Ninety percent of the United Kingdom's biodiversity lies in its
Overseas Territories, and the Chagos archipelago is by far the most biodiverse marine area in the United Kingdom's waters. Its habitats include extensive shallow limestone reefs and associated environments, about 300
seamounts and a deep sea trench - an underwater canyon more than 4,900 m (16,000 ft) deep.
Coral One of the most unusual aspects of the Chagos marine environment is its extremely healthy and diverse
coral cover, which is dense even in deep water and on the steep outer slopes of reefs. The area hosts 220 species of coral including the
Ctenella chagius, a variety of
brain coral believed to be endemic to the atoll, and
staghorn coral which is important to protecting low-lying islands from wave erosion. Chagos provides an important benchmark for coral conservation, and is a 'natural laboratory' in which we can study the functioning of these wonderfully complex ecosystems.
Fish The fish of the region are equally diverse, with at least 784 different species having been identified including the Chagos clownfish (
Amphiprion chagosensis) which is endemic to the archipelago. The strictly no-take Chagos Marine Reserve has freed Chagos' fish populations from fishing pressure within the reserve's boundaries. The marine reserve is an important refuge for overfished
pelagic species such as
manta rays,
sharks including
whale sharks,
tuna and is of particular importance for globally threatened species, such as the
silky shark.
Turtles The islands of the archipelago provide vital nesting sites for
green and
hawksbill turtles (
Chelonia mydas and
Eretmochelys imbricata). Since the hawksbill turtle is labelled ‘critically endangered’ and the green turtle ‘endangered’ on the
IUCN Red List, the Chagos populations are considered to be of international importance. It is estimated that 300-700 hawksbill and 400-800 green turtles nest annually across the 55 islands of the archipelago.
Mammals Environments of Chagos Archipelago provides rich biodiversity and support varieties of
cetacean species within the vicinity, such as three populations of
blue whales and
toothed whales (such as
sperm,
pilot,
orca,
pseudo-orca,
risso's and other dolphins such as
spinners). Regional extinct
dugongs were historically seen in the archipelago, hence the
Sea Cow Island was named due to the presences of the species.
Birds The breeding seabirds of the Chagos are considered to be of international importance. though the presence of human-introduced rats on several of the other islands severely hinder seabird nesting on these. Five species are considered to be breeding in internationally significant numbers: the
sooty tern (
Sterna fuscata); the
brown and
lesser noddy (
Anous stolidus and
Anous tenuirostris); the
red-footed booby (
Sula sula) and the
wedge-tailed shearwater (
Puffinus pacificus).
Coconut crabs The world's largest terrestrial
arthropod, the
coconut crab (
Birgus latro) is abundant on the islands of the Chagos archipelago, with an overall density in the conservation area on Diego Garcia of 298 crabs per hectare – the highest ever recorded. ==Scientific research==