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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.

The capture of a specimen of Gourretia denticulata (Lutze, 1937) (Decapoda, Ctenochelidae) from the Cadiz littoral zone, represents the first record of the species for Spain and for European Atlantic waters and confirms its presence in the East Atlantic Ocean. Some data on anatomic considerations and distribution are given.

A population of the hermit crab Diogenes pugilator from the northeastern Atlantic (Barbate, Straits of Gilbraltar, Spain) was studied. The results show continuous recruitment throughout the year, within a size range from 0.4-1.02 mm in cephalothoracic shield length, as a consequence of the prevalence of ovigerous females throughout the year. The sex ratio was in favor of females. Results of size-frequency distributions from different locations show that there was no clear segregation of sizes by sexes or depths. Five cohorts can coexist, although a decrease in the number of specimens was observed in the second year studied, perhaps induced by human influences in the area (the building of a new harbor and the restoration of the beach). The values of the Von Bertalanffy parameters (K and Loo) showed slight differences between sexes and between years within sexes, as is shown in the φ' index analysis. The estimated maximum longevity of the individuals in the population studied was about 2 years, but normally the cohorts lived in the study area for approximately 1 year.