Mangroves

Published in Swara Magazine May/June 1986

Although muddy and smelly, the mangrove swamps along East Africa's coast are hosts to a distinct and well-adapted community of plants and animals.

It is true that mangrove swamps sometimes stink. The water-logged silt is devoid of oxygen. The bacteria that live there decompose dead plant and animal remains and release unpleasant gases, such as hydrogen sulphide, as a by-product. However, it is also true that mangrove swamps can be beautiful. The mature trees often have an elegant park-like appearance, especially at the edges of the forest in estuarine areas. Their foliage is striking and the associated animals fascinating. The mud and smell are soon forgotten when on the trail of mud-skippers or fiddler crabs.

A mangrove is not one kind of tree but can be one of several species, some closely related but others in different families. They are grouped together under one name because they all share the same habitat, much in the same way that we refer to 'alpine plants' or `desert plants ‘. The mangrove habitat is very distinct and harsh. The swamps develop in sheltered bays and estuaries opening out to the sea and are found along most tropical coastlines of the world. Along the Kenyan coast there are a number of sites, the best developed being in the Lamu area, but a rich growth is also found bordering the sheltered creeks near Mombasa Island. There is one small patch in the Mwachema estuary, within easy reach of Diani and Tiwi beaches, which has many features of the larger forests, including a delightful and thriving fiddler crab colony. Living on the borderline between land and sea is a precarious existence. The dangers are not immediately obvious; there are no crashing waves nor extremes of temperature but rather the more subtle extremes of salinity, lack of oxygen for the roots and an unstable substrate of soft silt. In their own way, each can be deadly to potential colonisers. Each of these must be overcome and this has been accomplished successfully by only a few organisms.

Walking into a mangrove forest from the seaward side, the first trees to be encountered will be the early colonisers, such as Sonneratia, which needs very little soil and can grow on bare coral rock. They can tolerate their roots being in sea water for a longer time than the other trees. Continuing towards the land the mud-level rises as material is caught up in the roots. Conditions thus become more favourable for other trees such as Avicennia, whose seedlings are able to obtain a hold and which can survive in low nutrient conditions. These grow up through the Sonneratia trees and later displace them.

The process is repeated by the tall Rhizophora mangroves further towards the land, unless the soil is very infertile. This process results in a succession of different species and can reach a balanced state in about fifty years, although the forest may still be advancing toward the sea until wave-erosion halts it.

Not only does the depth of the mud change but also the salinity. While the outskirts are regularly washed by every tide and possibly by a freshwater river, the landward side will be less frequently covered, maybe only by the high spring tides every fortnight. The sea water soaks into the mud as the tide recedes and evaporation increases the salt level. High salinities have the same effect as the driest desert: living cells have the water drawn out of them by the process of osmosis. The later colonisers are adapted to this while the earlier ones are not. Nevertheless, there is usually a bare, sandy area immediately along the landward side of the forest. This area presents insuperable problems to living organisms. It is only reached by the very highest spring tides. The resulting evaporation gives a high salinity, alternating with fresh water whenever it rains, then drought when it dries again. Further inland the true land plants are found; grasses, succulents and coconut palms are common. All can survive in the light, sandy soil and are tolerant of salt to some extent. The mangrove forest provides the frame-work of a community which includes a relatively small variety of animals. In some areas it is impossible to put a foot down and not step on masses of horn shells, Terebralia palustris, many of which are occupied by hermit crabs. These and others browse off decomposing leaves and seaweeds around the roots.

Many other crabs are also herbivores, including the charming fiddler crabs, Alca species. They are frequently found on the seaward side of the forest, just away from the trees. Each male crab has an enormous claw which is waved around in a manner thought to resemble a violinist. This encumbrance must have a very important function in the life of the crab. It is not used for feeding but, like the exaggerated horns of the giant elk and the oversized plumes of a peacock, they owe their existence to sex. They are signalling devices; their ritual gesticulations warn off competing males and their vibrations attract females. If a rival dares to approach too closely then battles occur, but little damage ensues—should an occasional leg or claw be lost regeneration can be rapid. I have heard of people harvesting these claws for food. As it is an annual event, regeneration occurs in time for the next crop—it must cause total confusion among the fiddler population. Their method of feeding is rather unexpected. The mouth parts are designed to filter out the edible, organic fragments from the inedible sand and mud on which they live, the latter being spat out. They are most active during the low spring tides in the daytime, so the full and new moon periods are the best times to go fiddler-watching.

