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Microcoleus Classification Essay

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Aquat. Sci. 68 (2006) 338–354
                                                 Aquatic Sciences
DOI 10.1007/s00027-006-0868-8
© Eawag, Dübendorf, 2006

Overview Article

Biodiversity and its conservation in the Sundarban
Mangrove Ecosystem
Brij Gopal* and Malavika Chauhan

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India

Received: 15 December 2005; revised manuscript accepted: 9 March 2006

Abstract. The Sundarban, covering about one million ha    darban has been extensively exploited for timber, fish,
in the delta of the rivers Ganga, Brahmaputra and Megh-   prawns and fodder. The regulation of river flows by a se-
na is shared between Bangladesh (~60 %) and India      ries of dams, barrages and embankments for diverting
(~40 %), and is the world’s largest coastal wetland. The   water upstream for various human needs and for flood
area experiences a subtropical monsoonal climate with    control has caused large reduction in freshwater inflow
an annual rainfall of 1,600–1,800 mm and severe cy-     and seriously affected the biodiversity because of an in-
clonic storms. Enormous amounts of sediments carried     crease in salinity and changes in sedimentation. Heritiera
by the rivers contribute to its expansion and dynamics.   fomes (locally called Sundari, from which Sundarban
Salinity gradients change over a wide range of spatial and  derives its name), Nypa fruticans and Phoenix paludosa
temporal scales. The biodiversity includes about 350 spe-  are declining rapidly. During the past three decades, large
cies of vascular plants, 250 fishes and 300 birds, besides  parts of the remaining Sundarban have been protected for
numerous species of phytoplankton, fungi, bacteria, zoo-   wildlife, particularly tiger, through the creation of several
plankton, benthic invertebrates, molluscs, reptiles, am-   sanctuaries and a biosphere reserve. Parts of the Sundar-
phibians and mammals. Species composition and com-      ban in both India and Bangladesh have been declared
munity structure vary east to west, and along the      World Heritage sites. However, its biodiversity continues
hydrological and salinity gradients. Sundarban is the    to be threatened by a growing human population that not
habitat of many rare and endangered animals (Batagur     only places pressure on its biological resources, but also
baska, Pelochelys bibroni, Chelonia mydas), especially    impacts on the freshwater inflows from upstream areas.
the Royal Bengal tiger (Panthera tigris). Javan rhino,    Oil exploration in coastal areas is also emerging as a new
wild buffalo, hog deer, and barking deer are now extinct   threat. Further threats arise from global climate change,
from the area. Large areas of the Sundarban mangroves    especially sea level rise. The future of the Sundarban will
have been converted into paddy fields over the past two    depend upon the management of freshwater resources as
centuries, and more recently into shrimp farms. The Sun-   much as on the conservation of its biological resources.

Key words. Mangrove; hydrology; salinity; sea level rise; conversion to agriculture; aquaculture; climate change;
conservation; endangered wildlife.

* Corresponding author e-mail: brij@nieindia.org
Published Online First: September 4, 2006
                                                             Overview Article
Aquat. Sci. Vol. 68, 2006                                                                339

