Mangrove Vegetation

The mangrove flora of the world is represented by about 65 species. Since there is little confusion about the true mangroves and the mangrove associates, it is difficult to give the exact number of mangrove species in the world. If the vivipary and breathing roots were taken into consideration, there would be 55 species in the world (Chapman 1970). Most of the species are strongly represented in South East Asia and the Eastern coast of Africa.

The floral diversity of mangroves in India is great. The Indian mangroves are represented by approximately 59 species (inclusive of some mangrove associates) from 29 families. Of the 59 species, 34 species belonging to 21 families are present along the west coast. There are a few species of which are indigenous to the west coast, e.g., Sonneratia caseolaris, Sueda fruticosa, Urochondra setulosa etc. The East coast of India and the Andaman and Nicobar islands show a higher species diversity as well as unique distribution of mangrove flora. The east coast is represented by 48 species belonging to 32 genera.

 Few commonly found plant species in Mangrove ecosystem are:

Rhizophora apiculata (Red Mangrove): This evergreen tree is a front mangrove species and grows well in sheltered areas rather than open seas exposed to the wave action. It generally grows upto 3-5 mts. Leaves are about 10-20 cms long and 2.5 - 7.5 cms broad. Propagules are about 10-15 cm long.

Rhizophora mucronata (Red Mangrove): This is a useful species for mangrove plantation. One may often notice this plant as a front mangrove where the shore is well protected. It has a high growth rate and is economically important. The species is quite similar to R. apetala except that the leaves are slightly broader and the propagules are about 50 to 60 cms long and spearlike.

Bruguiera gymnorhiza (Broad leaf orange mangrove): This evergreen tree grows upto 8-12 -mts in height. Leaves are 9-12 cms in length and 3.5-4.5 cms. broad. Roots are characteristically thick, rope-like and filled with air. They are called "cable roots".

B. parviflora (Small leaf orange mangrove): Essentially a back mangrove species. It appears as a shrubby tree growing upto 3-5 mts. As in B. gymnorhiza cable roots are present. It is a useful tree for commercial extraction of tannin. The leaves are supposed to be used for treating high blood pressure.

Sonneratia alba (Mangrove Apple): A large tree of upto 30 m in height, it prefers non swampy intertidal zones. This is a front mangrove species and prefers open areas with some wave action. It has thick, pointed and long pneumatophores. The apple like fruits are edible and used in pickles.

Avicennia marina: Avicennia spp have the highest salt tolerance of mangrove trees. These are shrubby trees with a height of 3 to 7 meters. This is one of the dominant species found throughout the coastline.

Avicennia officinalis: Of the three dominant species of Avicennia this is the tallest, growing upto 8-12 mts. The wood is used as timber and fuel. Extraction of tannin is still done at some places. Leaves useful as fodder for cattle.

Acanthus ilicifolius (Shore purslane): One may notice this attractive plant in the back mangrove zones. Its shrubby nature and spiny leaves make it an outstanding species. It is versatile, and flourishes on mud-flats, slopes and even ridges with different inundation and salinity. Its widespread sub-surface and aerial root system checks soil erosion and it often serves as a distress source of fuelwood. The blue flowers are also a source of nectar for honey bees. The flowers and leaves are used for decoration in Kerala. 

Aegiceras corniculatum (River mangrove): A densely flowering shrub. It grows in both non-swampy intertidal and tidal zones and even on soils only partially consolidated. Salt tolerance of this species is comparatively low and grows only in the areas where there is good mixing of freshwater at least for a few months. It seeds profusely between January and March and fruits are curved and very finely pointed. The nectar produces fine quality honey.

Phoenix padulosa (Sea Date): This palm species is also called as sea date and is a relative of the common date. It prefers mature, porous, well consolidated soils in the tidal zones but has a wide level of salinity tolerance. In Sunderbans, this species forms a major zone along the upper reaches of the delta. It is said to be the ideal habitat for tigers. Fruits are smaller than the common date, shining black when ripe and available in summer, from March to July. It is used as a fuel and its trunk is used for constructing traditional hutment. It grows naturally only on the East Coast.

Nypa fruticans (Golpatta): This is a characteristic palm species and resembles a shrunken coconut tree. Nypa has become a rare plant in Sunderbans where it was once the commonest palm. Over-exploitation of its leaves, which provide isothermic roofing, has been a major cause of its extinction in W. Bengal. It is still present widely in the Andaman & Nicobar Islands. It prefers well consolidated but moist tidal zones with a low to medium level of salinity preferably with freshwater mixing. It fruits throughout the year and the clustered fruits are similar to the Toddy fruits.

