Decapod crustacean communities from calcareous seaweed andPosidonia oceanica(rhizome stratum) in shallow waters

Quantitative decapod studies carried out on calcareous seaweeds and Posidonia oceanica beds (mainly rhizome stratum), from shallow coastal waters of Almeria (S, SE Spain), revealed that these biotopes represent part of the habitat of the same decapod crustacean community. A gradual transition from the decapod community in Posidonia to those in calcareous concretions is shown. This transition appears to depend on the relative developments of, and the interrelations between, both biotopes, as well as on the influence of a number of extrinsic factors such as the presence of crevices, photophilic seaweeds and sedimentation. However, when these two biotopes are well developed and independent they could be recognized by their differing quantitative species compositions, that is by the high dominance of a small number of and different species.