Introduction                                  area lies in Brazil, Australia and Nigeria (Spalding et al.,
                                        1997). While practically all mangroves occur in small
Mangroves1 are intertidal forested wetlands confined to             patches that develop in deltaic habitats, the mangroves in
the tropical and subtropical regions (Tomlinson, 1986).            the Ganga-Brahmaputra-Meghna Delta, shared between
The total global area of the mangroves is estimated at             India and Bangladesh, are the only contiguous and larg-
only 18.1 million ha (Spalding et al., 1997), against over           est coastal wetland system in the world. Popularly known
                                        as Sundarban2, they currently cover about one million ha
570 million ha of freshwater wetlands including peat-
lands globally (but excluding paddy fields; Spiers, 1999).           area, greater than the combined area of Wadden Sea wet-
Although mangroves have been exploited for many cen-              lands that are shared between Denmark, Germany and
turies, our scientific understanding of these wetland for-           the Netherlands.
ests remained poor until the 1970s (Lugo and Snedaker,               Exploration of the Sundarban mangroves dates back
                                        to the 16th century (Rollet, 1981). A large bulk of pub-
1974; Blasco, 1975; Chapman, 1976). During the past
three decades or so, these wetland forests have received            lished literature exists on the Sundarbans of both India
increasingly greater attention which is reflected in an             (Naskar and Guha Bakshi, 1987; Chaudhuri and Choud-
exponential increase in the number of publications (Elli-           hury, 1994; Guha Bakshi et al., 1999) and Bangladesh
son, 2002). Several recent publications have examined             (Seidensticker et al., 1991; Hussain and Acharya, 1994;
issues concerning ecology, management and conserva-              IUCN-BD 2002, Islam and Wahab, 2005) covering many
tion of mangroves (Robertson and Alongi, 1992; Ricklefs            aspects of their habitat characteristics, flora, fauna (par-
and Latham, 1993; Ellison et al., 1999; Kathiresan and             ticularly fisheries), utilization and management, yet very
Bingham, 2001; Macintosh and Ashton, 2002; Ellison,              little is known about the functional aspects of this ecosys-
2002; Linneweber and de Lacerda, 2002; Vannucci,                tem. In this review, we examine the current state of our
2003; Saenger, 2003).                             knowledge of Sundarban’s biodiversity, various factors
  The biodiversity of mangroves has also been of in-             threatening it, and the recent conservation measures. This
creasingly greater interest, firstly because of the Conven-           is preceded by a brief overview of the geology, climate,
tion on Biological Diversity, and secondly, because the            soils and hydrology which are the major driving variables
mangrove ecosystems are among the most threatened by              of mangrove biodiversity in this region. It may be noted
the global climate changes, particularly the sea level rise          here that since 1947 the Sundarban mangroves are di-
(Macintosh and Ashton, 2002, 2004). Mangroves are               vided between India and Bangladesh (formerly East Pa-
relatively well known for their floral diversity which is            kistan), and the two parts differ considerably in the nature
comprised of only 65–69 species of vascular plants               and extent of investigations, conservation and manage-
which have several specific adaptations to the dynamic             ment. They also differ substantially in the level of human
coastal environment (see Kathiresan and Bingham, 2001)             exploitation over more than a century. This makes it dif-
and among the fauna, their fisheries (both fishes and crus-           ficult to integrate the information of the entire Sundar-
taceans) resources are better understood. The global pat-           ban. Therefore, we frequently refer to the Indian and
terns of biodiversity in mangroves also present an inter-           Bangladesh parts of the Sundarban separately.
esting picture. Whereas the latitudinal pattern of
mangrove flora is normal in as much as the highest spe-
                                        The Sundarban mangroves
cies richness of plants occurs around the Equator and
declines at higher latitudes – both north and south (Duke
et al., 1998; Ellison et al., 1999), the longitudinal distri-         The Sundarban (21º 30′ to 22º 40′ N, 88º 05′ to 89º 55′ E)
bution is quite ‘anomalous’ with high concentrations in            comprises essentially of numerous islands formed by the
the Eastern Hemisphere between 90º E and 135º E (Rick-             sediments deposited by three major rivers, the Ganga,
lefs and Latham, 1993; Ellison and Farnsworth, 2001).             Brahmaputra and the Meghna, and a dense network of
Interestingly, the mangrove-inhabiting molluscs follow a            smaller rivers, channels and creeks. The maximum eleva-
similar pattern (Ellison et al., 1999).                    tion within the Sundarban is only 10 m above the mean
  More than 41 % of the world’s mangroves occur in              sea level. The western and eastern limits of the Sundar-
South and Southeast Asia of which Indonesia alone ac-             ban are defined by the course of the River Hooghly (a
counts for 23 %. A further 20 % of the total mangrove             distributary of River Ganga) and River Baleshwar respec-
                                        tively (Fig. 1). The River Harinbhanga (known as
                                        Ichamati or Raimongal in Bangladesh) demarcates the
  The term ‘mangrove’ is used variously referring to plants (Macnae, 1968;
  Unesco, 1973), their communities (Blasco, 1975; Hamilton and Snedaker,
  1984) and the whole ecosystem. Macnae (1968) reserved the term
  ‘mangal’ to describe the mangrove plant communities. In this paper, we     Often referred to in plural as ‘Sundarbans’ because the forest comprises
  follow Arroyo (1979, 1997) who used the term mangrove for the entire      of hundreds of islands separated by rivers and creeks, or to mean both the
  ecosystem, its plant communities as well as the plants inhabiting them.    Indian and Bangladesh parts together.
340    B. Gopal and M. Chauhan                         Biodiversity of Sundarban Mangrove Ecosystem

