Population Dynamics and Dispersal Pattern of Fiddler Crabs in the Mangroves Areas of Karachi Coast

Abstract

Mangroves inhabit a pool of diversity, creates habitat for diverse community of organism ranging from bacteria and fungi to fish, shrimps, birds and mammals. The mangroves the unique inter tidal structure habitat have been focus of descriptive and experimental investigations of the relative importance and interaction of physical and biological functions in communities as their inter tidal structured habitat is a unique system. Decapods crustaceans are associated with mangrove community of which brachyuran crabs predominantly form significant feature of this community. Crabs population is believed to have an important effect on nutrient dynamics, sediment structure, microbial activities and meiofaunal abundance. Of these, fiddler crabs belonging to family Ocypodidae forms one of the largest parts of macro-faunal biomass. The present study was initiated to study the distribution and abundance of fiddler crabs and associated crabs and fauna in the mangrove areas along the Karachi coast. Well representative samples were collected from two transect in the mangroves areas from two stations at sand spit (backwater mangroves), and one station at Korangi creek. Regular monthly collections of fiddler crabs samples were collected from March 2001 to February 2002 from each station. Each monthly collection includes data on the number crabs, number of burrows and number of mangrove roots (pnematophores), numbers of molluscan shells (potamides cingulatus), along with associated fauna and flora residing in the each quadrate. The size of burrow openings was also noted. Data of the environment parameters like: salinity, temperature and PH were collected simultaneously from each quadrate at each site. Sediment samples were also collected and brought back to laboratory for further analyses of sediment properties. In the laboratory crabs and other organisms when present were sorted, sexed and identified up to species level. The crabs in each quadrate were sized (up to nearest mm) and weighed (up to nearest grams) for biomass measurements. The abundance and distribution of crabs their relationship with crabs burrows, breeding periods, sex ratio, handedness and dimorphism were studied. So far eleven species of members of family Ocypodidae and one species of Xanthid crab have been collected and identified from our collection. The species of crabs were identified following Crane (1975) and Tirmizi and Ghani (1996). Four species of fiddler crabs identified were Uca annulipes, Uca lacteal, Uca urvillie and Uca inversa sindensis. The other associated crabs species were Macropthalmus pectinipes, Macropthalmus boscii, Macropthalmus depressus, scopimera crabicauda, serenella indica, IIyoplax frater, Dotilla blandfordi and Nasima dotilliformes belonging to family Ocypodidae and one species Eurycarinus orientalis of family Xanthidae. Four species of fiddler crabs with respect to their distribution and abundance in zones and sympatric association with other crabs were observed from low tidal region to high tidal region in the mangrove area. Transect analyses data showed that Uca lacteal along with U annulipes were distributed in all zones except with in mangrove forest in Sandspit back water mangrove area. Whereas U urvillei was randomly collected from the study area and they were found to be distributed in land ward edge of the mangroves under the shades. U. Lactea and U annulipes were found to be sympatric with U urvillei at Sandpit back water mangrove area. There were definite trend in the species distribution of these crabs. Some species showed patchy distribution with each patch representing separate species, while other species over lapped in their distribution. Different species inhabited different niches on the same site and were found to be sympatrically associated. Our results show that sediment characteristics play an important role in the distribution and abundance of fiddler crabs. This have interesting implication on the inter specific distribution of different species of Uca at both sites. The densities of burrows were estimated to range from 20 to 2100 m2. The crab burrows were usually greater in number than the number of crabs collected from all three stations, except, at few instances when the number of crabs exceeded the number of crab burrows. Small sized crabs were generally collected close to low water marks and these patterns were also reflected in the size of burrow openings. The seasonal variability in the burrow abundance and sizes were observed. The number and size of burrows are likely related to reproductive cycle and recruitment of these species. During the present study and it was observed that the fiddler crabs along the Pakistan coast breeds from April to October. Study of sex ratios showed that male crabs were more abundant than female crabs except during breeding periods when more female crabs were collected. Fiddler crabs are well known for their outstanding sexual dimorphism and hypertrophy. The male crabs distinguished by the presence of asymmetrical enlargement of one of the chelipeds, while the female chelipeds are small and nipping. Our results show the presence of either left or right enlarged chelipeds or handedness was purely occasional and not apparently related with size or season. Our observation on the spatial and temporal variation is first comprehensive study from mangrove area. Knowledge of ecology of fiddler crabs from our mangrove habitat will help us to understand the extent of utilization of mangroves by these crabs and their specific role in the ecosystem they inhabit. Fiddler crabs are likely playing a major role in channelling energy within the system and also exporting it out from the system (for example predation of birds resident and migratory) on the fiddler crabs personal observation.