Heritiera fomes (Sundari): This plant is locally called as sundari in W. Bengal. The name Sunderban perhaps has been derived from the abundance of this species in the Gangetic delta. Once a widely spread species, it has now become almost extinct in W. Bengal. It produces timber of excellent quality which is said to be more expensive than teak.

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Animals in the Mangrove World

Through the specialised root systems and other morphological adaptations, mangroves form dense forests on the shore lines, creating a secured habitat for a variety of fauna. Since mangals are transition ecosystems, they give refuge to terrestrial, marine/brackish water as well as purely intertidal organisms, making itself a richly diversed ecosystem. The muddy or sandy sediments of the mangrove forest are home to a variety of epibenthic, infaunal, and meiofaunal invertebrates. The mangal may play a special role as nursery habitat for juveniles of fish whose adults occupy other habitats (e.g., coral reefs and seagrass beds) Because they are surrounded by loose sediments, the submerged mangroves roots, trunks, and branches provide niche for epifaunal communities including bacteria, fungi, macroalgae, and invertebrates. The aerial roots, trunks, leaves and branches host other groups of organisms. A number of crab species live among the roots, on the trunks or even forage in the canopy. Insects, reptiles, amphibians, birds and mammals thrive in the habitat and contribute to its unique character. The mangrove fauna could also be classified as a) Aquatic, b) Semi aquatic and c) Terrestrial.

As far as the Indian subcontinent is concerned, the fauna of the mangrove environment is rather poorly understood. Most of the studies conducted are on vertebrate fauna. Very few reports are available on micro and macro fauna.

A brief account of fauna in Mangrove ecosystem

Zooplankton
The zooplankton in the mangrove areas mostly includes crustacean larvae. Larvae of several species are found in large quantities. This is obvious because mangroves are the breeding ground for a variety of organisms. Food in the form of suspended solids is plenty, while shelter is sought in the complex root-systems of plants.

Insects
Insect fauna of mangroves has not been adequately researched in India. Hardly any information is available about the insects in mangrove areas. But some work has been conducted in W. Bengal and Orissa where honey collection is one of the major tribal activity. Collection of fine quality mangrove honey is a major occupation for tribals. The common honey bees found here are Apis dorsata (rock bee) and Apis mellifera (European bee).

Butterflies and moths are also commonly found in the mangrove ecosystem. Several species of butterflies and moths have been reported in mangrove areas.

Salmona is a butterfly which is associated with the mangrove associate, Salvadora.

Another moth, Hybloea puera, has recently been observed to be infesting large tracts of Avicennia marina on the Western coast.

Molluscs and Crustaceans
Mangroves are a paradise for aquatic animals like molluscs and crustaceans. About 20 species of shellfish and 229 species of Crustaceans belonging to Indian mangroves have been recorded. However, much work is needed on this subject. The reason for high density of these animals lies in the high deposits of silt and detritus in the mangrove environment. In addition to the rich sediments brought by the rivers, the leaves shed by mangrove trees also add in the organic constituents of mangrove soil. As a result, a rich source of food is created which is utilized by detritovorous organisms like crustaceans and molluscs. Many crustaceans in the mangroves make burrows which are used for refuge, the feeding, as a source of water or for establishing a territory necessary for mating. Some may filter water through their burrows, feeding on suspended detritus and plankton while others may breed there. These burrows play an important role in the mangroves, aerating, draining and turning the dense waterlogged soil - a direct benefit to the plants which in turn give them shelter.

Mangrove Crab (Scylla cerata) : Scylla serrata, the large edible swimming crab, inhabits the muddy bottom of mangrove estuaries, as well as coastal brackish water. Due to its association with mangroves it is known as the Mangrove Crab or the Mud Crab. This is a commercially important crab and it is trapped in special nets throughout the country. Due to its habit of cutting stems of young plants it is a pest for young plantations.

Fiddler Crab (Uca sp.) : Fiddler crab is probably one of the first animals one sees in a mangrove area. Fiddler crabs are charcteristised by the males which are armoured with a single huge pincer (claw) which is used as a courtship display tool rather than for protection. The other pincer is small in the males and is used for feeding. Females have two small pincers of equal size. Fiddler crabs are semiterrestrial and prefer to stay on protected sand and mud beaches of bays and estuaries. Their burrows are located in the intertidal zone, and at low tide the crabs come out for feeding and courting. There are 62 known species of fiddler crabs in the world.

Fiddler crabs are preyed upon by fish, large crabs, some mammals and birds. In the Indo-Pacific region a species of snake called "mangrove snake" goes down into a fiddler crab's burrow and hunts the crab.

Telescopium telescopium : Like the mangrove crab, this characteristic mollusc is also strongly associated with mangrove environment and is an indicator species for mangroves. The name is derived from the typical telescope-like structure of the shell. This animal belongs to Class Gastropoda of Phyllum Mollusca.