border between India and Bangladesh. About 60 % of the      ban in the two countries often differ considerably. Ac-
mangrove forests lie in the Khulna District of Bangla-      cording to recent estimates, the area of the Sundarban in
desh and the rest in the 24-Paragnas District of West       Bangladesh is 599,330 ha (1978 Landsat data; Rahman et
Bengal (India). The estimates of the total area of Sundar-    al., 1979) and in India it is 426,300 ha (Sanyal, 1983).

                                 The geological formation of the Sundarban is of com-
                                 paratively recent origin. Several geomorphological
                                 changes since the Tertiary period that included tectonic
                                 movements in northwestern Punjab and the southeastern
                                 flow of the River Ganga, resulted in the deposition of
                                 sediments in the Bengal Basin and development of the
                                 Sundarban Delta (Wadia, 1961). It is noteworthy that the
                                 Ganga and Brahmaputra together carry the world’s larg-
                                 est sediment load to the oceans (Coleman, 1969; Milli-
                                 man and Meade, 1983; Milliman et al., 1995). Neotec-
                                 tonic movements in the Bengal Basin between the 12th
                                 and 15th century AD further resulted in an easterly tilt
                                 (Morgan and McIntire, 1959). During the 16th century,
                                 the R. Ganga changed its course to shift eastwards and
                                 join the Brahmaputra (Deb, 1956; Blasco, 1975; Snedak-
                                 er, 1991). Later, in the mid 18th century, the combined
                                 Ganga (now called Padma) and Brahmaputra again tilted
                                 eastwards to empty into the R. Meghna (Williams, 1919;
                                 Snedaker, 1991). This continuing tectonic activity greatly
                                 influenced the hydrology of the deltaic region because of
Figure 1a. Location of the Sundarban in the Ganga Brahmaputra
                                 changes in the sedimentation patterns and the reduction

Figure 1b. The Sundarban mangroves are shared between India and Bangladesh. Mangroves once extended up to the Dampier-Hodges line
– the inward limit of tidal influence. Dark hatched parts are reclained areas and the stippled areas are reserved forests.
                                                Overview Article
Aquat. Sci. Vol. 68, 2006                                                341

