An exploratory study on grapsid crab zonation in Kenyan mangroves

Despite earlier efforts to understand the role played by grapsid crabs in mangroves, their importance in the structuring and functioning of such systems is fully appreciated, particularly with regard to small-scale studies. The present study provides some new data on the interaction between mangroves and crabs, namely the link between the distribution of particular mangrove tree species and the distribution of certain crab species at the assemblage level. Floristic and faunistic relevés were made in Gazi Bay (2 sites) and Mida Creek (3 sites), 140 km apart on the Kenyan coast, along five transects in a series of quadrats covering the width of the mangrove belts. Zonation of both mangrove vegetation and brachyuran fauna was described and height above datum and distance to the mainland (limit of non-flooded area) measured. The relationship between the presence and the absence of crabs and trees was analysed using detrended correspondence analysis. Summarized, the mangrove tree zonation pattern contains four assemblages with a particular dominant species: first a landward Avicennia marina zone, followed by a mixed zone with Ceriops tagal, Rhizophora mucronata and Bruguiera gymnorrhiza, and finally a R. mucronata zone and a Sonneratia alba zone, both of which can mix with seaward A. marina. Ordination results show that the distribution of Neosarmatium meinerti and Sesarma ortmanni is linked to the landward A. marina zone, that of Neosarmatium smithii, Sesarma guttatum and Sesarma leptosoma corresponds to the R. mucronata zone and that of Metopograpsus thukuhar and Sesarma elongatum to the seaward A. marina and S. alba zone. There appears to be one major underlying factor in the zonation of both crabs and trees, with most likely a complex multiple causality. In certain cases the association between crabs and trees are causal, whereas in other cases it can be the result of an independent restriction to the same zones by a common cause.