LARGER FORAMINIFERA FROM THE CHAGOS ARCHIPELAGO - THEIR SIGNIFICANCE FOR INDIAN-OCEAN BIOGEOGRAPHY

The Chagos Archipelago is a series of atolls situated in the centre of the Indian Ocean close to the equator and geographically isolated by surface water currents and deep water barriers from other areas of shallow water environments. The region is exposed to small tides and periods of strong winds. The combined effects of these is to cause relatively high energy conditions to exist in the shallow waters around the reefs; the bottom sediments are coarse grained and they form a mobile substrate. Although the coral faunas are diverse, seagrasses are rare. The larger foraminifera (> 1.4 mm) are principally Amphisorus hemprichii, Heterostegina depressa and Operculina ammonoides. Many of the tests are damaged due to postmortem transport and consequently depth distributions are distorted. However, A. ammonoides is confined to the deeper waters of the lagoon. Other taxa > 1.4 mm in size are widely distributed in modest abundance. Overall, the diversity of the fauna is lower than that observed in atolls of the Mauritius and Comoro island groups. This is attributed to geographic isolation.