in freshwater inflows. Most rivers (distributaries) other    sands, which form sand dunes, occur mainly along the
than the Hooghly, that contributed to the formation of the   coast.
Ganga Delta (from west to east: Muriganga, Saptamukhi,       Similarly in the Bangladesh Sundarban, the soil is a
Thakuran, Matla, Gosaba and Bidya), have lost their      silty clay loam with alternate layers of clay, silt and sand.
original connections with the Ganga because of siltation    The surface is clay except on the seaward side of islands
(Fig. 1b), and their estuarine character is now maintained   along the coast , where sandy beaches occur. In the east-
by the monsoonal runoff alone (Cole and Vaidyaraman,      ern part, the surface soil is soft and fertile whereas it is
1966). Thus the delta-building process has nearly ceased    harder and less suitable for tree growth in the west
in the west, but has accelerated in the eastern part. The   (Choudhury, 1968). The soil pH averages 8.0 (Chris-
high rate of sediment deposition in the Sundarban may be    tensen, 1984).
understood from the estimated increase in the land area
by more than 800 km2 in 80 years between 1793 and 1870
(Richards, 1990). Recent studies by Chakrabarti (1995),    Climate
however, show that the mangroves are the dominant geo-     The climate of the area is characterized by relatively high
morphic agent in the evolution of tidal shoals and their    temperature and humidity (>80 %) throughout the year,
accretion to the main landmass.                and well distributed rainfall during the monsoon season.
                                Temperatures rise from a daily minima of 2–4 °C in win-
                                ter to a maximum of about 43 °C in March and may ex-
Soils                             ceed 32 °C during the monsoon. Recent reports suggest
The land is moulded predominantly by tidal action. An     that the air temperature over the Sundarban and adjacent
intricate network of waterways, of which the larger chan-   parts of the Bay of Bengal are gradually increasing (Huq
nels (often 1.5 to 2.0 km wide), run in a generally north-   et al., 1999; Agrawala et al., 2003). The cold season lasts
south direction, intersect the whole area. Innumerable     from about the middle of November to the end of Febru-
small khals (= creeks) drain the land at each ebb tide.    ary and is followed by the summer from March to May.
Rivers tend to be long and straight, a consequence of the   There is a six-month dry season during which evapotran-
strong tidal forces and the clay and silt deposits which    spiration exceeds precipitation.
resist erosion. Easily eroded sands collect at the river      The rainfall over the Ganga-Brahmaputra deltaic re-
mouths and form banks and chars, which are blown into     gion decreases from east to west and from south to the
dunes above the high-water mark by the strong southwest    north. In the Bangladesh region, mean annual rainfall
monsoon. Finer silts are washed out into the Bay of Ben-    varies from about 1,800 mm in Khulna, north of the Sun-
gal, but mud flats are formed on the leeward side of the    darban, to 2,790 mm on the coast. The average annual
dunes where they are protected from wave action. These     rainfall in the Indian region is only 1,661.6 mm. It de-
mudflats become overlain with sand from the dunes, and     creases from 1,805 mm in the south on Sagar Island to-
develop into grassy meadows. This process of island-      wards Kolkata in the north. Most of the rainfall (about
building continues for as long as the area on the wind-    74 % of the total) occurs during the southwest monsoon
ward side is exposed to wave action. With the formation    period (June–September). Some precipitation is received
of the next island further out, silt begins to accumulate   in the latter half of the hot season and in October.
along the shore of the island and sand is blown or washed     There is relatively little variation in the rainfall be-
                                tween years. During the first half of the 20th century, the
away (Seidensticker and Hai, 1983).
  Most of the soils derived from alluvial deposits are    highest and lowest annual rainfalls were only 142 % (in
azonal with little or no profile development (Chaudhuri     1933) and 62 % (in 1935) of the normal respectively.
and Choudhury, 1994; Sarkar et al., 1999). Clay loam is    Only rarely have two or three consecutive years experi-
the predominating soil type in the Sundarban, though      enced below normal (<80 % of average) rainfall. On an
silty and sandy loams also occur in many areas. Clays     average there are 80 rainy days (>2.5 mm rainfall) in a
with or without muck occur in swamps and alluvial lakes.    year.
Alluvial soils along the coast, and especially in the Sun-     Winds are generally light to moderate with a slight in-
darban area, show a white efflorescence of sodium chlo-     crease in force during the summer and monsoons, but in
ride, as they are impregnated with salts by tidal estuaries.  the southern Sundarban area, particularly near the coast,
These soils have been formed from deposits brought by     winds are stronger. Winds blow mostly from directions
tidal currents. Numerous tidal flats have been formed af-    between the south-east and south-west during May to Sep-
ter the headwater flow through the deltaic distributaries    tember. In October, winds vary in direction. During the
of the R. Ganga were silted up. The parent deposits are    winter, winds blow mainly from the north-west. In March
either rich in calcium or magnesium, or consist of half-    and April they blow from the south and south-west.
decomposed organic matter. The coastal soils are usually      Thunderstorms are common during summer after-
classified as saline, non-saline and alkali soils. Pure     noons. These may be in association with severe squalls
342    B. Gopal and M. Chauhan                        Biodiversity of Sundarban Mangrove Ecosystem

