The importance of both mangrove flora and fauna (especially crabs) has been well documented. Brachyuran mangrove crabs are highly diverse and show a plethora of adaptations to this ecosystem, rendering them important for global biodiversity as well as interesting scientific subjects. They are most probably keystone species as their removal alters ecosystem structure and function (Smith et al., 1991). Two studies were carried out in Gazi Bay and Mida Creek, Kenya. One focused on the zonation of mangrove crabs and its relationship with the environment, the second narrowed down on the salinity tolerance of two species. Transect surveys describing crab zonation in conjunction with biotic and abiotic factors showed that factors controlling mangrove brachyuran crab distribution is a complex matter and is probably dependent on a combination of many environmental factors. However, species richness was negatively correlated to salinity and temperature and positively correlated to pH (pH lower than 7). The riverine forest studied, Dabaso, had higher species richness in general and also a higher salinity. This forest type probably has reduced stressors on other levels as well, allowing the higher species richness in spite of elevated salinities. Each zone within a mangrove forest has its own diversity and fauna (i.e. they are not faunistically equivalent), and each should be treated as separate sub-biotopes when considering mangrove crab fauna. Scope for growth experiments and osmo-regulatory studies showed that the Neosarmatium species inhabiting Kenyan mangroves differ substantially in their salt tolerances. N. smithi has a reduced salt tolerance and inhabits a narrow salinity range in the field, whereas N. meinerti has a wide salt tolerance and inhabits a wide salinity range in the field. Although N. meinerti can withstand high salt concentrations, it is not probable that populations could be fully functional in hypersaline areas due to the increased energy expenditure of osmoregulation. Both of these species occur in high densities, generally, their distribution stopping where the other one starts, more or less covering the entire mangrove forest. Both have wide diameter burrows extending down to about one meter and a very rough estimate of excavated was calculated at 72 m3/ha throughout the forest by these species. If, due to increasing salinity these populations decreased or disappeared, the mangrove forest would be seriously impacted. Groundwater has been shown to affect estuarine salinity regimes, if it is altered in East Africa, the aforementioned scenario may occur. It is unlikely that N. meinerti would be seriously affected, but N. smithi most probably would. Moreover, N. smithi cover a much wider area in both fringe and riverine forest types, this and their lower salinity tolerance, makes them the more critical species. In light of these findings, a similar tool to the BioSalinity index developed for the Gulf of Mexico (Christensen, et al., 1997) would be useful for these mangrove ecosystems. Additionally, The habitat of juvenile C. camifex has been described and hypotheses drawn on why it parasites adult burrows. Ecological notes on N. smithi include the observation of Merguia oligodon co-inhabiting their burrows and the possible significance of this. |
