Megabenthic decapod crustacean assemblages on the Galician continental shelf and upper slope (north-west Spain)

Abstract

The structure of megabenthic decapod crus- tacean assemblages on the Galician (north-west Spain) continental shelf (100 to 200 depth) and upper slope (200 to 500 m) was analyzed based on surveys carried out in autumn and spring, from 1980 to 1987. Forty species belonging to 19 families were caught. The portunid crab Polybius henslowii, a species with pelagic phases, was the most abundant species, but displayed strong spatial and temporal fluctuations. Other dominant species were the Norway lobster Nephrops norvegicus, the portunid Lio- carcinus depurator, the galatheids Munida intermedia and M. sarsi, and the shrimps Solenocera membranacea, Plesionika heterocarpus, Pasiphaea sivado and Dichelo- pandalus bonnieri. Total abundance and biomass (aver- age values excluding Polybius henslowii = 255 in- dividuals and 2.06 kg/30 min tow) and species richness and diversity, H' (6.85 species and H' = 1.45 per tow) displayed a significant positive correlation with depth, and strong interannual fluctuations. The factors de- termining community organization were depth and, to a lesser extent, spatial structure. There was clear evidence of bathymetric zonation, dierentiating between species characteristic of the slope (D. bonnieri and Pasiphaea sivado), shelf-slope edge (Macropipus tuberculatus, Pon- tophilus spinosus, Munida sarsi, S. membranacea, Pro- cessa spp.) and shelf (L. depurator, Macropodia tenuir- ostris, Paguridae and Chlorotocus crassicornis). The spatial zonation was related to changes in oceanography and sediment along the continental margin. Goneplax rhomboides, N. norvegicus, C. crassicornis and Alpheus glaber are benthic species which generally exhibit bur- rowing behaviour, and they were found mainly in the southern area where there are fine sediments due to the outwelling from the Rias Baixas. Dierent benthopelagic shrimps (Pontophilus spinosus, Plesionika heterocarpus, Processa spp. and Pasiphaea sivado) were typical of the zone just north of Fisterra, characterized by a con- vergence of water masses bringing about an increase in productivity due to upwelling. The benthic anomuran and brachyuran crabs Munida intermedia, M. sarsi, L. depurator and Macropipus tuberculatus were char- acteristic of the northwestern zone between Fisterra and Estaca, where the infauna reaches high biomass despite coarser sediments with a lower concentration of organic material than in the southern area. Lastly, both the Paguridae and Macropodia tenuirostris were species ty- pically found in the waters in the northern shelf. Based on interannual changes in assemblage structure, two periods could be distinguished: between 1980 and 1984, when Polybius henslowii, D. bonnieri and Pasiphaea si- vado had abundance peaks; and another period from 1985 to 1987 when L. depurator, Munida intermedia, M. sarsi and Macropipus tuberculatus increased in abundance.