Aggregates of the Polychaeta Phyllochaetopterus socialis were recorded for the first time off Gemlik Bay in the Sea of Marmara, in August 2008. These were found at about 53 m depth, on the soft sediments around the sub-sea part of the North Anatoly Earthquake Fault. Samples were landed by beam trawl. Total wet weight of the mats was 58 kg. Forty-three species belonging to 10 phyla associated with P. socialis were recorded from the whole landing materials. Echinodermata and Gastropoda were the most abundant (12 and 8 species, respectively). Shannon–Wiener diversity and Margalef species richness indices were applied to analyse the structure of benthic communities. Presence and abundance lognormal distribution of the species and communities in mat habitat are shown. The hydrographic environment of the water mass is also discussed.

The assemblage and the temporal changes of benthic macrofauna inhabiting the Phyllochaetopterus socialis (Polychaeta: Chaetopteridae) aggregates were analysed during 2004 and May 2005 in the Mar del Plata harbour, Buenos Aires (Argentina). Species composition, abundance, richness and diversity were estimated from monthly or bimonthly samples obtained by SCUBA diving. Multivariate (MDS) and SIMPER analyses were applied to analyse the structure of benthic communities. In the mats, a total of 57 species (invertebrates and chordates) belonging to 11 phyla were associated to P. socialis: crustaceans and molluscs were the most abundant species and crustaceans and polychaetes were the most diverse groups (14 and 11 species richness, respectively). The mean abundance ranged from 79.67±57.47 ind/250 ml to 718.75 ind/250 ml (mean±SD). The amphipod Monocorophium insidiosum, the gastropod Anachis isabellei and the tanaidacean Leptognathia sp. were the most representative species in the mats. Richness and diversity in P. socialis aggregates were higher than in surrounding sediments. Results show significant differences among months in composition, abundance and feeding guilds of the macrofauna associated with the mats. The assemblage of benthic macrofauna with permanent and temporary organisms in P. socialis aggregates suggests the existence of microhabitats in the mats that provide substrate, refuges for predation and availability of food increasing richness and diversity. These results allow considering P. socialis as an autogenic ecosystem engineer in the Mar del Plata harbour.