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Exposed marine beaches are physically rigorous habitats in which macrofauna community patterns have been well correlated with physical factors such as grain size, beach slope and wave/swash processes. In this context, the swash exclusion hypothesis (SEH) gained wide acceptability in explaining the control of species abundance and diversity of the sandy beach macrofauna by swash climate, determined by wave height and beach face slope. This hypothesis predicts a consistent increase in species richness, abundance and biomass from reflective to dissipative conditions. However, predictions of the SEH have not been adequately tested at a population level. Here, demographic and life history characteristics of the intertidal mole crab Emerita brasiliensis were compared between populations of 2 microtidal exposed sandy beaches with contrasting morphodynamics during 22 consecutive months. The major prediction of the paradigm does not hold for the mole crab: total abundance, as well as that of males, females, males with spermatophores and megalops did not differ between beaches. Moreover, the reflective beach population presented higher male growth rates in size and lower natural mortality. Other population processes and life history traits gave support for the SEH: the dissipative beach population presented (1) higher abundance of ovigerous females, female growth rates, fecundity, egg production potential, size at maturity, as well as weight at size; and (2) lower female mortality and burrowing rates (for both sexes). The SEH seems to be particularly useful in explaining female somatic growth and reproductive features, which could be related to a major amount of food availability in dissipative beaches. However, other competing, but not exclusive, hypotheses are needed to explain variations in population abundance and other processes such as male growth and survivorship. The empirical evidence from several recent studies on the Uruguayan coast showed that sandy beach populations that co-occur in contrasting environments are less sensitive to variations in beach morphodynamics, which should not be considered the primary factor affecting the abundance and some life history traits. The importance of alternative population regulation processes and mechanisms are stressed.

Context. The ESA Rosetta spacecraft will reach the short-period comet 67P/Churyumov-Gerasimenko in 2014. Orbiting strategy, orbiter safety conditions, landing scenarios and expected results from dust collectors depend on models of the 67P dust environment. Many papers already tackled this matter, analysing a limited set of observations, and therefore often reaching conflicting conclusions. Aims. We consider a set of observations representative of all ground-based and IR (thermal infrared) Spitzer data collected over the last three perihelion passages, to determine the 67P dust environment after the end of the gas drag on dust (at about 20 nucleus radii) consistent with available 67P gas and dust coma photometry, images of the dust coma, tail and trail, at optical and IR wavelengths. Methods. In order to obtain the best fit to 67P data, we consider three independent tail and trail simulation codes (developed by three independent groups), which parametrise cometary dust by the quantity β, the ratio between solar radiation pressure and gravity forces. GIADA, the dust monitor instrument of the Rosetta orbiter, will provide an experimental determination of the β-dust mass relation. Results. A 67P environment model based on a perihelion-symmetric dust velocity and on a perihelion-asymmetric dust size distribution, is consistent with all available data. During most Rosetta operations, the dust cross-section is dominated by mm to cm-sized grains, while the ejected dust mass is dominated by grains larger than a few mm, with a dust-to-gas ratio of 3 around perihelion. Conclusions. 67P onsets its activity at Sun-distances r h ≥ 3.4 AU; the dust geometric albedo is 0.04 ± 0.02; at 3.0 AU, 10 g grains escape the nucleus gravity field (10 kg grains at perihelion) with a dust mass-loss rate of 10−40 kg s −1 (500 kg s −1 at perihelion); 67P's activity depends on seasons, with the northern heminucleus (rich in large grains and CN depleted) active before perihelion.

Exposed marine beaches are physically rigorous habitats in which macrofauna cornmunity patterns are related to physical factors such as sedimentary parameters and wave/swash processes. In this context, a consistent increase in species richness, abundance and biomass from reflectwe to dissipative conditions has been widely reported, and proposed as a paradigm of sandy beach ecology. Here, we examine this hypothesis on the demography and life history characteristics of the sandhopper Pseudorchestoidea brasiliensis of Uruguay. Abundance, population structure by sex and size, individual growth, natural mortality, fecundity, female maturity and size at maturity, and the lengthweight relationship were compared between populations of 2 microtidal exposed sandy beaches that differed widely in physical characteristics (i.e. grain size, slope, penetrability and water content), during 20 consecutive months. Contrasting with the predictions of 'the sandy beach ecological paradigm', the population of P brasiliensis at the reflective beach presented (1) higher abundance both for males and females; (2) higher egg production potential and recruitment levels; (3) lower natural mortality; and (4) no major differences in individual growth and estimated Life span to those of the dissipative beach population. On the contrary, growth in weight, individual fecundity and average size at maturity were higher for the dissipative beach population. We conclude that population level responses to variation in sandy beach morphodynarnics may markedly Mfer from community level responses, and thus macroscale, worldwide community patterns could not necessarily characterise life history and dernographic variations of individual species in a similar manner. We suggest that the paradigm of the forces generating patterns in sandy beach communities has underestimated the importance of population regulation rnechanisms in these communities. KEY WORDS: Sandhopper. Amphipoda. Population demography. Life history. Sandy beaches .

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Understanding the relationships between beach morphodynamics and descriptors of macrofauna assemblages has been a major step in theoretical development in sandy beach ecology. However, the role of morphodynamics in shaping life-history traits is still uncertain. Here, we test the predictions of the habitat harshness hypothesis (HHH) on the life-history traits of the sandhopper Atlantorchestoidea brasiliensis, based on information compiled from 7 Uruguayan sandy beaches spanning a continuum from reflective to dissipative states, over a period of almost 2 yr. A. brasiliensis showed clear population responses to physical variables in a trend opposite to that predicted by the HHH, including an increase in abundance (total, ovigerous females and juveniles) and individual sizes from dissipative to reflective beaches. A generalized additive model explained 46.1% of the deviance in sandhopper abundance and retained all 4 physical descriptors in the model as significant. Sandhopper abundance increased with grain size and beach face slope, and decreased with low values of water content and compaction of the sand. Per capita rate of recruitment increased linearly with adult abundance, i.e. there was a positive density-dependent effect of adults on recruitment rates, which exponentially decreased from reflective to dissipative beaches. Only the proportion of ovigerous females increased towards the dissipative domain. We conclude that the HHH did not accurately predict spatio-temporal fluctuations in population features of A. brasiliensis, particularly because of a limited appreciation of the role played by life-history strategies in explaining population responses to the environment. Our results have important implications for sandy beach macrofauna, particularly supralittoral organisms, and suggest a novel hypothesis (termed here the 'hypothesis of habitat safety'), which states that the combination of narrow swashes and steep slopes makes reflective beaches a more stable and safer environment for supralittoral species.

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