Determination Of Thermal Tolerance, Density And Distribution Of The Mangrove Crabs, Perisesarma Guttatum (Sesarmidae) And Uca Urvillei (Ocypodidae) At Gazi-Bay, Kenya

Abstract

Mangrove crabs are important in ecosystem functioning including; bioturbation of the soil resulting in soil particle size distribution, sediment aeration, reduction in sediment salinity and nutrient recycling and thus are fundamental for the viability of mangrove forests which are in turn important to the coastal communities’ livelihoods. Mangroves are intertidal forested wetlands confined to the tropical and subtropical regions. They play important role as habitat for animals, provision of wood and non-wood products and a source of medicine to coastal people. The effects of climate change on ecosystems are real and thus the mangrove ecosystem are also threatened by this phenomenon. Therefore, the question that arises is what effects, if any, would increases in temperature have on those animals resident in the mangrove sediment. The aim of this study was to determine the thermal tolerance, density and distribution of the mangrove crabs Perisesarma guttatum and Uca urvillei at Gazi-Bay on the Kenyan coast. Field activities included collection of data on density and distribution of the two crab species and on environmental variables in the Rhizophora mucronata, Ceriops tagal and Avicennia marina monospecific stands. In the laboratory, the crabs were maintained at temperatures between 17-37°C and respiration rate measurements was performed in closed chamber systems after the crabs were acclimated for 8 hours at 27°C. The results indicate that the temperature ranges for P. guttatum and U. urvillei adult crabs were 27-31°C and 27-33°C respectively beyond which the crabs got stressed as indicated by increased metabolism. This suggest that P. guttatum is more sensitive to temperature variation than U. urvillei which has a wide thermal tolerance window. The highest average densities of U. urvillei and P. guttatum crabs recorded were 66.25±7.7/m² and 11.75±4.1/m² respectively in the R. mucronata zone. Very low densities of U. urvillei of 0.88±0.25/m² were found in C. tagal and 0.47±0.28/m² in A. marina zone while density of P. guttatum was 1.9±1.8 in A. marina and was not significantly different between C. tagal and R. mucronata zones (P>0.05). The densities of U. urvillei and P. guttatum were strongly related to temperature, fine sand and organic matter using stepwise regression analysis (P