CROCODILE ADVENTURE & BIRD WATCHING
 

 
  Inhabitants of inter-tidal zones throughout the tropics and subtropics, mangroves are a most curious collection of plants. It is easy to make the erroneous assumption that the different kinds of mangrove trees are closely related species adapted to the unique conditions in which they grow. Even their English common names (red mangrove, black mangrove, white mangrove, buttonwood mangrove, etc.) lend to the idea that these are merely different species of the same plant family, as if they were maples or oaks. In reality, the mangroves are a wonderful example of convergent evolution a situation in which totally unrelated organisms have evolved certain similarities simply because those are the characteristics best suited for making use of a particular resource. And in this case the resource is a place where a plant might grow if it can overcome the two major difficulties faced by mangroves: the salinity of the sea water which saturates the ground they grow in and the absence of oxygen in that same saturated mud. In Costa Rica there are seven species of mangrove trees from four very different plant families: the red mangroves (Rhizophora mangle and R. harrisonii, Rhizophoraceae), the black mangroves (Avicennia germinans and A. bicolor, Verbenaceae), the tea mangrove (Pelliciera rhizophorae, Theaceae), the white mangrove (Laguncularia racemosa, Combretaceae), and the buttonwood mangrove (Conocarpus erectus, Combretaceae). In Costa Rica, any and all of these plants are called mangle (man-gley) and the association in which they grow a manglar. Of the mangroves found in Costa Rica, the red mangrove is the most easily recognized with its striking aerial prop roots, which often branch one or more times before reaching the ground. The primary function of these roots is not to support the tree but to aid in the aeration of the plant's sap system. The black mangrove, and to some extent the white mangrove, cope with the lack of air in the mucky substrate by developing vertical extensions from their roots which stick above the soil level and (at low tide) accomplish oxygen exchange. The tea mangrove has pronounced buttresses which act as aerators. The buttonwood mangrove effectively avoids this problem by growing on the back edges or higher ground within a mangrove swamp, thus reducing the likelihood of having the soil around its roots supersaturated except at extreme high tides. Mangroves have developed various ways to deal with the problem of high salt concentrations in the water around them. Some species secrete salt from their roots and/or leaves. Pacific coast black mangroves can be observed in the dry season with salt crystals along the outer edges of their leaves (secretion of salt through the leaves happens throughout the year, but is not usually observed in the rainy season because the rain washes off the salt).

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