The rocky intertidal zone around the city of Mar del Plata (SW Atlantic, 38° S–57° W) is characterized by dense mussel beds of Brachidontes rodriguezii. This intertidal community develops on natural and artificial hard substrates, including abrasion platforms in sewage‐impacted areas. A monitoring program, designed to assess the effect of sewage discharge on intertidal macrobenthic communities, has been conducted since 1997. During the spring season of 2008, a new spionid polychaete (Boccardia proboscidea) was found near the sewage outfall, forming large biogenic reefs. This is the first report of biogenic reefs being built by a non reef‐forming spionid polychaete in areas organically impacted by sewage discharges. The aim of this work was to evaluate the spatial–temporal dynamics (% cover and density of B. proboscidea) of these reefs. These biogenic reefs covered almost the entire impacted site, reaching a density of 650,000 ind·m−2. This phenomenon is unique in that there is no other record available worldwide of any other biogenic polychaete reefs that could be sewage‐induced. The presence and stability of these biogenic reefs is discussed in relation to increased organic contamination as a structuring factor.

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The intertidal zone around Mar del Plata’s sewage discharge (38° S, 57° W) is characterized by the presence of the non‐indigenous spionid polychaete Boccardia proboscidea. This species has been classified as tolerant to moderate and high levels of organic contamination. During early stages of colonization this species can reach very high densities without suffering from interspecific competition, building biogenic structures such as reef. The aim of this work was evaluate the recovery time of the reef to a small‐scale experimental physical disturbance. Five independent rocks with B. proboscidea reefs on them were selected randomly and a small‐scale disturbance was generated by corers (16 cm2). At the beginning of the experiment, six corers were collected in each reef to produce the disturbance. The original density of B. proboscidea in each reef was determined in these corers. The reefs with disturbed corer were sampled on successive days to assess the reef recovery time. The number of larvae, juveniles and adults was quantified. Polychaete reefs had very high densities before the disturbance (mean density: 1,021,250 ind m−2). Boccardia proboscidea reefs had a fast recovery rate after small‐scale disturbance. Four days after disturbance the density reached about 50% of the original density and after 5 days the disturbed reefs could not be differentiated from the undisturbed reef. The initial recolonization of disturbed patches occurs as a result of migration which appears to be driven by larvae and juveniles. These reefs can not be seen as biodiversity hotspots and the presence of the species demonstrates great environmental deterioration.

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Boccardia proboscidea is an alien polychaete species that has been introduced to the Southwestern Atlantic Ocean. This polychaete species has invaded intertidal benthic rocky shores, and builds reefs formed by thousands of individuals per square meter with densities up to 1,500,000 ind.Ám À2 in organically impacted sites. This phenomenon was first recorded in Argentina in 2008. This is the first comprehensive study on the morphology, habitat and reproductive biology of B. proboscidea associated with the sewage-induced abrasion platforms on the Argentinean coasts. Adult morphology of B. proboscidea from Mar del Plata is described and a comparison with material from other locations is made. Results showed that B. proboscidea from Mar del Plata, Argentina share the same diagnostic features as those of the individuals from the original description (California). Moreover, the results also indicated that the reproduction of this species involves a poecilogonous development (production of more than one type of offspring with both kinds of larvae) and implicates adelphophagy (nurse egg ingestion). Patterns of larval development varied both within a single brood and among broods of different females. Continuous reproduction of B. proboscidea coupled with a continuous supply of organic matter could explain the possible mechanism that led to the development of reefs.

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