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Abstract. The community structure of the macrobenthic (> 1 ram) fauna of an intertidal soft-bottom estuary, the Ria de Foz (Galicia, Northwest Spain) was investigated on 99 occasions during June, September and December 1984 and March 1985. The infaunal data were subjected to classification and ordination techniques. The analyses indicated that sediment characteristics (grain size, percentage silt-clay and organic-matter content) and tidal height are the most important factors governing the distribution and abundance of the intertidal communities. Three faunal assemblages were identified and examined in terms of species dominance, constancy and fidelity. In the clean sands of the outer estuary two main assemblages were distinguished; one, corresponding to the "boreal Lusitanian Tellina community", inhabited the middle and lower tidal levels; the other, typical of a biocoenosis of sands and slightly muddy mediolittoral sands, inhabited the upper tidal level. The third benthic assemblage occupied the muddy sands and mud of the inner estuary and displayed some features in common with the Cardium edule-Scrobicularia community described by Thorson.

Five species of Prionospio Malmgren, 1867, each with four pairs of branchiae, are studied from coast of the Iberian Peninsula. Two of these species, Prionospiocristaventralissp. n. and P.paraparisp. n., are new to science, whereas P.caspersi Laubier, 1962, P.fallax Söderström, 1920, and P.ehlersi Fauvel, 1928 have been previously recorded. Prionospiocristaventralissp. n. is characterized by having ventral crests present on chaetigers XI–XIX, dorsal crests and low crests on chaetigers X–XXXIV, triangular neuropodial postchaetal lamellae with pointed ventral edges on chaetiger II, oval neuropodial lamellae on chaetiger III, digitiform pinnules on the posterior face of the first and fourth pairs, and branchial pairs II and III are triangular. Prionospioparaparisp. n. is characterized by having rounded neuropodial postchaetal lamellae on chaetiger I, digitiform pinnules on the posterior face of the first and fourth pairs, branchial pairs II and III are cirriform, low dorsal crests on chaetigers VIII–IX, and oval neuropodial lamellae with enlarged dorsal edges on chaetiger III. A key is given to all Prionospio species with four pairs of branchiae known from the Iberian Peninsula coastline.

Re-assessment of spionid specimens from Iberian Peninsula initially assigned to Dispio uncinata Hartman, 1951 and Streblospio benedicti Webster, 1879 led to the recognition of two new species from the spionid genera Dispio and Streblospio from coasts around the Iberian Peninsula. Dispio elegans sp. nov. is characterised by having an oblanceolate-shaped prostomium. In addition, the first two notopodial postchaetal lamellae are serrated with digitiform papillae, the anterior neuropodial lamellae are smooth; all branchiae are almost completely fused to the notopodial lamellae, but with the tips free; the notochaetae on chaetiger 1 are smooth, alimbate capillaries; the ventral chaetae located in the position of the sabre chaetae on chaetigers 1-2 are smooth, alimbate capillaries, but becoming granulated on chaetiger 3; and the middle and posterior chaetigers are granulated and reticulated. Streblospio padventralis sp. nov. is characterized by lacking dorsal papillae between the branchiae of chaetiger 1; the hooks have 4-5 pairs of small teeth; sabre chaetae are present from chaetiger 3; the pygidium has two ventral lappets; and brooding structures are present in the coelomic cavities. We suggest raising the status of Streblospio benedicti japonica Imajima, 1990 to full species level as S. japonica Imajima, 1990. A key for Streblospio species is provided.