and occasional hail. These are commonly known as
nor’westers (because the associated squalls usually come
from the north-west) or Kalbaisakhi (the disastrous
winds of Baisakh, the first month of the Bengali calen-
dar). Storms result in heavy rain and a sharp drop in
  The storms often develop into cyclones that are usu-
ally accompanied by tidal waves up to 7.5 m high (Sei-
densticker and Hai, 1983). Available long-term records
show that cyclones over the Bay of Bengal adjoining the
Sundarban are increasing in their intensity, but decreas-
ing in their frequency of occurrence. This correlates with
the rising trend in temperatures mentioned earlier and has
a significant bearing on the extent of coastal flooding,
erosion and saline water intrusion due to storm surges
(Huq et al., 1999; Agrawala et al., 2003).

Hydrology and salinity regimes
The hydrology of the Sundarban is dominated by the
freshwater flows from Rivers Ganga, Brahmaputra and
Meghna, which exhibit very high seasonal variation in
                                Figure 2. Salinity zones in the Bangladesh Sundarban.
their discharge, and the tides which range in height from
2 to 5.94 m. Tidal influence extends to more than 50 km
inland from the shoreline and surges increase considera-
bly during the cyclonic storms.                blanks which may check the penetration of high tides to
   The freshwater flows from the rivers and the tidal in-   the interior of the islands. Irregular flooding of these
gress result in a gradient of salinity that varies both spa-  blanks by high tides, coupled with the capillary action of
tially and temporally. In general, the salinity is higher   the clayey soil and excessive heat during dry periods,
nearer the coast and the water is nearly fresh on the inland  results in the deposition of a salt crust on the soil surface,
side limit of the Sundarban. Similarly, the salinity de-    converting them into saline blanks. Lands on the sea
creases from west to east. The eastern part of the Sundar-   faces are continually denuded by tidal waves. Towards
ban in Bangladesh is oligohaline (<5 ‰ salinity) whereas    the west, new land formation occurs because of heavy
most of the Indian Sundarban is polyhaline (Fig. 2).      silt deposition from the Hooghly River and its distribu-
   During the past few decades, however, the sources of   tary, the Muriganga at their confluence with the Bay of
all rivers in the western part of the Indian Sundarban     Bengal. Thus, the habitat is characterized by a continu-
have progressively silted up, disconnecting the inflow of    ing erosion of banks, the formation of new islands, and
fresh water into the mangrove delta. Freshwater flows      ever-changing soil texture and salinity of water under the
are much larger from Brahmaputra and Meghna rivers       influence of tides.
on the Bangladesh side particularly in the Baleshwar        In recent years, the Farakka Barrage, built in 1974 on
River on the eastern side of the Sunarban (Seidensticker    the River Ganga within India, has also affected freshwa-
and Hai, 1983). The reduced freshwater flows in western     ter flows into the Bangladesh part of the Sundarban.
parts of the Sundarban have resulted in increased salinity   Similarly, the construction of several dams and barrages
of the river waters, and has made the rivers shallower     in the Damodar River catchment area and on the Ganga
over the years. At the same time, during ebb tides, the    have resulted in a decreased silt load and less deposition
receding water level causes scouring of top soil and cre-   of detritus in the estuaries downstream.
ates an innumerable number of small creeks, which
normally originate from the centre of the islands. The
ebb tide eroding action is stronger in some islands than
others within the Sundarban. The receding water carries
large volumes of silt which is deposited along the banks    The large spatial and temporal variability in hydrological
of rivers and creeks during high tides. This results in in-  regimes (both freshwater inflows and the tides), topogra-
creasing the height of the banks as compared to the inte-   phy and texture of the substratum, the salinity, and their
riors of the islands. Over time, such eroded channels     interactions, result in very high habitat heterogeneity in
extend further inwards into the islands and form muddy     the mangrove ecosystems, and thereby ensure an equally
                                                 Overview Article
Aquat. Sci. Vol. 68, 2006                                                   343

