Abstract: In this study, the distribution of crabs in Pichavaram was recorded from December 2001 to November 2002 at monthly intervals. The species present on the substratum and on the vegetation area were recorded in quadrant each measuring 1 m2. There are about 36 crabs species are distributed in Pichavaram mangrove environment. Crabs belonging to the family Grapsidae and Ocypodidae are most dominant forms. Substrate suitability; effects of tidal inundation and distribution of mangrove plants were the possible factors that could influence zonation and abundance of the crabs in the Pichavaram mangroves. The reason for depletion of crabs and their conservation measures are also discussed.
INTRODUCTION
Mangroves are unique inter-tidal ecosystem of tropics and sub-tropics, which support genetically diverse groups of aquatic and terrestrial organisms. The mangrove ecosystems are of great ecological and economic significance in coastal protection, enrichment of water quality and in production of fishery resources (Kathiresan and Bingham, 2001; Kathiresan, 2003). Crabs are among the most predominant species in many mangrove forests. They constitute about 80% of the macrofaunal biomass and can reach densities of 80-90 m-2. The crabs depend directly on mangrove areas for survival, by feeding on leaves and liter. They have a significant role in detritus formation, nutrient recycling and dynamics of the ecosystem, together with numerous annelids and nematodes living in the sediment. The digging behavior by crabs enhances aeration and facilitates drainage of mangrove soils. They are adapted to the sediment conditions, tidal fluctuations and varying salinities found in the mangroves. Distribution and zonation studies of crabs in Indian mangrove are scanty. Joel et al. (1985) studied the distribution and zonation of 29 crab species in the Pulicat Lake, while Chakaraborthy and Choudhury (1992) observed 18 species in the virgin mangrove island of Sunderbans. However, nobody attempted the distribution of crabs in Pichavaram mangroves. Hence the present study was concentrated on the distribution of crabs in Pichavaram mangrove environment.
MATERIALS AND METHODS
Description of Pichavaram Mangroves
The Pitchavaram mangrove forest (Lat. 11°26`N; Long. 79°48`E) is located between the Vellar and Coleroon estuaries (Fig. 1). The forest occurs on 51 islets, ranging in size from 10 m2 to 2 km2, separated by intricate waterways that connect the Vellar and Coleroon estuaries. The southern part near the Coleroon estuary is predominantly mangrove vegetation, while the northern part near the Vellar estuary is dominated by mud flats. The Vellar estuary opens into the Bay of Bengal at Parangipettai and links with the Coleroon River, which is a distributary to the river Cauvery. Pitchavaram mangrove is influenced by mixing of three types of waters; (1) neritic water from the adjacent Bay of Bengal through a mouth called Chinnavaikal, (2) brackish water from the Vellar and Coleroon estuaries and (3) freshwater from an irrigation channel (Khan Sahib canal), as well from the main channel of the Coleroon River. The mangrove covers an area of about 1100 ha, of which 50% is covered by forest, 40% by waterways and the remaining filled by sand-flats and mud flats (Krishnamurthy and Jayaseelan, 1983). The tides are semi-diurnal and vary in amplitude from about 15 to 100 cm in different regions during different seasons, reaching a maximum during monsoon and post-monsoon and minimum during summer (Muniyandi, 1986). The raise and fall of the tidal waters is through a direct connection with the sea at the Chinnavaikal mouth and also through the two adjacent estuaries. The depth of waterways ranges from about 0.3 to 3 m (Muniyandi, 1986).
| Fig. 1: | The Pichavaram mangrove area |
Field study was carried out in the swamp of Pichavaram from December 2001 to November 2002 at monthly intervals. The study was undertaken in both creek and fringe mangroves. The species present on the substratum and on the vegetation area were recorded in quadrant each measuring 1 m2. Crab burrow density was recorded in 5 randomly placed one square meter quadrates as per Jones (1984). Crabs were collected at low tide and preserved in 70% alcohol. The species were identified following the description of Williams (1984) and Sethuramalingam and Ajmalkhan (1991).
RESULTS AND DISCUSSION
Distribution of Crabs in Pichavaram Mangroves
In the present study 36 species of crabs are recorded in Pichavaram environment (Table 1). Crabs belonging to the family Grapsidae and Ocypodidae were the most dominant forms, which play a vital role in recycling the nutrients, enhancing the role of decay of plant materials and litter processing. Crab population was high during monsoon and post monsoon months in Pichavaram mangroves. Factors such as low salinity, possible physical changes in the substrate composition and availability of maximum organic carbon during monsoon and post monsoon seasons may be attributed for greater abundance. Sethuramalingam (1984) divided Pitchavaram mangrove areas into five zones based on the distribution of crabs. For instance, in Sesarma zone, Sesarmid crab species were dominant as other zones also had their specific crabs. Zonation of Pichavaram mangrove environment based on crab distribution (Table 2).
