Quantitative studies dealing with the ecology of the Pycnogonida (Arthropoda) are scarce. In this work, the pycnogonid fauna present in the seagrass meadows (Zostera spp.) of an estuarine environment in NW Spain are studied, including spatial distribution and intra-annual seasonal variations of density and diversity. Quantitative sampling showed that the pycnogonid taxocoenosis associated with seagrass meadows was rich and diverse. The best represented species in terms of distribution and abundance were Callipallene tiberi, Achelia echinata and Anoplodactylus pygmaeus. Abundances showed a strongly seasonal pattern, with the highest values in September and lowest in March. Temporal variations in the population dynamics of A. echinata suggest a strong coupling with the life story of the seagrass Zostera marina, which in turn determines the availability of food resources for the pycnogonid assemblage.