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Prey selection by predators is of interest to community ecologists. By choosing some prey over others, predators affect prey population dynamics and the strength of food webs. According to Optimal Diet Theory (ODT), as prey density increases predators will select more profitable prey. Thereby, prey population dynamics can also affect predator behaviour. Prey profitability involves the prey energetic value, energetic costs associated to predation (time spent searching, capturing and handling prey) and certain characteristics such as body size, abundance and behaviour. We examined the effect of the blue mussel (Mytilus edulis) association with conspecifics at different densities on prey selection and behaviour by the seastar Asterias rubens. Contrary to ODT prediction, when mussels were tightly clumped in high densities, the size range of the mussels consumed by A. rubens did not differ of the size range of the mussel population. Mussel association at high density caused seastars to feed locally, reducing their exposition to risks. Moreover, preying without selecting for a specific mussel size did not result in a decrease of net profit, rather it was increased. We concluded that under certain circumstances, not selecting prey results in an increase of net profit at prey size. We discuss our results in the context of the mussel industry where the effect of prey selection on the prey population dynamics is of importance. Mussel size selection by seastars, as described in previous works, will impact the size distribution of mussels within cultured populations affecting growth of the population and reproductive output among others, thus increasing the impact of seastar predation beyond that of just removing mussels from the population. However, this study shows that seastars prey upon availability at high densities, typically encountered in cultivated populations, and therefore their effect on mussel population is restricted to removal of mussels without affecting the size distribution of the population. • According to Optimal Diet Theory (ODT), as prey density increase predators will select more profitable prey. • Contrary to ODT, when mussels are clumped at high densities A. rubens fed on mussels according to availability. • Size selection is a trade-off between energy yield and predation energy costs, which is affected by prey behaviour. [ABSTRACT FROM AUTHOR]

Environmental stress plays an important role in determining ecosystem functioning and structure. In estuarine areas both tidal and seasonal salinity changes may cause osmotic stress on predators, affecting their behaviour and survival. The interaction between these predators and their prey may affect performance, thus influencing predator impact on prey populations. The common starfish, Asterias rubens , inhabits estuarine areas, such as the Dutch Wadden Sea, that exhibit large seasonal variation in salinity (10–32 PSU). In those areas A. rubens exerts top down control on its prey, thus representing an important shellfish predator. This predation may impact on cultured and natural shellfish populations. However, the effects of osmotic stress on A. rubens performance may influence its effect on prey. Although the effect of salinity in A. rubens survival has been extensively studied, the impact on its predation behaviour and acclimation capacity remains unclear. In this study, we analyse the performance of A. rubens preying on mussels ( Mytilus edulis ) after a salinity decrease and monitor its acclimation capacity over a period of 22 days. Our experiments demonstrated that salinity affected performance by reducing feeding activity and altering size prey selection. Moreover, as acclimation occurred, A. rubens predation performance improved in all sub-lethal treatments. We conclude that osmotic stress caused by decreasing salinity potentially influences A. rubens distribution, abundance, and potential impact on prey populations. However the magnitude of the change in salinity (from 31 to a minimum of 10 PSU) and its timescale (3 weeks) mediate this effect. [ABSTRACT FROM AUTHOR]

Abstract: It is not known whether and by what factors spatial heterogeneity in mussels (Mytilus edulis L.) affects mussel production in human-created mussel beds. In a field experiment, the same number of mussels was relayed on four different areas within plots of the same size, resulting in four treatments with different mussel densities. Density, individual weight and spatial structure of mussels were followed per treatment. The uniformly placed mussels on different areas redistributed into new patches, but mussels did not spread out over a larger area. Initial mussel density affected redistribution and mussel survival. At high densities mussels redistributed into a uniform matrix or in a few larger patches, that showed larger losses than at low densities, where mussels redistributed into a high number of patches. Growth rate and condition index of the mussels did not differ between treatments and no relation was found between treatment and number of foraging shore crabs, which was the major predator of mussels in this experiment. We hypothesise that the relation between initial mussel density and mussel loss after relaying is associated with redistribution, with less competition for space when mussels are positioned at the edge of a mussel patch. The very high mussel losses that we observed in the experiment within four weeks after relaying were the major factor in biomass development. Mussel bed formation concerns mussel growers and managers involved in natural mussel bed restoration. Initial mussel survival determines the success of these activities. The present study shows the effects of mussel relaying on spatial redistribution for the first time under field conditions, and underlines the importance of edge effects in understanding mussel loss in redistribution. Mussel survival after relaying will be higher when the mussels are distributed homogeneously and in relatively low density. [Copyright &y& Elsevier]