Fish : Mangroves are the breeding and nursery grounds for several species of fish. There are a total of 105 species of fish which are typical mangrove dwellers in India. Besides, many other species visit the mangrove environment frequently or occasionally. Some common species are - scats, milk fish, mudskippers, mullets, cat fish, perches, etc.

Mud skippers are one of the fish which live on the mud flats associated with mangroves shores. The mud skipper is a fish well adapted to alternating period of exposure to air and submersion and is frequently seen hopping along the mud at the water's edge They are well-comouflaged and able to change colour to match their background. It respires under water like other fish but out of the water gulp air. When submerged it swims like a fish but on land proceeds by a series of skips. Some of them can even climb trees using their fused pelvic (rear) fins as suckers and their pectoral fins as grasping 'arms'.

When a mud skippers is out of water it carries in its expanded gill chamber a reserve from which to extract oxygen. After a few minutes, when this reserve is exhausted, it is replenished from pool or from water in the burrows which they dig. The mud skipper's most noticeable feature is a pair of highly mobile eyes perched on top of the head to increase the field of view and to enable it to see both under and over the water.

Reptiles

Estuarine or Saltwater Crocodile (Crocodylus porosus): This is the largest crocodile found in India or in the world. The other two types of Indian crocodiles are freshwater species and are called the Marsh Crocodile (Crocodylus palustris) and the Gharial (Gavialidae gangeticus). It normally attains a size of 6 mtrs and the largest record has been of a male over seven mtrs. The estuarine crocodile inhabits the estuarine areas with mangrove cover and swampy grasses. Its main food is dead and decaying matter in the estuary and is a major scavenger of the estuaries. They are also known to hunt fish and small animals. However, its nuisance value as a man eater has not been established. Nevertheless, it is the most infamous crocodile species in India.

Sea Turtles : Olive Ridley (Lepidochelys olivacea) is the most common sea turtle in Indian waters. Large nesting sites are found in Orissa, the largest nesting site ever recorded is Gahirmatha near Bhitarkanika. Besides the Olive Ridley other turtles in Indian waters are the Green Turtle (Chelonia mydas), Hawksbill Sea Turtle (Eretmochelys imbricata), Bibron's soft shell turtle (Pelochelys bibroni), Batagur turtle (Batagur baska) and the Leatherback turtle (Dermochelys coriacea) of which the last two species are endangered. Sea turtles are valuable for meat, shell and plastron.

Water Monitor Lizard (Varanus salvator) : The Water monitor is one of the largest lizards in the world, growing upto 3 mtrs. In India it is found in association with the estuarine crocodile. They are a major predator of crocodile and turtle eggs. Due to overkilling and very long periods of incubation (8-9 months) it has become endangered in India.

Birds

Birds are a prominent part of most mangrove forests and they are often present in large numbers. The mangrove habitats offer rich feeding grounds for many of the large and more spectacular species as well as a multitude of small birds.

Both aquatic as well as tree dwelling birds are commonly found in mangroves and associated areas About 177 species of resident and migratory birds are found in the mangrove forests. The most common among these are Kingfishers, herons, storks, sea eagles, kites, sand pipers, Curlews, terens, ducks etc. Flamingoes flock the exposed mud flats, during the low tides. They use mangrove environs as breeding and feeding grounds.

Mammals

Royal Bengal Tiger (Panthera tigris tigris) : This is an example of adaptation at the highest level in the animal kingdom and is one of the unique resident species of mangroves of the Sunderbans. The hunting cat of hard ground is converted to a fishing cat of marshes in the Sunderbans. There are about 250 tiger now present in Sunderbans and total protection has been offered by including the area under Project Tiger. The tigers here also hunt for spotted deer, wild boars and water monitor lizards.

Dugong (Dugong dugong): The Dugong or the Sea Cow, though a frequent mangrove visitor, is not an exclusive mangrove dweller. It is found on the coastal shallow waters where sea grass is abundant. It is a herbivorous sea mammal feeding alone or in a herd, in the day or night. It has a life expectancy of about 50 years. Dugongs are on the verge of extinction in India.

Otters : Otters are also visitors of mangroves and often frequent them in search of food and shelter. They are elegant and swift swimmers. Being hyperactive they are constantly on the move. Their favorite foods are sea urchin, crab, clam, mussels, squid, octopus and fish. The peculiar thing about sea otters is that, while most otters come ashore for breeding, the sea otters breed in the water itself.

Crab eating macaque : The crab eating macaque is an endangered species of monkey. In India it is found only in the Andaman and Nicobar islands. This monkey has adapted to the coastal environment and dwells amongst the mangrove trees. Another adaptation is its skill in catching crabs which helps it to survive in the mangrove habitat.

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