastreidae has experienced low levels of diversification and includes 6 extant genera. The genus Horastrea is monospecific meaning there is just a single species within it, that being Horastrea indica. This species has no close relatives.

Horastrea colonies are described as ‘massive’, a description that describes colonies as spherical in shape, not necessarily large in size. Horastrea indica is pale-brown in colour with blue-grey oral discs (centre of the corallite).
Horastrea is a colonial, stony coral 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.

The reproductive method of this species is currently unclear. The most common method used by reef building corals is that of broadcast spawning, however the Siderastreidae family contains a diverse group of species that have not been thoroughly studied.

An overview of hard coral ecology can be found here.
Colonies of Horastrea grow in sandy reef habitats at depths of up to 20m.
Horastrea is endemic to the West Indian Ocean and occupies a limited range along the southeastern coasts of mainland Africa, from Somalia to Mozambique and around the islands of Madagascar, Mauritius and Reunion.
Population Estimate
No specific population estimate has been made for this species however it is now considered uncommon.
Population Trend
The rarity of this species means that its population trend is unknown. Globally 19% of coral reefs have already been lost. Currently 75% of coral reef habitats are threatened, a figure that includes 65% of Indian Ocean reefs.

Analysis of coral reefs in Kenya found that the 1998 mass bleaching event had a dramatic effect on local corals in the region with cover dropping from as much as 45% down to as low as 10%. Since this time however the reefs have recovered at an average rate of 2% a year.
Vulnerable (VU) 2012.2 IUCN Red List
Horastrea indica is likely subject to many of the threats that face coral reefs worldwide. An overview of these threats can be viewed here.

Research is required to identify the threats which are of particular risk to this species. Bleaching as a result of temperature extremes is considered a real concern as the reef habitats within Horastrea’s range were severely impacted by high mortality in the mass bleaching event of 1998.

A major threat to all corals is the alarming rate of habitat loss. This loss is due to a range of stressors that vary spatially and temporarily. An overall increase in reef exposure to all known risks has occurred in recent years and as a result habitats are increasingly too hostile for animal growth.

Much of the coastline in this species range is subject to high levels of human disturbance which will pose a threat to this species. In Madagascar for example, rapid population growth has resulted in unsustainable consumption of marine resources. Furthermore, land use changes in terrestrial habitats are resulting in sedimentation and eutrophication within coral reefs systems.

Conservation Underway
No species specific measures have been proposed for this species. Positively however, some protection is offered to all corals through their inclusion in CITES Appendix II.

The number of marine protected areas (MPAs) and Locally Managed Marine Areas (LMMAs) within Horastrea’s range has also increased in recent years as Madagascar pledged to have 100,000km2 of marine waters protected by 2012. MPA’s such as Velondriake are being developed to integrate local communities and livelihoods with conservation efforts, in the process proving that habitat conservation and sustainable resource management can be economically advantageous.

Additionally, a range of ongoing regional initiatives aim to improve reef conservation around East Africa and the Indian Ocean. The programme CORDIO (Coastal Ocean Research and Development in the Indian Ocean) for example, conducts research on coastal and ocean ecosystems to inform the conservation of coral reefs and fosters the integration of science, practice and policy at all levels.

This project supports in-country EDGE Fellows to help conserve relevant EDGE species

This project will carry out the first population study of this Vulnerable species in Madagascar.

The objectives are to provide more understanding of the ecology of Horastrea indica and to integrate findings into the Velondriake management plan, to ensure adequate protection for the species.

Conservation Proposed
With no suggested conservation measures, the species would benefit from further research into all aspects of its ecology, therefore allowing for scientifically informed conservation proposals.

More effective management of existing MPA’s, provision of alternative sustainable livelihoods, and better management of marine resources would also benefit Horastrea corals. Alongside increasing the effectiveness of existing marine reserves, it is essential that new sites are offered legal protection as currently less than 3% of marine areas are protected globally, way short of the 2020 target of 10% set by the 2004 Convention on Biological Diversity.

Additional MPAs are proposed around Madagascar (totaling 1 million hectares) and research has been devoted to identifying priority regions, many of which lie within the known range of this coral.
Associated EDGE Community members

NGO dedicated to working with local communities to conserve threatened marine environments.

Rado works for Blue Ventures in Madagascar and is looking at the EDGE coral Horastrea indica.


Bruggeman, J. H. et al. 2012. Wicked Social–Ecological Problems Forcing Unprecedented Change on the Latitudinal Margins of Coral Reefs: the Case of Southwest Madagascar. Ecology and Society. 17 (4): 47.

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

Dublinsky, Z. and Stambler, N. 2011. Coral reefs: An ecosystem in transition. Springer. London, UK.

McKenna S.A. and G.R Allen, eds. 2003. A Rapid Marine Biodiversity Assessment of Northwest Madagascar. Bulletin of the Rapid Assessment Program 31, Part 1: Zooxanthellate Scleractinia of Madagascar, Conservation International, Washington, DC.

Obura, D. 2011. The Western Indian Ocean a treasure trove in its own right. Accessed on 06 December 2012.

Sheppard, C., Turak, E. & Wood, E. 2008. Horastrea indica. 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. Australian Institute of Marine Science, Townville, Australia.

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

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