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Aim We evaluated whether patterns of species diversity (α, β and γ) of rocky shore assemblages followed latitudinal gradients (i.e. LDGs) along the South American coasts, and tested hypotheses related to potential processes sustaining or disrupting the expected LDG pattern at various spatial scales. Location Coasts of South America. Taxon Macroalgae and sessile/slow‐moving macrofauna on intertidal rocky shores. Methods We evaluated changes in species composition across 143 sites. The degree of replacement and loss of species at different spatial scales (i.e. coasts, regions and sites) were estimated to help distinguish among ecological, historical and evolutionary hypotheses for explaining LDGs. Furthermore, components of diversity and taxonomic distinctness were measured, and variability in these measures was decomposed using analysis of covariance. Finally, we examined relationships between diversity and a suite of environmental and anthropogenic variables to identify potential mechanisms that may be responsible for the reported spatial relationships. Results Species composition varied with latitude, and this variability was relatively consistent on both coasts. At all spatial scales, replacement of species was the dominant phenomenon (>95%), rather than loss in the total number of species (<5%). LDGs were strongly dependent on the diversity component and the spatial scale: generally, positive for regional β‐diversity, negative for α‐diversity and site β‐diversity. Sea surface temperature (SST) was the variable that best explained patterns of diversity along both coasts (14%–22%), but other regional and local environmental variables associated with river discharges, upwelling, confluence of currents, tides and anthropogenic pressures also accounted for an important portion of variation (5%–14% each). Main conclusions Species diversity of South American rocky shores followed, with interruptions, LDGs. The trend of those LDGs, however, depended on the scale and metric used to describe diversity. It is proposed that patterns of LDGs at various scales are not the result of a single overarching process but are strongly influenced by local and regional processes. Although the most evident environmental gradient was the decrease in SST towards the south, it was demonstrated that regional and local environmental variables were also important for understanding the increase in regional β‐diversity towards the tropics.

For many mussel species, the model of planktonic development followed by metamorphosis and settlement in the benthic habitat is complicated by the existence of planktonic post-metamorphic stages and/or pediveliger benthic stages that can relocate after initial settlement. This has led to the long-standing hypothesis of 'primary' settlement from the plankton onto intertidal algal substrate followed by 'secondary' relocation to mussel beds. Here, we investigate settlement of the intertidal mussels Perumytilus purpuratus and Semimytilus algosus in central Chile to test this hypothesis and explore physical drivers. Our results indicate that: (1) these species do not have planktonic post-metamorphic stages, (2) larvae typically arrive to the intertidal zone at a size 80-150 µm larger than the largest planktonic larva, which based on growth rates, corresponds to a 3-20 d delay, (3) there are no differences in pediveliger sizes between different algal substrates, mussel beds, or artificial collectors, and (4) there is no evidence that larvae metamorphose in the intertidal and grow in alternative habitat before relocation to mussel beds. In 2 summers, daily settlement of both species was tightly and positively associated with wave height, despite large inter-annual variability in wind conditions. Our results reject the primary-secondary settlement hypothesis and support a new settlement model in which, after metamorphosis beyond the surf zone, the negatively buoyant settlers become semi-benthic and readily sink to the bottom. There, they can be transported onshore through the surf-zone by wave-driven near-bed transport. The process of tumbling under the surf may take from a few hours to several days, with larvae arriving at the shoreline in a wide range of sizes at any given time. For some larvae, relocation continues in the intertidal zone for months.

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