Pioneer Zone. The pioneer zone is that area where seedlings of Avicennia and Sonneratia grow and which is covered by almost all high tides. The sediment is essentially the same as in the open areas and the fauna is also similar, although fewer strictly marine animals are found. Snails such as Syncera, Salinator and Fairbankia may occur on the wet surface of the sediment (fig. 2.38).

      Mudskippers are common and large- and medium-sized fiddler crabs Uca can be very abundant. Various species of polychaete worms, a few bivalve molluscs and the peculiar peanut worm (fig. 2.39) which can store oxygen in its coelomic fluid when the tide is out (Brafield 1978) live permanently in the soil of this zone.

      The abundance and type of fauna living on the mangrove trees depends largely on the age of the tree—older ones have denser populations consisting of more species. Littoraria snails (fig. 2.40) occur on almost all the vegetation, sometimes up to 2 m above the soil surface. Some species are generally found on leaves while others are found mainly on bark (Reid 1986). Various authors have also noticed that individuals of a species can be pale coloured in trees and dark on the ground (Sugondo 1978; Anon. 1980a; Cook 1983) presumably as a means of camouflage. Populations of sedentary animals encrust the lower stems of trees as they grow. These animals typically include barnacles with the larger Balanus amphitrite below and the smaller Chthamalus withersii extending higher; oysters, commonly Crassostrea cucullata; and the small black mussels Brachyodontes sp. which are attached to the tree by 'byssus' threads. A total of 15,401 animals, 60% of which were mussels of the genus Brachyodontes, have been found on a single Avicennia tree (Tee 1982). These attached fauna may be eaten off the lower stems by the carnivorous snails Thais and Murex (fig. 2.41) which may move in groups decimating their prey (Broom 1982). Barnacles and mussels sometimes suffer 50% mortality in the first 10 m above the soil surface but the number of dead animals decreases upward, indicating that predation higher up the tree is less (Tee 1982).

      Figure 2.39. Peanut worm, Phascolosoma from the pioneer zone of the mangrove forest.

      Figure 2.40. Littorina scabra (left) and L. carinifera (right). Scale bar indicates 1 cm.

      Around the roots of mangrove trees can sometimes be found a bright green sea anemone which, when disturbed, buries itself very quickly, so that collecting a specimen is a very frustrating task (Hickson 1889).

      Two or three species of hermit crab are found in this zone. These crabs have lost the hard protective carapace over the rear part of their bodies and depend for protection on finding empty shells. The combination of security and mobility of the adopted shells is clearly advantageous and hundreds of hermit crabs can sometimes be seen on a beach. Suitable shells are, unfortunately for the crabs, a limited resource and this affects growth and reproduction. It has been found that crabs with roomy shells put their energy into growth and do not reproduce, whereas crabs with tight shells, stop growing and put their energy into reproduction. The scarcity of shells leads to active competition between species of hermit crabs (Bertness 1981a, b).

      When water inundates the pioneer zone it is sometimes possible to find the pond skater/water strider Halobates, one of the very few insects that has adapted to the marine environment (fig. 2.42) (Anderson and Polhemus 1976; Cheng 1976).

      Eroded Banks. The edge of mangrove forest along a winding estuary is often marked by a nearly vertical bank 1-1.5 m high instead of a sloping pioneer zone. This is caused by currents sweeping away the consolidated sediment and is most obvious on the outer bend of rivers, where the current flows faster than on the inner bend. The bank may be broken in places and mangrove trees at its edge often fall into the sea as the sediment is eroded slowly away. The top of this bank is usually at, or slightly above, the mean high water of neap tides, and may sometimes be left 9-10 days without tidal cover. The sediment of an eroded bank resembles that of the mangrove forest floor behind it, with less fine sand (commonly about 65%) than in the pioneer zone. The bank is burrowed into by various crab species (Berry 1963).

      Figure 2.41. Thais (left) and Murex (right), carnivorous snails of the mangrove forest. Scale bars indicate 1 cm.

      True Mangrove Forest The fauna of this zone differs in several respects from that in the preceding zones. Most of the ground is very flat and the sediment surface is exposed to the air for an average of 27 days per month. Since the trees provide heavy shade, however, the humidity is very high, and this together with the poor drainage means that the soil rarely dries out. There is also abundant leaf litter and other organic matter so that detritus feeders

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