Abundance and distribution of mangrove crabs in relation to environmental parameters such as soil salinity , soil pH soil organic carbon content, soil sulphur comtent and soil nitrogen content has been described by previous workers. Both gualitative and gualitative methods have been used to estimate the abundance of crabs in mangrove ecosystems and the dependence of mangrove crabs on the environmental condition has been described. Salinity ,water content in the soil ,ph ,total percentage values for carbon, nitrogen and sulpphur content of the soil and soil particle are some of the factors which have shown correlations with the distribution and abundance of crab. Macintosh1984 found that the low shore crab, Metaplax elegans could not tolerate salinities below 10%, whereas Persesarma onychophorum, inhabiting higher shore zones, colud tolerate several days at 5%. In the present study, the soil salinity varied between 60% to 80% and E. vesicolor and S. guttatum preferred low salinity range (57-67%) while the unidentified juvenile crab and M. thukuhar preferred higher salinity (80-82%).
Soil and water pH values also show correlation with the crab community structure. Low- pH conditions can be detrimental to crabs because they depend on calcium carbonate formation for exoskeleton and shell building. Low-pH values can cause exterior exoskeleton and shell erosion in crustraceans and hence they tend to avoid such places . Soil pH values in the upper zone of Kadolkele mangrove varied from 4.1 to 4.4 while it ranged between 5 and 6 in the lower zone. M. thukuhar and the juveniles of unidentified of all crab species were found to be high in the lower area where soil pH value is higher soil pH levels. This is in agreement whith the fidings of Plaziat. S. guttatum and S. smithii have shown positive correlation with soil pH in the Kadolkele mangrove.
Carbon, nitrogen and sulphur content of the soil are organic portions in the detritus and are important food sources. However, in humid areas with large amounts of detritus, most of the nitrogen and sulphur present is organic. Many crabs have been observed feeding on the sediment. The bacteria and meio-fauna present in the sediment may also be a good source of the nitrogen and sulphur for some macro-fauna species. Olafsson et al.have observwd positive correlation between crab biomass and soil nitrogen content which merit further attention in relation to the apparent importance of these soil organisms in the nutrition of common members of the crab fauna. Unfortunately, this study could not study the soil nitrogen contents due to lack of facilities, but positive correlations observed in abundance of all species of crabs with the soil carbon contents show their dependency on detritus based food web.
Presence of mangrove vegetation that provides a habitat structure and food for mangrove crabs is another factor determining their distributionAtshton et al showed that the crab community structure was correlated with the tree and seedling community structure indicating their importance in supporting crab diversity or vice versa. Some sesarmid. Floral zonation in Kadolkele mangrove is closely associated with the zonation pattern described by boaden and seed in East Africa. In the present study it is evident that floral zonation has an influence on the spread of crab species . M. thukuhar and unidentified juvenile crabs were found restricted to Rhizophora/bruguiera zone, while E. vesicolor and S. guttatum were confined to the Avcennia/Lumnitzera zone . Metopograpsus messor and S. smithii on the other hand could inhabit both zones.