Crabs live in burrows dug in mangrove mud and they alter the structure and chemistry of the sediment in ways that affect wider range of mangrove organisms, other than their direct competitors, predators, or prey. Mangrove crab diversity is very high with an estimated 275 species from six families of brachyurans (true crabs) associated with the mangrove ecosystems. Eighteen crab species in Kenyan mangroves alone are making burrows. They are the "ecosystem engineers"?? that create new habitats and change the availability and quality of food, shelter and refuge for other constituent species in mangrove ecosystems. Beneath the surface, mangrove soils are tunneled by crabs into a complex of burrows that behaves like a system of conduits carrying water, dissolved nutrients and air to these primarily anaerobic soils. These interconnected burrows are the habitats of mangrove crabs and provide them shelter from extreme environmental conditions and refuge from predators. The burrowing activity of the crabs can alter the topography and micro-hydrological patterns within the mangrove area. The changes to the physical, chemical and biological nature of the ecosystem as a result of the presence of crabs (or any other organism with that capacity) is collectively called bioturbation. In mangrove ecosystems, bioturbation caused by crabs is a life-supporting process.
Sediment lining the burrows are oxic as oxygen in water may diffuse to this layer, hence become habitats for nitrifying (ammonium-oxidising) and denitrifying (nitrate-reducing) bacteria, thus allowing nitrification-denitrification processes in the oxic-anoxic interface associated with burrow lining. Through this process, crab burrows can effectively remove nitrogen from the aquatic ecosystem in the form of gaseous nitrogen (N2
) and nitrous oxide (N2
O). Part of the nitrates formed may be transported to other parts of the mangrove and become available nitrogen for plant and bacterial growth. In mangrove areas in Mid Creek, Kenya, about 10% of the burrows of Sesarma meinerti (a sesarmid crab) are dug anew daily and hence the extent of aeration of mangrove soils down to a depth of approximately one metre is considerable (Micheli et al, 1991). Left: Diagrammatic representation profile of a crab burrow in mangrove soils.
Mangrove crabs are predominantly herbivores, feeding on fallen or live leaves and propagules. They take leaves into the burrows and the uneaten ones are thus buried in mangrove soil by the crabs. While part of it is decomposed by aerobic bacteria in the aerobic lining of the burrows, some are decomposed in the anaerobic zone of the soil by sulfur and iron- reducing bacteria. The burrowing habit of crabs extensively increases the reactive anaerobic zone of the mangrove soils thus facilitating the decomposition and mineralization and provision of nutrients to mangrove life. Their absence will aggravate the plant nutrient deficiency in mangrove soils and in turn mangrove primary productivity and the life-supporting capacity of the ecosystem.
Nearly half the water that move into the mangrove areas with tide, could at some stage be held inside the burrows, indicating its potential in exchanging ground and surface water and hence salt and other dissolved plant nutrients. The presence of crab burrows therefore helps to reduce soil salinity, preferred by non-secretor mangroves like species of Rhizophora
(Stieglitz et al, 2000). By aerating the soil, they may also support establishment of mangrove seedlings in aerobic soils which are less stressful than the normal mangrove soils that are anaerobic (Youssef and Saenger, 1996). Neosarmatium meinerti
and N. smithi
in Kenyan mangroves have shown to be negatively affected by hypersalinity, indicating that natural and anthropogenic processes that lead to salinity rises will affect these crab populations and therefore sustenance of the ecosystem (Gillikin et al, 2004). References
Gillikin, David Paul, De Wachter, Bart and Tack, Jurgen F., 2004. Physiological responses of two ecologically important Kenyan mangrove crabs exposed to altered salinity regimes Journal of Experimental Marine Biology and Ecology
, 301(1): 93-109.
Micheli, F. Gherardi, F. and Vannini, M., 1991. Feeding and burrowing ecology of two East African mangrove crabs. Marine Biology
Stieglitz, T., Ridd, P. and Muller, P., 2000. Passive irrigation and functional morphology of crustacean burrows in a tropical mangrove swamp. Hydrobiologia
Youssef, T. and Saenger, P., 1996. Anatomical adaptive strategies to flooding and rhizosphere oxidation in mangrove seedlings. Australian Journal of Botany 44(3): 297 - 313.