Mauritius island is almost completely enclosed by an extensive reef system covering an area of 300 km2. The open areas are breaches 18 km wide in the south and 13 km wide in the west plus gaps near river mouths. Formation of coral reef around the island is estimated to be ⁓5000 years ago ; the oldest coral in Mauritius is the 1000-year-old brain coral in the Blue Bay marine park. More than 160 species of corals have been identified in Mauritius .
Mauritian waters provide an adequate environment for the growth of the beautiful reef around the island.
Mauritius is mainly enclosed by fringing reef that surrounds the coastline of the island leaving a shallow lagoon in between. The distance between the shore and the reef does not go beyond 5 m except on the eastern coast which is on higher grounds.
However, on the coast of Mahebourg, barrier reef is present extending from Ile de la Passe to ⁓9 km northwards. Fringing reef is found in its inner layers for over a distance of 1.5 km laterally along the shore. A 2 km (width) by 20 km (depth) canal separates the inner reef from the rest of the mould.
This is the backside of the reef that is in direct contact with the open ocean. As such, it is subject to pressure from the incoming waves and acts as a cushion against them. The reef front of Mauritius displays a rugged outline with pits and valleys in between. The foot of the reef may reach ⁓ -60 m.
Different coral species grow at different depths of the sea:
This is the upper surface area of the coral reef that can be seen from above. Mauritian reef exhibits a closed network formation reaching 20 m on average, crisscrossed by rills that allow the free passage of water from the open ocean into the lagoon.
The calm side of the reef is covered by stony corals, sea urchins and other echinoderms while the windward segment is dominated by skeletal coral and sea anemones.
It is the part of the reef that faces the coast. The width of the reef varies from place to place but in general, can be as big as hundreds of metres, being especially larger on the east coast. The depth of the reef is less than 1 m during spring tide but can go as deep as 20 m where canals are present.
Only isolated colonies of Acropora develop due to siltation from the land.
The lagoon of Mauritius is characterised by the presence of canals and basins along the seafloor such as at Blue Bay. They are often as wide as 0.5 m to 1.5 km and as deep as 6 m to 35 m. Polyps grow on the sides of the reef while the middle part is covered with sediment and hinterland materials that prevent polyp growth.