Among all the zones the dominant representatives were Sesarmid group of crabs. It included 13 species; of which, Grapsus strigosus and G. tenuicrustatus were very rare. The dominant species of Sesarma zones were S. brockii, S. plictum and S. andersoni. The other crab species recorded from these zones were Neoepisesarma tetragonum, M. mederi, Metapograpsus messor, M. maculates. Brachyuran taxa were dominant among other crab species distributed in Pitchavaram mangrove forest.
| Table 1: | Crabs of Pitchavaram mangroves |
| Table 2: | Zonation of crabs |
Distribution and zonation of crabs were based on the substratum, water level and floral distribution. It included 13 species; of which, Grapsus strigosus and G. tenuicrustatus were very rare. The dominant species of Sesarma zones were S. brockii, S. plictum and S. andersoni. The other crab species recorded from these zones were Neoepisesarma tetragonum, M. mederi, Metapograpsus messor, M. maculates. Brachyuran taxa were dominant among other crab species distributed in Pitchavaram mangrove forest. Distribution and zonation of crabs were based on the substratum, water level and floral distribution.
Habitat Preference of Crabs in Pichavaram Mangroves
Pichavaram was found to be sandy along its vertical transect, with patches of mangrove vegetation. Table 3 shows that each species of mangrove crabs have its characteristic pattern of abundance and habitat preference.
Distribution of Crabs in Various Vegetative Zones of Pichavaram Mangroves
Based on the distribution of plants Pichavaram mangrove environment was divided into Rhizophora zone, Avicennia zone, back mangrove zone and non-saline zones. The crabs are distributed in different vegetative zones (Table 4). Maximum number of crabs was distributed in Avicennia and Rhizophora zones. Sesarma brockii and other sesarmid crabs were present along the high water of neap tides and found to be sheltered amidst Avicennia marina. This is attributed due to the presence of rich nutrients in the Avicennia leaves when compared to other mangrove leaves (Rajendran, 1997).
Most of the juveniles and females of smaller crab were distributed along the lower shore regions. The females and juveniles of Uca annulipes were distributed close to the water edge along with their neighbours Macrophthalamus depressus and M. erato, which may be due to their lesser capacity to tolerate desiccation. The sand dominated substrata were suitable for Portunus sanguinolentus and Thalamita chaptali. From the study, it was confirming that substrate suitability; effects of tidal inundation and distribution of mangrove plants were the possible factors that could influence zonation and abundance of the crabs in the Pichavaram mangroves.
Reason for Depletion of Crabs
The population of commercially important mangrove crabs viz., Scylla serrata, S. tranquebarica, Portunus sanguinolentus and P. pelagicus are declining day by day due to indiscriminate fishing of berried females and also fishing of under sized crabs. Pesticides used in agricultural operation in and around Pichavaram mangroves were also posing serious threat to the crab population in general and larval farms in particular. The levels of HCH (Hexachlorocyclohexane) and DDT (p,p, -dichlorodiphenyltrichloroethene) were higher during wet season, although the trend was more pronounced with HCH than DDT. The levels of HCH were 630 ng L-1 in the water of Pichavaram mangroves in wet season and the levels were rather higher in global comparison. Similarly HCH occurs in high concentration in the sediments of Cauvery and Coleroon riverine systems, which also supplies to Pichavaram mangroves (Rajendran and Subramanian, 1997). Invariably the crab samples of mangrove contain the compounds more than other species of aquatic animals collected from nearby areas (Ramesh et al., 1992). In recent years mangrove areas have been converted to shrimp ponds. The discharged effluents from the shrimp farms are also cause serous threat to crab population. So increase in pollution around mangrove ecosystem by man made chemicals due to aquaculture practices is now adding pressure on crab population (Subramanian, 2000). Hence conservation of mangrove crabs is paramount importance.
| Table 3: | Crabs, their abundance and habitat |
| Table 4: | Distribution of crabs in different vegetation zones |
Conservation Measures
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A single female crab can produce up to eight million eggs in one mating season, so try to minimize their capture. |
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Berried crabs should be released back if at all they are caught in the nets. |
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Juveniles and undersized crabs should not be caught. |
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Fishing of crabs should be strictly banned during their peak-breeding season. |
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Awareness should be created among the local people and fishermen regarding the depletion of the crabs. |
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Extension programmes of crab conservation should be carried out through media. |
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Wastewater released into this complex area should be treated well before its release into the natural waters. |
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To maintain the population, sea ranching of commercially important crab species should be practiced |