Another characteristic and delightful little creature of the mangrove swamps is the mud-skipper, Periophthalmus koelrenteri. Dr Smith, an authority on Western Indian Ocean fish, maligns them as being 'rather repulsive' and 'in keeping with their environment'! Any fish that can live out of water, climb trees and skim over the surface of the water with its tail going like an outboard motor, deserves more than that. The bulbous cheeks contain water, which is washed over the gills for breathing, thus making it independent of continuous immersion in the sea. Its pectoral fins are formed into a sucker to hold on to the mangrove stems. Thus, it is able to occupy an under-exploited niche, scavenging over the mud, with conspicuous success. In some ways their mode of life probably resembles that of the amphibious fish of the Devonian era which were able to drag themselves out of the drying out pools to reach safety, thus initiating the colonisation of the land even though these ancient fish were in fresh water and had primitive lungs.

In other parts of the world sea snakes sometimes shelter in mangrove roots during egg-laying, but they do not appear to occur in Kenya. Land snakes can be found in the branches, searching for birds and eggs. These branches also support a variety of other animals: small snails browsing on algae on the bark, birds nesting in the branches and catching insects, which abound as do spiders. Mammals often scavenge at low tide; mongoose, jackals and cats have all been seen. On the whole there are few organisms which bridge the gap between land and mangroves. It is a well-defined habitat, self-contained and self-perpetuating.

The mangrove trees themselves are quite distinct and easily recognisable. Flowers vary from the large, white, `pom-pom' like blooms of Sonneratiato the small, open inflorescences ofAvicennia. Pollination is usually by insects but bats probably visit Sonneratia. The fruits and seeds which develop must be able to resist immersion in sea water, as do coconuts, and they must be able to germinate and grow very rapidly; otherwise they will be quickly washed away by the next tide. The method varies but Rhizophora, Ceriops and Bruguiera seeds germinate on the tree to produce a long, rubbery radicle (young root) up to 40cm long. When this falls from the tree it could become impaled in the mud, but growth here would cause the tree to be in competition with the parent; so they also float and can be carried long distances by the currents until being washed up in a suitable, sheltered bay along the coast.

These seedlings are very common on beaches among the debris. Sonneratia, on the other hand, has a rounded, fleshy fruit which bursts open in the sea, releasing masses of small, slimy seeds.

Mangroves cope with the high salinities of their environment in different ways. Some, such as Avicennia, absorb salty water and excrete the salt through specialised cells on the leaves; this can easily be seen as a powdery deposit. Others, such as Rhizophora, use up metabolic energy to absorb water from the mud while leaving the salt behind. A final, specialised feature of mangroves is one designed to cope with the lack of oxygen around the roots. They require some oxygen therefore several mangroves have roots which periodically send up pneumatophores to the surface which allow oxygen to circulate to the cells below. These are very marked in Avicennia andSonneratia.

Apart from their biological interest, mangroves have made an important economic contribution to the Kenyan coast. Since the collapse of the slave trade, one of the main reasons for dhows to visit East Africa has been for the Rhizophoratrunks, known as boriti, which make excellent building poles. The bark of this tree also contains considerable amounts of tannin.Avicennia wood is used to make the ribs of dhows and all have been used for firewood and charcoal production. The trees also reclaim land and stabilise the coastline. The forests are an important feature of the Kenyan coast and contribute a great deal towards its richness and beauty. They are an asset worth preserving.

Bibliography

Isaac, W. E.&F. M. — Marine Botany of the Kenya Coast. Angiosperms. Journal of the E.A.N.H.S. Vol. XXVII No 1 (116) Jan. 1968.

All photographs and drawings (scanned from the original article in Swara) are by Brian Maudsley, who would like to thank the National Museum, Nairobi for permission to use material from the East African Herbarium.

Brian Maudsley lived in Kenya from 1952 to 1964, then returned to England to do a botany degree at Liverpool University. Since that time, he has been teaching biology in Peru, England, Kenya and now Jordan. During their last two years in Kenya, he and his wife, Venita, gathered information for a guide-book on the flora and fauna of the Kenyan coast. He has written two previous articles for Swara, `Corals' (May/June 1984) and 'Defence and Deception on the Reef (January/ February 1985).