The spionid polychaete Pygospio elegans Claparède 1863 is a small tube-building opportunist, commonly found in the benthic communities of organically enriched boreal estuaries. The species may reproduce both sexually and asexually; larvae may develop either planktonically or benthonically. Population densities of up to 100,000m⁻² have been recorded. In the macrotidal Somme Bay, N. France, the Pygospio population regularly exceeds densities of 100,000m⁻²; an abundance of nearly 600,000m⁻² was noted in December 1993. Raised sedimentary structures are formed in the presence of dense arrays of Pygospio-built tubes: so-called "tube-beds". A multidisciplinary study was made of these structures and the Pygospio population that formed them. An extended period of sexual activity was discovered in the Somme Bay Pygospio population; there was no asexual activity. All larvae developed planktonically, hatching at an early stage in development. This life-history provides a massive larval availability for tube-bed foundation and an optimal spatio-temporal vantage for the re-colonisation of defaunated areas in the strongly hydrodynamic Somme Bay. Laboratory experiments suggested that this reproductive mode in the Somme Bay population was fixed. A similarly non-poecilogonous response was observed in individuals taken from Ryde Sand (Isle of Wight), which exhibited extended larval brooding. Although no morphological variation was noted among Pygospio specimens from populations from Europe and the USA, this apparent reproductive inflexibility within Pygospio elegans populations suggested the possibility of cryptic, sibling speciation. Sampling revealed that the approximate threshold tube-bed forming Pygospio density in the Somme Bay was 50,000 m⁻². Pygospio had a significant effect on sediment physico-chemical characteristics and dense aggregations of the spionid stimulated the proliferation of the microbial and meiofaunal communities. Pygospio density affected the structure of the associated macrofaunal community: diversity peaked between Pygospio densities of 50,000m⁻² and 200,000m⁻² as tube-bed formation offered an attractive resource and refuge from erosion. Beyond Pygospio densities of 200,000m⁻², the community became more dominated by species able to tolerate the spionid's spatial monopoly.