28 The ability of the invasive mussel Xenostrobus securis to activate defence mechanisms in response to the 29 novel predatory dogwhelk Nucella lapillus was explored using field and laboratory-based approaches. The 30 importance of the origin of mussels was investigated in relation to different environmental conditions and 31 levels of predation pressure (high and low). In the field, the responses of mussels were clearly asymmetrical, 32 as only individuals caged with dogwhelks at the site of

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... gh predation risk underwent phenotypical changes 33 (stronger attachment, thicker shells and heavier adductor muscle). By contrast, shell growth was faster in 34 mussels held in cages without dogwhelks at the high predation risk site, suggesting trade-off patterns 35 between growth and other energy-demanding actions. Nevertheless, X. securis activated inducible 36 morphological defences without any detrimental effect on soft tissue growth (i.e. condition index). 37 In the laboratory, the role of temperature on phenotypic responses of mussels exposed to dogwhelk was also 38 evaluated. Mussels originally from the site of low predation risk showed a weaker response to the predator N. 39 lapillus probably because of difficulties in correctly identifying predator cues. At higher temperatures, 40 mussels secreted stronger byssal threads regardless of their origin, while condition was poorer, shells thinner 41 and gametogenesis activated more rapidly, particularly in presence of dogwhelks. In summary, X. securis 42 appears to be highly capable of activating protective mechanisms in marine environments within its 43 geographical range of expansion through improved fitness. 44