Sustainable aquaculture and aquatic resources management

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Mangrove crabs: the ecosystem engineers and keystone species

Posted on 28/10/2009 | 10947 reads | Tags: Mangroves
Diagrammatic representation profile of a crab burrow in mangrove soils
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 (N2O). 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 111: 247-254.

Stieglitz, T., Ridd, P. and Muller, P., 2000. Passive irrigation and functional morphology of crustacean burrows in a tropical mangrove swamp. Hydrobiologia, 421:69-76.

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.

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