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The density, size, and morphology of coastal invertebrate species often predictably vary with latitude. In this study we evaluate whether the density, size, and morphology of the pulmonate limpet, Siphonaria lessonii, in the Southwestern Atlantic varies in agreement to the predictions of the abundance-centre hypothesis (i.e., higher densities by the center of the species range), Bergmann´s rule (i.e., increasing body size with latitude), and Allen´s rule (i.e., decreasing body surface to volume ratio with latitude). Our results indicate that the upper density bounds of S. lessonii decrease with latitude across our study range, which may either contradict expectations of the abundant-centre model (i.e, if assuming a Temperate Southwestern Atlantic/Magellanic distribution; 27°S in the Atlantic to 42°S in the Pacific) or represent a partial trend within a broader scale, abundant-centre pattern (i.e., when considering a panmictic distribution from the Temperate Southwestern Atlantic to the Temperate Southeastern Pacific; 27°S in the Atlantic to 5°S in the Pacific). Limpet shell length (i.e., a surrogate of body size) was not significantly related to latitude (cf. Bergmann´s rule). The two principal components of morphological variation in limpet shells – which increased with decreasing shell surface to volume ratio – showed positive and negative relationships with latitude (i.e., contradictory support to Allen´s rule). From these analyses, we conclude (a) that the size and morphology of S. lessonii are primarily controlled by local conditions rather than broad-scale climatic or environmental gradients and, (b) that geographic marginality does not means reduced performance in this species at least as it concerns to abundance and maximum size. Fil: Soria, Sabrina Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales ; Argentina. Grupo de Investigación y Educación en Temas Ambientales, Estación Biológica Las Brusquitas; Argentina Fil: Teso, Silvia Valeria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales ; Argentina Fil: Gutierrez, Jorge Luis Ceferino. Grupo de Investigación y Educación en Temas Ambientales, Estación Biológica Las Brusquitas; Argentina. Universidad Nacional de Mar del Plata; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Arribas, Lorena Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto de Biología de Organismos Marinos; Argentina Fil: Scarabino, F,. Universidad de la República; Uruguay. Museo Nacional de Historia Natural; Uruguay Fil: Palomo, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales ; Argentina. Grupo de Investigación y Educación en Temas Ambientales, Estación Biológica Las Brusquitas; Argentina

Ecosystem engineers can modify habitat, creating structural microhabitats. This structural complexity can affect species richness. Marine ecosystem engineers are able to produce local effects in combination with environmental variables (e.g. to create more humid habitat during low tides). We tested the hypotheses that if there is a relationship between mussel morphology and environmental factors, mussels would be larger at warmer than at cooler locations, and in areas where 2 species of mussels overlap, size and biomass will decrease and density will increase. At a smaller scale, we predicted that there is a relationship between the assemblage structure and hardness of the substratum and sediment content of a mussel bed. Using a nested design, we measured density, biomass and size of 2 species of mussels, Brachidontes rodriguezii and Perumytilus (Brachidontes) purpuratus, and diversity of species associated with mussel beds at 2 rocky intertidal sites at each of 4 shores along the northern Argentinean coast. These variables were correlated with oceanographic conditions and local characteristics. Significant correlations were found between intertidal assemblages and local factors. The largest specimens of B. rodriguezii and P. purpuratus were found at the warmer shores. In areas where they overlapped, size, biomass and density of P. purpuratus were lower, although B. rodriguezii did not change. The mean abundance of invertebrates associated with a mussel bed showed significant differences among shores. These 2 species of mytilids coexist over a small area, and although these species are very similar in their biological and ecological function, the fauna associated with their matrices are very different. Fil: Arribas, Lorena Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales; Argentina Fil: Bagur Creta, Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales; Argentina Fil: Klein, E.. Universidad Simón Bolívar. Laboratorio de Sensores Remotos e Instituto de Tecnología y Ciencias Marinas; Venezuela Fil: Penchaszadeh, Pablo Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales; Argentina Fil: Palomo, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales; Argentina

Non-indigenous marine species often change the abundance and diversity of native species in coastal ecosystems. On the SW Atlantic coast, the macroalgae Ahnfeltiopsis sp. (Rhodophyta, Phyllophoraceae) and Schizymenia dubyi (Rhodophyta, Schizymeniaceae) have invaded the intertidal rocky shore of Mar del Plata, Argentina (38°S, 57°W). To study the spread and ecological associations of these invasive species, algal abundance, biomass and biodiversity of benthic assemblages at three different tidal levels were examined during five years. Sparse Ahnfeltiopsis sp. thalli (3 % cover) were detected in February 2007 at the three tidal levels. By January 2011, its cover had increased to 11 %, while its biomass showed a 27-fold increase. S. dubyi was detected at the lower intertidal level in January 2010 with a cover of 2 %. By January 2011, it had increased to 5 % and spread to the other intertidal levels. The presence of these two non-indigenous algae modified the substrate and the structure and composition of the benthic assemblage. The constant increase in the algal biomass and presence along the intertidal suggest that the effect will be greater in the future. Moreover, the effects of these exotic algae could potentially displace Brachidontes rodriguezii—an important ecosystem engineer that creates microhabitat for a large number of organisms on these shores.

Intraspecific competition for space is generally invoked as the chief process limiting crowding in sessile or highly sedentary marine invertebrates. However, the mechanisms by which high conspecific density induces individual removal or mortality, in turn restraining crowding in these organisms, generally remain uninvestigated. Here we illustrate that mussel crowding in a southwestern Atlantic rocky intertidal shore is limited by a combination of wave action and space limitation. Brachidontes rodriguezii mussel beds at this site occur primarily as a single layer of individuals because wave forces remove multilayered mussel hummocks quickly after they develop. Mussels in hummocks show lower attachment strength than those in the single-layered matrix. Accordingly, wave conditions associated with the passage of cold fronts (i.e. transition zones from warm air to cold air accompanied by moderate to strong winds and wave action, with 7 d average recurrence times based on historical weather data) cause detectable mussel dislodgment in a high proportion of hummocks but have virtually no impact on single-layered areas. Since wave action is the proximate cause of mussel dislodgment, upper limits to crowding in this species would not be fixed to a particular level of space occupation (i.e. as predictable from inter-individual interference alone) but would be variable in space and time depending on wave exposure. This example suggests a mechanism of population control where the impact of a physical factor on population size is larger at higher population density and supports early hypotheses about the occurrence of density-dependent population control by physical factors when the availability of safe sites is limiting. Fil: Gutierrez, Jorge Luis Ceferino. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Cary Institute of Ecosystem Studies; Estados Unidos Fil: Palomo, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Bagur Creta, Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Arribas, Lorena Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina Fil: Soria, Sabrina Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; Argentina