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Figure 2.42. Halobates, one of the few insects adapted to the marine environment. Scale bar indicates 1 cm.
After Anderson and Polhemus 1976
About 75% of the fauna of this zone is not found in the other zones (Frith et al. 1976). The fauna is best divided into three groups: tree, ground surface, and burrowing.
Perhaps the most striking change observed in going towards the dry lands is the rapid decrease in encrusting animals on the lower stems and trunks of the trees (fig. 2.43). The remaining tree fauna is mobile, for these animals can to some extent determine their immersion in water. It is largely composed of snails such as species of Littoraria and Nerita (fig. 2.44) which are also found in the pioneer zone. Further back from the sea Cerithidea obtusa and Cassidula are found, and even further inland are species of air-breathing pulmonate snails (fig. 2.45) (Budiman and Dar-naedi 1982). Most of these feed on algae growing on the sediment and move up trees when tides wet the ground, but Littoraria very rarely leaves the tree trunks. All these snails are able to breathe efficiently in air and the pulmonate snails, such as Ellobium, have lungs.
Figure 2.43. Relative abundance of four species of tree-dwelling animals along a transect through mangrove forest near Roraya, Tinanggea, on the south coast of Southeast Sulawesi, a - barnacle Balanus sp.; b - mussel Brachyodontes sp.; c - snail Littoraria 'scabra'; d - barnacle Chthamalus sp.
After L. Clayton pers. comm.
Figure 2.44. Nerita birmanica. Scale bar indicates 1 cm.
The encrusting animals on a tree also show a vertical zonation depending on their tolerance to desiccation. In a case examined in Southeast Sulawesi, the small Chthamalus barnacle appeared to have greater tolerance than the larger Balanus barnacle (fig. 2-46).
The animals seen on the sediment surface comprise mostly crabs that have emerged from their burrows, and snails, although the medium-sized mudskipper Periophthalmus vulgaris can be common. Among the snails, actual distance from the sea seems to matter less than details of ground conditions. In wetter areas such as where drains into small gullies, Syncera brevicula can be more common than anywhere else in the mangrove forest.
Molluscs at the seaward edge of the mangrove forest comprise a mixed sample of gastropods and bivalves. Further back in the mangrove forest, however, carnivorous and filter-feeding molluscs disappear. The vast majority of the molluscs in the true mangrove forest feed by grazing on algae or micro-organisms on the soil surface. Little is known about mollusc reproductive behaviour in mangrove forest but most have internal fertilization of eggs and, unlike many aquatic snails, have eggs which develop directly into tiny snails rather than water-borne larvae.
Figure 2.45. Air-breathing snails of the inland parts of mangrove forest, a -Ellobium auris-judae; b - Pythia pantherina; c - Auriculastra subula; d -Cassidula sulculosa. Scale bar indicates 1 cm.
Nearly all the mangrove crustaceans and worms make burrows which reach down to the water table. Many different types of tunnels are constructed (fig. 2.47) and the elliptical tunnels of the edible crab Scylla serrata may slope down from the bank of a river for as far as 5 m. In general, these burrows serve as: a refuge from predators at the surface, a reservoir of water, a source of organic food, a home for pairing and mating which is defended, and a place for brooding eggs and young, although no single species uses its burrow for all these purposes.
Figure 2.46. Vertical distribution of barnacles on a Rhizophora tree 10 m from the seaward margin of mangrove forest near Roraya, Tinanggea, on the south coast of Southeast Sulawesi, a - Balanus sp.; b - Chthamalus sp.
After L. Clayton pers. comm.
In the landward areas of true mangrove forest, the first signs are seen of an animal which itself is rarely seen, the mud lobster Thalassina anomala (fig. 2.48). This animal builds volcano-like mounds of mud which can reach over a metre high (fig. 2.47), and feeds on mud, digesting the algae, protozoa and other organic particles within it. The burrow below the mound is up to three metres long, extending down below the water table. The entrance leading to the main burrow is generally plugged with layers of earth. The habit of burrowing deeply in generally plugged with layers of earth. The habit of burrowing deeply in anoxic mud, closing itself off in poorly oxygenated air and water suggests that it may have evolved means of anaerobic respiration (Malley 1977).
The bivalve mollusc Geloina (fig. 2.49) lives buried in mud and can occasionally be found in this zone, but is more common in mangrove forests on islands in or near river deltas. Geloina is remarkable in its ability to feed, respire and breed so far from open water and at levels where it is sometimes not covered by tidal seawater for several weeks at a time.
Figure 2.47. Different types of animal burrows in mangrove forest areas. The horizontal line indicates the water table. A - crab Scylla serrata (with transverse section of burrow); B - mud lobster Thalassina anomala; C - crab Uca spp. (burrows may reach water table nearer the low-tide level); D - crab Sesarma spp.; E - peanut worm Phascolosoma; F - large mudskipper Periophthalmodon schlosseri; G - smaller mudskipper Boleophthalmus bodarti; H - pistol prawns Alphaeus spp.; smaller mudskipper Periophthalmus vulgaris.
From Berry 1972
Rivers, Streams and Gullies. The banks and beds of water courses in the mangrove forest have a fauna which is generally distinct from that on the forest floor. For example, many of the forest floor species of polychaete worms and crabs are missing, whereas juvenile fiddler crabs and some snail species are more common. The edible crab Scylla serrata18 occurs in firmer sediment near the larger streams and rivers in the mangrove forest where it is caught in traps (Anon. 1980a).
Terrestrial Margin. Far back from the sea, the soil of the mangrove forest is covered by fewer and fewer tides and suffers longer and longer intervals of exposure. Unlike other types of shore, there is virtually no wave action in mangrove forest to carry seawater higher than the true tidal level, because the wave energy is absorbed by the abundant tree trunks and roots. Thus, animals in the terrestrial margin live on a salt-impregnated soil but are covered by seawater only at irregular and infrequent intervals. Insects, snakes, lizards and other typically terrestrial animals are much more common here than further seaward. Sesarma and some large crabs occur here and in the Nypa palm swamps behind, where a small bivalve mollusc Enigmonia aenigmatica is found in association with the palms (Kartiwinata et al. 1979). The crabs' basic requirement is that their burrows must reach down to the water table. Where there is moving water some snails, such as Fairbankia and Syncera, and the large mudskipper Periophthalmodon schlosseri, may be found.
Figure 2.48. The nocturnal mud lobster Thalassina anomala.
Figure 2.49. The bivalve mollusc Geloina ceylonica.
From Berry 1972
Only one estimate of biomass