                                Table 1. Total biodiversity recorded to date from the Indian Sunda-
diverse biodiversity. There has been some discussion
                                rban mangroves.
about the characteristic mangrove plants and a distinction
is usually made between the “true mangrove” and “man-     Group of Organisms     No. of   Reference
grove associate” species. However, following the defini-                  species
tion of wetland species by Gopal and Junk (2000, 2001),
                                Flowering Plants      105    Naskar and Mandal (1999)
here we consider all those species which depend directly    Pteridophytes
or indirectly upon the mangrove habitats, or on any other    (Ferns & Fern allies)
organism living in the mangrove as mangrove organ-       Algae            150    Sen et al. (2000)
                                Lichens           32    Santra (1999)
  An assessment of the total biodiversity in Indian Sun-   Vertebrates         445    Das and Nandi (1999)
darban is given in Table 1. Overall, there are relatively    Chondrichthyes       22
                                 Osteichthyes (Fishes)   154
small differences between the species composition of the
                                  Amphibians        8
Indian and Bangladesh parts, largely because the deve-
                                  Reptiles        58
lopment of the mangrove system has been dictated by         Birds         163
similar processes. However, the overarching gradients of      Mammals         40
salinity and freshwater that occur across the Sundarban    Phylum-                Das and Nandi (1999)
from west to the east, are clearly reflected in the distribu-   Sarcomastigophora     45
                                 Aplicom plexa       24
tion of biota. The Sundarban in the two countries differs
                                 Myxozoa           4
also in the extent of management interventions. It must     Ciliophora         31
also be emphasised that the Sundarban mangrove in India
has been explored frequently and in greater detail, par-
                                 Porifera          1    Mukherjee (1975)
ticularly floristically, whereas the mangroves within       Cnidaria          33    Mandal and Nandi (1969)
Bangladesh were poorly investigated until recently        Ctenophora         2    Nandi et al. (1993)
                                 Platyhelminthes      41    Chaudhuri and Chaudhury
(Chaffey et al., 1985). Recent surveys provide consider-
able information on several groups of plants, fishes, rep-     Turbellaria        1    Anon (1995)
tiles, amphibians and birds (Ismail, 1990; Hussain and       Monogenera        21
Acharya, 1994; Seidensticker, 1991; IUCN-BD, 2002a,        Trematoda        13
                                  Cestoda          6
b, c). Differences in the nomenclature followed in the
                                 Nemathelminthes      68
two countries also sometimes make comparisons diffi-       Acanthocephala       3
cult. The following account is based largely on the Indian    Nemertinea         2
                                 Rotifera          4
part of the Sundarban, with frequent comparisons with
                                 Mollusca         143
the information available from Bangladesh.
                                 Sipuncula          2
                                 Echiura           3
                                 Annelida          78
                                  Polychaeta        69
Species diversity
                                  Oligochaeta        6
Floristic diversity. Mangrove plants are usually divided      Hirudinea         3
into ‘true mangrove’ and ‘mangrove associate’ species.      Arthropoda        476
                                  Crustacea        240
Globally, Duke (1992) recognised 69 species (belonging
                                  Insecta         201
to 26 genera and 20 families) of true mangroves (major
                                  Arachnida        33
and minor, sensu Tomlinson, 1986) though recently         Merostomata        2
Kathiresan and Bingham (2001) recognized only 65 spe-      Entoprocta         1
                                 Bryozoa           3
cies (22 genera and 16 families). Of these, at least 30 true
                                 Brachiopoda         1
mangroves occur in the Indian Sundarban. Debnath and       Chaetognatha        4
Naskar (1999) identified 36 species as true mangroves.      Echinodermata       20
The Bangladesh part of the Sundarban differs mainly in      Hemichordata        1
the relative abundance of various species. Whereas mem-
bers of the Rhizophoraceae and Avicenniaceae generally
dominate most other mangrove areas, the Bangladesh
part of the Sundarban has the greatest abundance of Ster-   only one species of Bruguiera (B. parviflora), whereas
culiaceae (Heritiera) and Euphorbiaceae (Excoecaria).     Lumnitzera racemosa and Barringtonia sp. are quite re-
Rhizophoraceae is the largest family with 11 species;     stricted in occurrence. Interestingly, the Sundarban sup-
four genera (Rhizophora, Bruguiera, Avicennia and Son-     ports fewer species than other mangrove areas in India
neratia) are represented by four species each and 5 gen-    and Southeast Asia. For example, there are 45 mangrove
era (Xylocarpus, Excoecaria, Thespesia, Derris and       species recorded from the Andaman and Nicobar Islands
Tamarix) have three species each. In Bangladesh there is    in the Bay of Bengal (Deshmukh et al., 1991a) and 55
344    B. Gopal and M. Chauhan                        Biodiversity of Sundarban Mangrove Ecosystem

