MEGABENTHIC DECAPOD CRUSTACEAN ASSEMBLAGES ON THE GALICIAN CONTINENTAL-SHELF AND UPPER SLOPE (NORTH-WEST SPAIN)

The structure of megabenthic decapod crustacean assemblages on the Gal ician (north-west Spain) continental shelf (100 to 200 depth) and uppe r slope (200 to 500 m) was analyzed based on surveys carried out in au tumn and spring, from 1980 to 1987. Forty species belonging to 19 fami lies were caught. The portunid crab Polybius henslowii, a species with pelagic phases, was the most abundant species, but displayed strong s patial and temporal fluctuations. Other dominant species were the Norw ay lobster Nephrops norvegicus, the portunid Liocarcinus depurator, th e galatheids Munida intermedia and M. sarsi, and the shrimps Solenocer a membranacea, Plesionika heterocarpus, Pasiphaea sivado and Dichelopa ndalus bonnieri. Total abundance and biomass (average values excluding Polybius henslowii = 255 individuals and 2.06 kg/30 min tow) and spec ies richness and diversity, H' (6.85 species and H' = 1.45 per tow) di splayed a significant positive correlation with depth, and strong inte rannual fluctuations. The factors determining community organization w ere depth and, to a lesser extent, spatial structure. There was clear evidence of bathymetric zonation, differentiating between species char acteristic of the slope (D. bonnieri and Pasiphaea sivado), shelf-slop e edge (Macropipus tuberculatus, Pontophilus spinosus, Munida sarsi, S . membranacea, Processa spp.) and shelf (L. depurator, Macropodia tenu irostris, Paguridae and Chlorotocus crassicornis). The spatial zonatio n was related to changes in oceanography and sediment along the contin ental margin. Goneplax rhomboides, N. norvegicus, C. crassicornis and Alpheus glaber are benthic species which generally exhibit burrowing b ehaviour, and they were found mainly in the southern area where there are fine sediments due to the outwelling from the Rias Baixas. Differe nt benthopelagic shrimps (Pontophilus spinosus, Plesionika heterocarpu s, Processa spp. and Pasiphaea sivado) were typical of the zone just n orth of Fisterra, characterized by a convergence of water masses bring ing about an increase in productivity due to upwelling. The benthic an omuran and brachyuran crabs Munida intermedia, M. sarsi, L. depurator and Macropipus tubercalatus were characteristic of the northwestern zo ne between Fisterra and Estaca, where the infauna reaches high biomass despite coarser sediments with a lower concentration of organic mater ial than in the southern area. Lastly, both the Paguridae and Macropod ia tenuirostris were species typically found in the waters in the nort hern shelf. Based on interannual changes in assemblage structure, two periods could be distinguished: between 1980 and 1984, when Polybius h enslowii, D. bonnieri and Pasiphaea sivado had abundance peaks; and an other period from 1985 to 1987 when L. depurator, Munida intermedia, M . sarsi and Macropipus tuberculatus increased in abundance.