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The 115 granite and coral islands of the Seychelles are like emeralds, scattered across the ocean just south of the equator. The many coral islands of the archipelago stretch in an arc, almost all the way to the east coast of Africa.
The Northern Coral Islands (Northern Coral Group) is a collective term for the islands that are located within the Seychelles reef plain, but are not made of granite like the other inner islands. The Northern Coral Islands include Bird Island and Denis Island.
The Southern Coral Islands (Southern Coral Group) are the isolated islands located outside of the Seychelles reef plain, consisting of coral, and include groups such as the Amirantes, as well as islands including Coëtivy and Platte.
Coral reefs are sea formations that are formed of cnidarians, and are the largest living organisms on earth. The total area of present-day coral reefs is around 600,000 km² (230,000 sq. miles). Coral islands and reefs are restricted to warmer seas, as they need a temperature of between 18 and 30 °C to survive.
A coral island is caused by permanent changes in sea levels. As coral can grow all the way up to the surface of the water, if the sea level drops then these can become islands, often in the form of an atoll.
In its initial stages, a coral island is still a very inhospitable habitat, with few nutrients available for animal life. A fresh water supply is also an issue for land plants. Therefore, the first animals to settle on coral islands tend to be creatures such as sea turtles and birds who rest and nest there, but find their food in the sea. They then bring plant seeds onto the islands in their feathers or they are excreted in their faeces.
Other seeds may be brought through the air or, as with the buoyant seeds of many mangrove plants, wash up on the shore. The first plants to successfully colonise a coral island are usually simple species, able to cope well without water or with salty water supplies, as well as without nutrients. When they die and decompose, their remains contribute to the formation of compost, which then improves the water retention in the soil, helping other plants to grow.