                               Table 2. Rare, threatened and endangered flora of the Indian Sunda-
species from Bhitarkanika in the delta of the Brahmani
and Baitarni rivers in Orissa on the east coast of India
(WWF-I, 2001).                        Family       Species             Status
  The total flora of the Sundarban has also been esti-
                               Rhizophoraceae   1. Rhizophora apiculata     Occasional
mated differently in India and Bangladesh. In India, the
                                         2. Bruguiera parviflora     Occasional
total vascular flora (including mangrove associates) is                              Occasional
                                         3. Ceriops decandra
estimated at 100 species representing 34 families and 57                             Occasional
                                         4. Kandelia candel
genera. comprises of 30 species of trees, 32 shrubs, and   Meliaceae      5. Aglaia cucullata       Rare
the rest are herbs, grasses, sedges and two ferns. Many              6. Xylocarpus mekongensis    Threatened
                                         7. Xylocarpus granatum     Threatened
terrestrial upland plants within the Sundarban area have
                               Sterculiaceae    8. Heritiera fomes       Threatened
apparently not been included because Seidensticker and
                               Rubiaceae      9. Scyphiphora hydrphyllacea  Very Rare
Hai (1983) recorded 334 plant species representing 245
                                         10. Hydrophyllax maritime    Very Rare
genera from the Bangladesh Sundarban. This record lists
                               Tiliaceae     11. Brownlowia lanceolata    Occasional
numerous epiphytes and climbers, which include 13 or-
                               Arecaceae     12. Nypa fruticans        Occasional
chids and several ferns. Several species in the Indian
                               Acanthaceae    13. Acanthus volubilis      Very Rare
Sundarban do not occur in Bangladesh part, whereas
                               Papilionaceae   14. Cynometra ramiflora      Rare
many other species have been reported from Bangladesh
                                         15. Dalbergia spinosa      Rare
alone (Choudhury, 1968; Ismail, 1990). Interestingly,
                               Sapotaceae     16. Manilkara hexandra      Rare
some species listed earlier from Bangladesh by Prain
                               Rutaceae      17. Atalantia correa       Very Rare
(1903) and Choudhury (1968), such as Bruguiera sexan-
gula, Rhizophora apiculata and Sonneratia alba, were
not recorded by Chaffey et al. (1985) and it is feared that
these species may have disappeared from the eastern part   Botrychia, Catenella and Caloglossa form a turfy cover-
of the Sundarban. Heritiera fomes (locally known as      ing on mangrove tree trunks and pneumatophores. At a
Sundari, the most important timber species from which     lower level on the same trees, species of Caloglossa occur
Sundarban derives its name),which is abundant on the     along with Chaetomorpha and Rhizoclonium. Species of
Bangladesh side, is not common on the Indian side where    Lyngbya, Oscillatoria and Microcoleus are common on
it is considered endangered. Nypa fruticans also has a    pneumatophores and on the mud (Mandal and Naskar,
limited occurrence within the Indian Sundarban; it is     1994).
rapidly disappearing because of extensive exploitation.
Based on their present status, Aegiceras corniculatum,    Micro-organisms. The diversity of bacteria and fungi in
Kandelia candel, Rhizophora sp., Sonneratia acida, Son-    the Sundarban has not been examined in detail, although
neratia apetala and Sonneratia caseolaris also require    some reports mention various microorganisms in the
