Eastern African Marine Ecoregion

The Eastern African Marine Ecoregion and its Biodiversity

The inshore waters of the eastern African marine ecoregion support around 1,000 different seaweeds, several hundred sponge species, over 200 coral species, more than 3,000 species of molluscs (oysters, cockles, mussels and clams), over 300 species of crabs, at least 50 species of starfishes, over 100 species of sea-cucumbers and more than 1,500 species of fish. As studies continue, the number of species recorded for these waters continues to rise, but already we know that the eastern African marine ecoregion supports an incredibly rich species composition, easily exceeding 11,000 species of plants and animals.

About 15% of the marine plant and animal species occurring in the region are pan-tropical, that is they can be found in all warm seas. Between 60 – 70% are found only across the Indo-Pacific, a vast area of similar tropical conditions that stretches from eastern Africa to the eastern Pacific Ocean islands of Hawaii and Polynesia, whilst 10 – 15% of the eastern African marine life is found nowhere else on earth. These species (including several species of corals, starfish, molluscs and fish) are said to be endemic to the region. All of the larger animals and plants of the ecoregion that have been described so far can be seen with the naked eye, but there is also an abundance of minute life forms. These microscopic creatures include much of the plankton, bacteria and fungi. Though vital to all other life, these tiny creatures are much less well known, but could well number tens of thousands of species, and thus the larger creatures represent only a small percentage of the total marine biodiversity of the region. As research continues we will learn more. Even for the larger animals there is still insufficient information on species distributions and abundance. For example, the well-known, deep water, ancient fish, the Coelacanth, first discovered in 1938 by fishermen off South Africa, was thought to be endemic to the south-west Indian Ocean until the discovery in 1998 of a close relative of this fish off Sulawesi ( Indonesia), 10,000 km from its previously known population.

On the seashores, and in the shallow coastal waters, the habitats that contain the bulk of the larger plants and animals are easy to identify. These are the sand beaches and dunes, mangrove forests, river deltas, seagrass beds, rocky shores, mud flats, coastal lagoons, coral reefs and open waters. Each habitat has its characteristic compliment of species, though many species require more than one habitat. Fish and shrimps for example will move from one habitat to another for food, breeding or for refuge.

Beaches and coastal mud flats

The beaches and coastal mud flats of the region provide feeding and breeding areas for about 35 species of resident and migratory seabirds. Between October and March each year, hundreds of thousands of shorebirds fly from their breeding grounds in northern Europe to feed on the mud flats of the large mangrove estuaries of Lamu in Kenya, the Rufiji River in Tanzania and the Zambezi Delta in Mozambique. Other resident seabirds, nesting on small, isolated islands will scout the seas for hundreds of kilometres in search of sardines and anchovies to feed their young.

Seagrass beds

Further down the shore, seagrassesform extensive beds on mud and sand, to depths of 15m or more, though they are restricted in their depth range by the presence of light. These are the only true lowering plants to have colonised the sea and 12 species are found in the region. Seagrass beds are home to thousands of small species of animals and plants including seaweeds, sponges, worms, crabs, shrimps, marine snails, starfish, sea-cucumbers and fish. Some of these depend on the seagrass beds for shelter, food or as nursery grounds. Much larger creatures like the Dugong (or sea cow, a distant relative of the elephant) and marine turtles also feed on seagrasses.

Mangrove forests

Mangrove forests occupy the largest area of all these coastal habitats, typically around river estuaries. Ten species of salt-resistant trees, some reaching 20m in height, plus numerous shrubs and palms, form the mangrove forests of eastern Africa, all specially adapted to survive in sea water and root in mud or sand. Mangroves are one of the most productive habitats on earth. When exposed at low tide the forests teem with crabs, worms and snails, many of which are food to birds. At high tide mangrove forests attract hundreds of species of fish, crabs and shrimps which swim among the submerged branches and depend on the forests as feeding areas and nursery grounds for their young. The best developed forests occur around river mouths where they are important in trapping river sediments that would otherwise be washed out to sea. Kenya has a total area of mangrove of about 53,000ha (530 square kilometres) and Tanzania, with a total of 133,000ha (1,330 square kilometres) also contains the largest continuous mangrove forest of 53,000ha in the Rufiji River delta. Mozambique has by far the largest mangrove area in the region with 500,000ha (5,000 square kilometres) scattered along its 2,800km coastline, and the South African coastline supports a smaller extent of mangrove, the southermost forests of the continent.

Coral reefs

Coral reefs form extremely species-rich habitats, rivalled only by tropical forests in terms of their diversity and productivity. They are a community based on rock-forming coral animals and algae that exist and grow into large submerged mounds, slopes and islands. Non-reef building corals and seaweeds, soft corals and sponges are also important members of this community. The coral animals harbour microscopic algal cells (zooxanthellae) within their own cells that allow them to use sunlight to make food and so help to build their limestone structure. The depth of coral reefs is therefore limited by light penetration with few reefs extending deeper than 40 metres. Growth of the rock-like corals varies between a few millimetres to 10cm per year, depending on the species and water conditions. In the eastern African marine ecoregion fringing coral reefs are the most common type of reef, in many places forming continual stretches of 100km or more, and constituting most of the estimated total of 1,500km of reefs along these shores.

The open waters

The open watersof the region are important for large pelagic fish including various species of tuna and billfish, such as the black marlin whose distribution is restricted to eastern Africa and Australia. The region also provides crucial nursery or feeding habitats for 34 species of marine mammals, including the endangered Humpback whales that migrate from Antarctica each year into the warm waters off eastern Africa to give birth. The depths of the seas are little known, as specialist equipment is needed to explore and study this dark and hazardous world. Certainly animals unknown to science are yet to be discovered. The Coelacanth for example, is now known to live at depths of 50 – 400m in southern Mozambique and northern South Africa. Deep-water sharks, lantern fish and giant squid may also be found at these depths.

Typically the coast of the eastern African ecoregion is comprised of a mixed array of the coastal habitats described above. These various habitats are often closely connected physically and are linked both through the coastal waters and by the species they comprise.