Abstract: The diversity of the crabs in the Pichavaram mangroves for the first time has shown that there are 46 species of crabs from the five different stations. The neritic zone (Chinnavaikal) was rich in species composition (35 species). The freshwater zone was poor in species composition (14 species) and population density. Among the crab species Sesarma and Uca species were dominant in almost all the stations. Grapsid crabs were the most dominant species. Neritic and Avicennia zones showed more number of crab species but less number of species was noted in slightly saline zone. The substrate suitability, tidal inundations mangroves distribution and salinity were the possible factors for distribution of crabs in Pichavaram mangroves.
INTRODUCTION
Biodiversity issues in coastal zones are important for conservation of species and habitats. Brachyuran crabs, which contribute a bioenergetically significant faunal component, play a significant role in maintaining a steady state of the mangrove ecosystem. Zonation, distribution and salinity tolerance of both mangrove vegetation and brachyuran crabs was described in many mangrove forests crabs constitute maximum number of macrofaunal biomass in this environment (Augusto et al., 2005; Dahdouh-Guebas et al., 2004; Sean et al., 2005; Nordhaus et al., 2006; Diele and Smith, 2006). Two families of crabs are particularly associated with mangrove ecosystem, the Grapsidae (63 species) and the Ocypodidae (30 species) (Jones, 1984). However studies on biodiversity of crabs in Indian mangroves have not much attempted so far. Hence, the present study reports for the first time data on the number of crab species occurring in five different sites of the Pichavaram mangroves, their distribution, relative abundance and also zonation in relation to the mangrove species.
MATERIAL AND METHODS
Pichavaram mangrove (Lat. 11° 27’ N; Long 79° 47’ E) represents a heterogeneous mixture of mangrove elements. It lies between the Vellar and Coleroon estuaries, located at 250 km south of Chennai City on the southeast coast of India. Field study was carried out in the Pichavaram mangrove from February 2004 to January 2005 at monthly intervals. The study was undertaken in both creek and fringe mangroves in five sampling sites, varying in microclimatic niche (Fig. 1). 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 for identification (Rathburn, 1918, 1930; Chhapger, 1957; Sakai, 1976; Williams, 1965, 1984).
The general survey involved, collection of various species of crabs from the creek and fringe mangrove vegetations to describe their species richness. Density of the crabs were estimated random sampling in the Pichavaram islands by number of crabs active on the substrate enclosed within the quadrant or by digging the crabs from their burrows.
| Fig. 1: | Sampling stations in Pichavaram mangrove |
The atmospheric temperature, surface water temperatures were recorded using a Celsius thermometer and salinity also estimated (Stickland and Parsons, 1968).
To assess the species diversity of the crabs in the environment, the formula of Shannon and Wiener (1949) was used the species richness was calculated by simpson’s and evenness or equitability, in the distribution of individuals among various species was calculated using the program written by Bakus (1989).
RESULTS
Species Composition
A total of 46 species of crabs were recorded from five sites (Table
1). Maximum number of species (31) was observed during the postmonsoon especially
in January (31) and minimum (12) during the summer particularly in May.
Number of crab species recorded was maximum at station 5 (35) and minimum number (14) at station 1 (Table 1). Among the crab species Sesarma and Uca species were the most dominant in almost all the islets. Sesarma brockii, S. plicatum, Neoepisesarma tetragonum, Metapograpsus messor, Pseudograpsus intermedius, Macropthalmus depressus, Uca triangularis and U. annulipes were commonly found in all the five stations. But, Plugusia depressa, Graspus tenuicrustatus and Neoepisesarma medri were observed only in the core mangrove site (Periyakadavu station) and Portunus, Podapthalmus, Ocypods, Dotilla, Dromia and Leptodius species only in the neritic zone (Chinnavaikkal station) in the present study. The abundance of crab species among the stations is given in the following order.
Chinnavaikkal > Periyakadavuu >Alasi odai > Kanankeluthi canal > Khan Sahib canal.
Population Density
The crab population density varied between 595 and 885 individuals among
the stations. Minimum (595) was documented at station 4 and the maximum (885)
in station III (Table 1). Crab population varied significantly
between months and station-wise analysis.
Zonation
Higher number of crab species was observed in the Neritic zone (36) and
core mangrove zone (29). Very less number of species was noticed in the freshwater
zone (14). Among all the zones, dominant representative was Sesarmid group of
crabs (Table 3).
Based on vegetation, Pichavaram mangrove is divided into Rhizophora zone, Avicennia zone, back mangrove zone and non-saline or slightly saline zone. Among these zones, Avicennia zone was found to have more number of crab species (22) and very less number (1) of crabs in the non-saline zone (Table 4). Cardisoma carnifex was found commonly in all the zones except in Rhizophora zone while, Charybdis species were seen only in the Rhizophora zone (Table 5).
Salinity and Temperature
The salinity ranged from 0.6 to 36.2% during the study period (February
2004 to January 2005). The maximum salinity of 36.2% was reported in the month
of April and minimum salinity of 0.6% in December. Salinity varied significantly
between month and station-wise analysis.