conservation measures (Table 2).               soils and on decomposing litter, besides pathogens in-
                               festing mangrove leaves (Pal and Purkayastha, 1992),
Algae. The algal flora of the Sundarban is very poorly     fish, prawns and mammals (Das, 1999). Bacteria play a
known, but the available information suggests that the    significant role in the mangrove environments and the
Sundarban has a highly diverse algal flora comprised of    Sundarban is expected to harbour a diverse bacterial
both benthic and planktonic forms ranging from the fresh-   community which is yet to be explored, unlike other In-
water to marine environments. In the only study in Bang-   dian mangroves that are better known (Balasubramanian
ladesh, Islam (1973) reported 34 species of planktonic    and Ajmal Khan, 2002). Furthermore, 32 species of li-
and benthic algae. Various published reports on the algal   chens have also been recorded from the Indian Sundar-
flora provide only a patchy picture as they are based on    ban (Santra, 1998).
short-term surveys of small isolated areas (see Pal et al.,
1988; Santra et al., 1988, 1991). Sen et al. (1999) record-  Faunal diversity. The fauna of the Sundarban has at-
ed 80 species of algae (32 Cyanophyceae and 27 Chloro-    tracted much attention because of the huge economic
phyceae) from different parts of the Indian Sundarban.    importance of many species. Crustaceans account for the
These included only 7 species of diatoms, whereas Baner-   largest proportion of animal biomass, with an estimated
jee and Santra (1999) listed 48 species of diatoms from    annual harvest of 40 million kilograms of fiddler crabs
the Hooghly-Matla estuary alone. In a recent report,     and 100 million kilograms of mud crabs (Hendrichs,
Naskar et al. (2004), however, list 150 species including   1975), besides a considerable harvest of shrimps, prawns
15 species of Rhodophyceae and 2 species of Phaeophyc-    and lobsters. The Sundarban supports very rich estuarine
eae. Another study recorded 12 genera and 16 species of    and coastal marine fisheries. Among the diversity of in-
benthic algae belonging to the Cyanophyceae, Chloro-     sects, honeybees hold an important position for the pro-
phyceae, Chrysophyceae and Rhodophyceae. Species of      duction of large quantities of honey and beeswax. Also,
                                                  Overview Article
Aquat. Sci. Vol. 68, 2006                                                    345

                                 Table 3. Threatened fauna of Sundarbans
the Sundarban is the only mangrove forest with a species
of the tiger – the Royal Bengal Tiger (Panthera tigris).     Scientific Name          Common Name
The total faunal diversity of the Sundarban in listed in
Table 1.
                                 Muntjanus muntjack        Barking Deer
                                 Panthera tigris tigris      Royal Bengal Tiger
Fish. The fish fauna of the Bangladesh Sundarban in-        Felis bengalensis         Leopard Cat
cludes 53 pelagic and 124 demersal species (Sarker,        Felis viverrina          Fishing Cat
                                 Platinista gangetica       Gangetic Dolphin
1989; Rainboth, 1991). Of these, over 120 species have
                                 Orcaela brevirostris       Irrawady Dolphin
been recorded in commercial catches (Seidensticker and      Neophocaena phocaenoides     Little Indian Porpoise
Hai, 1983). The Indian Sundarban supports a similar        Manis pentadactyla        Chinese Pangolin
number of species (165 species; Sanyal, 1999). The fish      Reptilia
diversity is directly related to the salinity gradients in dif-  Crocodylus porosus        Estuarine Crocodile

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