Mangroves share a number of features that allow them to live in shallow marine waters. They are shallow rooted with widely spreading roots and have prop roots from the trunk and branches. The shallow roots may send up pneumatophores to the surface allowing the roots to receive oxygen. The leaves are succulent with water storage tissue and increased salt concentrations. They grow only on tropical and subtropical shores that are sheltered form wave action. mangrove forests may penetrate some distance upstream along the banks of rivers. Because of the mud bottoms, interstitial circulation is poor which leads to anoxic bottom conditions.
In the Caribbean there are three principal mangrove species plus a dozen or so woody halophytes. Mangrove trees have different degrees of tolerance for exposure and submergence of their roots to seawater, thus the tidal fluctuation results in zonation just as in other intertidal habitats.
The red mangrove (Rhizophora mangle) is the pioneer mangrove in the Caribbean (first to grow in newly exposed muddy shore). It grows as a dense tangle in the water along the edge of the shoreline. Their arching prop roots provide an excellent habitat for many invertebrates. Shoreward from the red mangroves, but still in damp soil, are the black mangroves (Avicennia mangle). The black mangrove fares best where there is occasional inundation, but not by diurnal tides. They are characterized by prop roots that extend from the lateral branches but do not produce the tangle of roots seen in the red mangroves. They also have many pneumatophores that extend upward from the roots around the base of the trunk.
REPRODUCTION
The reproductive potential of red mangroves is high. Flowers are produced throughout the year. The seed germinates and remains attached to the parent tree. Long greenish-brown seedlings called "droppers" hang like daggers from the branches. Eventually they fall into the water where they float upright and they may be dispersed by currents. When they encounter shore, the seedlings put out roots and begin to develop leaves.
Mangroves provide a habitat for many animals. The mangroves and their associated organisms are collectively called the mangrove community. Because of the vertical extent of the trees, terrestrial animals can occupy the upper levels of the community while truly marine organisms can occupy the bases.
Larger organisms of the Rhizophora, or red mangrove community can be grouped into three general categories.
1. Those which inhabit the roots. They represent the hard bottom component of the community.
2. Those that live in the soft sediment that accumulate around the base of the mangroves.
3. Animals that can enter and leave the mangrove community. This represents the transient component. They generally rely on the other two community groups for sustenance.
The dominant groups of marine animals in the mangrove forests are molluscs, certain crustaceans and some fishes.
Animals living above the waterline on the prop roots of the red mangrove
include the mangrove periwinkle, Littorina angulifera, the isopod, Ligia,
and the mangrove crab, Aratus pisonii
The periwinkle is a browser and feed on the lichens that grow on the bark of the red mangroves. The mangrove crab, Aratus, lives about the water line but will submerge and cling onto the roots below the waterline for protection.
a host of invertebrates life on the roots below the waterline. The mangrove oyster, Crassostrea rhizophorae, the flat tree oyster, Isognomon alatus, barnacles (Chthamalus and Balanus) and mussels (Brachidontes) are common.
Lower in the root region a number of encrusting attached organisms can be found including several species of sponges, bryozoans, hydrozoans, and the fan worm, Sabellastarte. Like the bivalves and bryozoans, fan worms are filter feeders.
Another component of the red mangrove community includes organisms that live on the soft mud bottom such as swimming crabs (Portunidae) and various bivalves. The crabs are transient. The mud is usually anaerobic but the bivalves have siphons that allow communication with the oxygenated water above the bottom.
Mud flats associated with mangrove forests are inhabited by a number of crabs. Some, such as the fiddler crab (Uca) burrow in the soft mud. Goniopsus cruentata, known as the ³mangrove tree crab², is often seen among the mangrove roots near the waterıs edge. It inhabits the mud flat but does not construct burrows, but may take refuge in other crabıs burrows such as the borrows of Ucides cordatus. U. cordatus is found in the mud flat near the shoreline. Young land crabs, Cardisoma guanhumi are also found burrowing in the mud flats but adult Cardisoma prefer drier habitats for borrowing.
Mangrove forests form nesting and roosting sites for many large birds.