All of the tropical oceans were joined together four to seven million years ago, before the continents drifted and Central America emerged above the sea to form a land barrier. Once this happened the Atlantic Ocean became separated and this isolation resulted in the evolution of a different group of corals. There are approximately 80 species of stony corals in the Atlantic Ocean compared to about 500 species in the Pacific Ocean.

Every island in the BVI is surrounded by coral reefs of varying size, health and composition. In addition to the stony corals which build the reefs, soft corals and seafans dominate many of the reefs. The Anegada Horseshoe Reef is the third largest in the Eastern Caribbean at 63km long, containing both patch reef and barrier reef. There are sixty-three popular dive sites in the BVI, which include fifty-seven coral reef sites and six artificial reefs that have been created by shipwrecks.

What are corals?
Corals are simple and primitive animals called polyps, like tiny sea anemones. They have a small fleshy body, with a ring of tentacles with stinging cells on top which catch tiny animals such as larvae for food. The polyps grow upwards, expanding their skeleton and while they grow they also divide by budding off identical twins, which also continue to grow upwards. They do this year after year, with some coral colonies living for hundreds of years.

Each species grows and buds in different ways, resulting in different coral shapes. Some grow branches, bushes or flat tables with polyps only one tenth of an inch or less in width. Corals with larger polyps may form domes or boulders with surfaces covered in star or flower shapes. Many deeper corals grow into sheets, whorls or scrolls which are designed to trap as much light as possible in these darker deep areas.

Many of these tiny animals need light to grow as their tissues contain millions of single-celled plant cells, called zooxanthellae which use light to make sugars through the process of photosynthesis. The sugars and oxygen made by the plant cells are used by the animals and the carbon dioxide and nutrients produced by the animals are used by the plant cells. This is called a symbiotic relationship.

Coral growth
Corals and other reef species create vast quantities of durable limestone rock as they grow, creating their own substrate, which becomes the reef. The skeletons of the largest corals are durable and form solid rocks, but even the fragile corals contribute to the reef-building process. Many become ground-up into sand when they die, which settles into cracks and crevices, where over many years, it cements into durable forms of limestone.

Branching corals grow faster than boulder corals, at a rate of up to 4 inches (10cm) per year. The growth rate depends upon a number of factors, including water temperature. A core through a large boulder coral will show growth rings, like tree rings, which can be counted to determine the approximate age of the coral.

Soft corals and other relatives
Soft corals are very similar, but instead of laying down a stony skeleton they establish a rubbery matrix of organic matter. Soft corals, such as seafans can be very colourful and contain polyps like stony corals.

Where are corals found?
The sea is warm enough for corals to grow between the Tropic of Cancer in the north and the Tropic of Capricorn in the south. The BVI lies within these areas and so coral reefs are found throughout the BVI’s marine environment.

Threats to corals
Approximately one third of the world’s reefs have been killed or damaged by misuse and overuse by humans. The major threats to coral reefs in the BVI are water pollution, sedimentation, coastal development, over-fishing, anchor damage and disease from sewage.

Sediment is washed down from the hillsides during rain storms and smothers the corals, preventing their ability to photosynthesize. Overfishing and coral disease can change the structure of the coral reef community and result in algae taking over, which can prevent new corals from settling and the reef from growing. Development along the coastline can result in the destruction of corals by landfill, sewage and overuse. Water pollution can be caused by untreated sewage from boats and waste that has been washed off the land, containing chemicals, oils, fertilizers and household sewage.

Coral Disease
Pollutants and sediment can cause coral diseases such as ‘black-band disease’ and ‘white-band disease’ which directly kill corals. One devastating coral disease in the Caribbean killed off almost all of the shallow-water Elkhorn coral, whose large branches provided an essential wave break along the coastline. The remains of these dead Elkhorn reefs can be seen throughout the BVI, with only a few areas beginning to grow back. 

Coral Bleaching
The effects of global warming are visible in the BVI as the ocean surface is warming. Corals respond to the heat by expelling the zooxanthellae plant cells that live within their tissues. As these give the corals their colour, the white limestone skeleton soon becomes visible and they appear “bleached”. A coral may survive for some time without the zooxanthellae and if the sea surface temperatures return to normal, the zooxanthellae will come back.

Bleached Coral Photo: S. Gore

Coral bleaching has been recorded in the BVI before, but these events are becoming more frequent and much more severe, with sea surface temperatures rising much faster in some areas around the world. In 2005 a major bleaching event began in September and October in the BVI, USVI and Puerto Rico with sea surface temperatures of 86°F / 30°C throughout the depth range of reefs. This is the warmest temperature ever recorded for the northeast Caribbean region and resulted in almost 90% of the BVI reefs being bleached. In addition to corals dying from long-term bleaching, it can result in less fish and other marine creatures that are dependent on coral for food and shelter.

NOAA Sea Surface Temperature October, 2005 (Dark Red Symbolizes Very Warm Water)

The Conservation and Fisheries Department monitor these bleaching events to learn more about their effect on BVI reefs and the long-term survival of bleached corals.