Importance of phenotypic plastic traits on invasion success: response of Xenostrobus securis to the predatory dogwhelk Nucella lapillus.
The ability of the invasive mussel Xenostrobus securis to activate defence mechanisms in response to the novel predatory dogwhelk Nucella lapillus was explored using field- and laboratory-based approaches. The importance of the origin of mussels was investigated in relation to different environmental conditions and levels of predation pressure (high and low). In the field, the responses of mussels were clearly asymmetrical: only individuals caged with dogwhelks at the site of high predation risk underwent phenotypical changes (stronger attachment, thicker shells and heavier adductor muscle). In contrast, shell growth was faster in mussels held in cages without dogwhelks at the high predation-risk site, suggesting trade-off patterns between growth and other energy-demanding actions. Nevertheless, X. securis activated inducible morphological defences without any detrimental effect on soft tissue growth (i.e. condition index). In the laboratory, the role of temperature on phenotypic responses of mussels exposed to dogwhelk was also evaluated. Mussels originally from the site of low predation risk showed a weaker response to the predator N. lapillus, probably because of difficulties in correctly identifying predator cues. At higher temperatures, mussels secreted stronger byssal threads regardless of their origin, while condition was poorer, shells thinner and gametogenesis activated more rapidly, particularly in the presence of dogwhelks. In summary, X. securis appears to be highly capable of activating protective mechanisms in marine environments within its geographical range of expansion through improved fitness.