Macrofauna associated with Phyllochaetopterus socialis aggregations in thesouthwestern Atlantic

Aggregates of Phyllochaetopterus socialis Claparede, 1868 have been found n ear the Rio de la Plata mouth (35 degrees 23' - 35 degrees 31' S; 55 degree s 36' W - 55 degrees 48' W) in November 1997. The depth of the sandy bottom was 11-12 in and the water salinity was 26-30 parts per thousand. Each agg regate (ca. 20 cm diameter) arose from shell or another small hard object. The macrofauna associated with these aggregates was studied and the results were compared with the available information on hard and soft bottom benth ic communities in this area. The more abundant taxa registered were Polycha eta, Mollusca, Bryozoa and Decapod Crustacea. Several sessile species (serp ulid polychaetes, thoracic cirripedia, bivalves, bryozoa and ascidiae) grow on the tubes. On the other hand, various polychaete and bivalve species, A nomura and Brachyura Decapod crustaceans and Ophiuroidea find refuge in the spaces between tubes. The macrofauna associated with the aggregates is mor e diverse than those found in the neighbourhood. The polychaete species bel ong to different trophic guilds; several species were recorded for the firs t time in the region and the distribution range of other species was extend ed. The importance of P. socialis aggregates in structuring marine benthic communities is discussed.