The atmospheric temperature ranged between 18. 2 and 37. 6° C. The surface water temperature varied from 16 to 36. 2° C. Maximum atmospheric temperature was recorded in the month of May 2004 and minimum in the month of December 2004. The highest surface water temperature was recorded during May, 2004 and the lowest during December 2004. Atmospheric and surface water temperature varied significantly between month of analysis but not station-wise analysis.
The statistical analysis indicates that the environmental variables such as salinity, atmospheric temperature and surface water temperature have tremendous influence on the population density of the crabs.
Species Diversity Index
The species diversity index varied between 2.674 and 3.388 among the five
stations. High species diversity index was noticed in station III and low species
diversity index at station II. The evenness values varied between 0.644 and
0.701 minimum value (0.644) was recorded at station IV and the maximum value
(0.791) at station III. The richness index in the stations fluctuated from 0.744
to 0.846. Minimum (0.774) was recorded at station I and the maximum (0.846)
at station III (Table 2).
| Table 1: | Station wise distribution of crabs |
| (No. of individuals in one square meter) | |
| Table 2: | Biodiversity indices obtained for crabs at different stations |
| Table 3: | Zonation of crabs based on the environment |
| Table 4: | Zonation of crabs based on the mangrove environment |
| Table 5: | Zonation of crabs in Pichavaram mangroves |
DISCUSSION
Of all benthic macrofauna inhabiting the mangrove swamps, brachyuran crabs are among the most important taxa with regard to species diversity and total biomass. Crabs depend directly on mangrove for survival and are adapted to the special sediment conditions, tidal fluctuations and varying salinities found in mangroves (Cocelho, 1967). The abundance and species richness of crab were examined in a subtropical mangrove forest in Moreton Bay Australia (Skilleter and Warren, 2000).
In the present study, as many as 46 species of crabs were recorded in the five different stations of Pichavaram mangrove. High number (35) of species was found distributed in the Chinnavaikal region and very less number (14) of species in Khan Sahib canal region. Chinnavaikal is a neritic zone, which promotes crab diversity. Sesarmid crabs are common in all the five stations but they are abundant in Peryakadvau is a dense mangrove area. But interestingly Portunus, Podaphthalmus, Ocypods, Dromia, Dorippe and Dotilla were observed only in the Chinnavaikkal region, which has more neritic influence.
Species composition and diversity were high during postmonsoon and less during summer. High floral and faunal diversities during postmonsoon season increase the availability of food along with maximum organic carbon due to rain, optimum salinity, also temperature and physical changes in the substrate composition. Ocypodid and Grapsid crabs are the dominant forms in the Pichavaram mangrove but in the intertidal zones of Hoogly estuary, Sundarbans Metaplex intermedia constitutes the dominant faunal component (Chakaraborthy et al., 1992). However, in the present study, Uca triangularis, U. annulipes and S. brockii are found to be the dominant species in the Pichavaram mangrove. The minimum density was observed during the summer is related to high temperature and salinity which may reduce availability of nutrients (Kathiresan, 2000; Ravichandran and Kannupandi, 2004). An increasing trend was reported in macrobenthic population towards postmonsoon in the tidal creeks of Sagar island (Bhunia and Choudhury, 1981) but the lowest population during the premonsoon from Sundarban mangrove (Chakaraborthy and Choudhury, 1994). Parulekar et al. (1980) also recorded more benthic faunal population in the estuaries and backwaters of Sagar island of Sundarban mangrove during the postmonsoon season.
Distribution and zonation of crabs are based on the substratum, waterlevel and floral distribution. The species components were dissimilar among the stations studied. The Ghost crab Ocypode macrocera preferred sandy substratum, whereas Macropthalmus species preferred only muddy substratum. Uca species was found in dry or grassy, elevated and muddy substratum. The Ocypode was found above the high water mark, while members of the sub family Ocypodidae were dwellers in the mud below high water neap (Snelling, 1959). In the present study high population density was noticed in station 3, which has dense mangrove and low population density was observed in station 1 lacking forest cover.
Habitat structural complexity influences the density and diversity of marine organisms and contributes to zonation, where the level of complexity varies significantly with tidal height (Loheer et al., 2000). Chinnavaikkal region is divided (Pichavaram) into five different zones based on crab species (Ravichandran et al., 2000; Ravichandran and Kannupandi, 2004a). In the present study also, maximum number of species was observed in the neritic zone and very less number of species in freshwater zone. Similarly in the Avicennia zone more number of species were recorded and least number of crabs in the slightly or non-saline zone. The leaves of A. marina contain rich nutrients and more palatable compared to other mangrove leaves (Lee, 1989; Rajendran and Kathiresan, 2000; Ravichandran and Kannupandi, 2004b; Ravichandran and Kannupandi, 2005). This perhaps may be the reason for maximum number of grapsid crabs present in the Avicennia zone.
In the present study, as many as 46 species of crabs were present in five stations. Species composition and population density were maximum in the core mangrove region (Periyakadvu region) and less in the fresh water zone (Khan Sahib canal region). Grapsid crabs are the dominant species among the crab taxa. The mangroves distribution, substrate suitability and tidal inundations were found to be the possible factors which influence zonation and distribution of crabs in the Pichavaram mangrove.
ACKNOWLEDGMENT
Thanks are due to the Authorities of Annmalai University for facilities and Ministry of Environment and Forest, Govt. of India for the financial assistance.