In tropical and subtropical areas a characteristic feature of the upper levels of estuaries and sheltered parts of the seashore is the mangrove swamp forest. This develops on mud flats which are exposed at low tide. The genus Rhizophora is a common mangrove tree which grows to large size, supported above the surface of the mud on a number of downcurving prop-roots resembling flying buttresses. These prop-roots contain air spaces which provide oxygen for the underground root system embedded in the waterlogged, oxygen-deficient mud. In some genera there are also aerial roots growing vertically out of the mud as slender, erect structures. There may also be 'pillar-roots' which support the branches. This elaborate rooting system reduces water movement and entraps and stabilizes the mud so that the mangrove forest tends to increase in extent, forming broad flat areas of swamp cut by drainage channels through which the sea flows with rise and fall of the tide. The spread and consolidation of the forest are assisted by a reproductive peculiarity of some mangrove species. These have seeds which germinate while still within the fruit borne on the tree, producing a long slender radicle growing downwards from the branch sometimes as much as 60 cm before dropping from the tree to stick into the mud. As the forest advances, the vegetation becomes zoned between land and sea with different species of mangrove at each level and different communities of smaller salt-marsh plants between the trees.
The mangrove swamp harbours a complicated community of animals. The roots of the trees provide a secure substrate for a variety of attached animals, especially barnacles, bivalves, serpulid worms and tunicates. Fish and free-living molluscs and crustaceans find shelter in the crannies between the roots. In the mud are large numbers of burrowing crabs, molluscs and fish, and the branches of the trees contain insects, lizards, snakes, birds and monkeys. Details of some shores of this type, common at low latitudes, can be found in references at the end of this chapter (Macnae, 1968; Sasekumar, 1974; Teas, 1983).
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