Behaviour is predicted to be a primary determinant of the success of the invasion process during the early phases of colonization. Comparing sympatric invaders and native species may provide a good approach to unravel behavioural traits involved in an invasion process. In this study, we carried out an experimental simulation of the introduction and the establishment phase into a new environment and assessed the expression of activity, alertness and habituation in a non-indigenous Mediterranean population of the South African nudibranchGodiva quadricolorcomparing its profiles with those of the sympatric nativeCratena peregrinaandCaloria quatrefagesi. Individuals of these three species were subjected to three behavioural tests: spontaneous activity, carried out in the introduction phase (immediately after sampling) and after a week of acclimatization; alert test, in which a potential threat was simulated by means of a tactile stimulus; habituation test, where the same alert test stimulus was repeated five times at thirty- minute intervals. Native nudibranch had repeatable traits, although with species differences perhaps related to their different ecological niches. The comparison with the invasive species highlighted its low repeatability in activity levels, suggesting higher plasticity, a strong tendency to locomotor activity, and a marked sensitization in the habituation test. Such traits could play an important and active role in the ongoing invasion process.

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