of aquatic mangrove fauna seems to have been made in the Indo-Malayan Region and that was for tree-dwelling aquatic fauna in four types of mangrove forest (table 2.9). The results show a general reduction of biomass with increasing distance from the sea. It is supposed that the turnover rate (time for one generation to replace another) must be quite rapid because most of the tree-dwelling aquatic fauna are in the lower trophic group (i.e., suspension feeders), none of which have long life spans (Tee 1982).
The biomass of molluscs in an Australian mangrove area was greater on the mudflats and pioneer zone where they tended to be filter-feeders, than in the forest where they were largely deposit-feeders. The crabs were very abundant in the open areas but they had a relatively low biomass. The biomass, density and diversity was highest in Avicennia forest but lower in Rhizophora forest (Wells 1984).
Terrestrial Fauna of Mangrove Forest
Insects, birds and mammals live chiefly in the canopy of mangrove forest. Ground-living animals such as rats, pigs and lizards only venture into the landward edge of mangrove forest for brief forays. Macaques will eat Sonneratia and other fruit and descend to the ground to search for crabs, peanut worms and other suitable food. Flying foxes Pteropus commonly roost in the mangrove canopy and others with feathery-tipped tongues such as the cave fruit bat Eonycteris and long-tongued fruit bat Macroglossus are important for the pollination of Sonneratia. In addition, species of insectivorous bats roost in the forest and different species feed in different microhabitats: over the canopy, just above the canopy, in the open beside the trees, and inside the forest. The composition of the guilds of bats feeding in each microhabitat can to some extent be predicted from their wing morphology and details of this from a study in northwest Australia can be found elsewhere (McKinzie and Rolfe 1986). Prediction of ecology and behaviour from gross and dental morphology can be useful tool for little known species of animal (Clutton-Brock and Harvey 1977; Kay and Hylander 1978).
From Tee 1982
The frog Rana cancrivora is common inland around lakes and other aquatic habitats, but is exceptional among amphibians in being able to live and breed in weakly saline water. The tadpoles are more resistant to salt than the adults and metamorphosis into adults will only occur after considerable dilution of the salty water (MacNae 1968).
Mangroves are inhabited by a variety of reptiles such as the monitor lizard Varanus salvator, the common skink Mabuya multifasciata, and the venomous yellow-ringed catsnake Boiga dendrophila with 40 to 50 narrow yellow rings around its body (fig. 2.50). Most snakes seen in or near mangroves are not in fact sea snakes (Hydrophidae) which live primarily in open water (p. 232). The prey of mangrove snakes comprises primarily small fish (Supriatna 1982). Potentially the largest animal of the mangrove swamps is the estuarine crocodile Crocodylus porosus, but persecution for centuries has reduced its number to a very low level and large specimens (they can exceed 9 m) are extremely rare around Sulawesi (p. 303).
The most conspicuous of the insects are mosquitoes, and the larvae of some species can live in water with a salinity of 13 ppt (MacNae 1977). Species of Aedes mosquitoes have been seen feeding on mudskippers but they are also attracted to human skin (p. 617). Mosquito collections made in an area of coconuts near the mangrove-fringed north coast of Bolaang Mongondow included large numbers of the potential vector of malaria Anopheles subpicta (Hii et al. 1985). Another potential insect hazard is the leaf-weaving ant Oecophylla smaragdina (fig. 2.51) which makes nests by glueing together adjacent leaves while they are still attached to the tree. The making of the nest is extraordinary because the adult ants have no means of producing a sticky secretion to join the leaves of the nest together. The larvae do have appropriate glands, however, and when a leaf is to be added to the nest or a tear repaired, some of the worker ants seize the leaf edges to be joined and hold them in the required position. Other workers enter the nest and collect larvae. These are held near the head in the workers' jaws and moved back and forth between the leaf edges, as they secrete the glue (Sarasin and Sarasin 1905).
Fauna of Beach Forests, Particularly Coconut Crabs
Virtually nothing seems to have been written about the fauna of beach forests, although travellers sometimes report seeing macaques in the trees.
An interesting snail Cochleoslyla leucophtalma has been observed in bushes near the beaches of Sangihe Island. This animal, about 2 cm across with a brown and white shell and an orangey-red body, lies across a leaf and bends its body over so the edges meet. It then starts to glue the edges together with mucus to form a bag. Before finishing, it lays large eggs inside the bag, completes the glueing and then eats a hole in the leaf blade which it covers with a very thin film of mucous. This is supposed to ensure that some air enters the leaf bag (Sarasin and Sarasin 1905).
Figure 2.50. Yellow-ringed catsnake Boiga dendrophila.
After Tweedie 1983
Figure 2.51. Oecophylla smaragdina which sews its leaf nest with larval secretions.
The beach forest animal about which most is know is the coconut crab Birgus latro (fig. 2.52). The most recent major studies of this interesting animal, the world's largest terrestrial arthropod, were conducted in the Marshall Island and the Mariana Island in the western Pacific (Helfman 1977; Amesbury 1980, 1982; Reese 1981), and it is on the results of those that much of the information below is based.
Figure 2.52. Coconut crab Birgus latro.
The coconut crab is a member of the land hermit crab family Coenobit-idae19 but, unlike the other members, it does not occupy a snail shell when adult. It was once widely distributed throughout the western Pacific and eastern Indian Oceans but now is restricted to small islands, particularly those uninhabited by man. In Sulawesi it now known only from Sangihe and the Kawio, Talaud, Togian and Banggai Islands, the Togian Islands being the most westerly part of its range in Indonesia20 (Reyne 1938; Anon. 1982b; Salm and Halim 1984) (fig. 2.53). Man has found the crab to be a desirable food item and it is easily caught even though it may be the largest animal on some of the islands it inhabits. In the Togian Islands, for example, crabs are occasionally collected by residents for their own consumption or for sale to visitors. Much more damaging though are the parties of non-residents from Gorontalo, Ampana and Poso who remove whole boatloads of crabs for sale in their respective towns as food or tourist curios. If this exploitation were regulated under an ecological management plan it could be sustained, but this is not the case (Anon. 1982b).
Most crabs are obliged to seek water for mating, but coconut crabs mate on dry land. The fertilized eggs, totalling tens of thousands, are carried under the crab's abdomen. Females release their eggs into sea water when the tides are highest; that is, around the full moon, and the larvae hatch in response. At this time eggs are most likely to be washed out to the open sea where predation is less and availability of plankton greater than over the reef flat. The larvae spend three to eight weeks as part of the oceanic plankton before the glaucothoe or transitional larva finds a small shell to shelter inside, like a hermit crab, and migrates to land. At this stage it may face severe competition with smaller species of hermit crabs for both shells and food. This larva moults several times and then experiences a metamorphosis to become a miniature adult. When its carapace is only about 2.5 cm across, at about two years of age, it gives up living in shells, a move which brings new opportunities and problems. It is sexually mature when the carapace is only 5 cm across but the largest crabs can weigh 5 kg with a thoracic length21 of 70 mm. Their large-clawed legs can span 90 mm. Females are smaller
