## 4 Discussion  
_Cerastoderma edule_ exhibited a burrowing vs. emergence pattern when subjected to salinity variations: (_a_) at the initial experimental salinity (= 20) individuals burrowed in the sediment, quite swiftly in some cases; (_b_) they displayed an emerging response during decreasing salinity trend; (_c_) specimens showed a burrowing response in the course of salinity increment; (_d_) when salinity variations were repeated the same behavioural responses were observed, however their magnitude was higher on the 1 st salinity variation cycle when compared to the 2 ndcycle. A clear relationship between changes in salinity and behaviour was observed, with cockles actively burrowing in the sediment column when water salinity is within the optimal performance range for this species [16] and emerging when salinity drop below a low salinity threshold (<10). Although this pattern was consistent throughout the experiment, the observed lower magnitude on the 2 nd cycle of salinity variation may indicate an altered physiological condition of specimens subjected to experimental conditions for too long [16; 29; 41; 42]. Salinity is one of the factors determining the biological features and spatial distribution of species on estuarine habitats [16; 29; 30]. In the Mondego estuary, increased mortality of the common cockle _C. edule_ and the peppery furrow shell _Scrobicularia plana_ was observed during flooding events, impacting the population structure and dynamics of these bivalves [8; 43; 44]. Low salinities also modified their behaviour (e.g., affecting burrowing/emergence behaviour, feeding activity, movement and valves opening/closure), impairing their overall activity [16]. Conversely, these species showed the best performance within a narrow optimal salinity range (20-25 for _C. edule_; 20-30 for _S. plana_) [16], and _C. edule_ population occupy preferentially downstream intertidal and subtidal sandflats [16; 39]. Figure 4: (**A**,**B**). Regression equations between salinity and proportion of buried individuals for treatments A (plot A.) and B (plot B.). _C. edule_ is better suited to higher salinity areas [7; 16] and is impaired by abrupt salinity declines [16], especially during long and intense flooding episodes [8; 24; 43]. Under these circumstances, organisms respond initially with behavioural changes, which in the event of prolonged exposure result in mortality [16]. On previous experimental studies [16] the absence of burrowing behaviour under low salinity conditions was considered a protection mechanism related to valve closure, retraction of sensitive body parts and an overall inactivity to avoid osmotic stress [16; 29; 34; 36; 45]. In an in situ personal observation during low tide, numerous cockles were found at the sediment surface in the Mondego south arm intertidal sandflat during a period of high freshwater inflow. This suggests that buried organisms actively emerged to the sediment surface during adverse conditions, putting themselves at risk. The pattern observed in this study seems to indicate a behavioural strategy to endure severe salinity declines. During riverine flooding events, increased freshwater inflow leads to reduced salinity and improved hydrodynamics, promoting sediment erosion and bedload transport of materials, with associated impacts on the populations of _C. edule_[8; 43; 45; 46]. In these conditions, cockles can be involuntarily or passively mobilized by currents or when the sediments they live in become eroded [47; 48; 49; 50]. With the observed behaviour, cockles emerge at the sediment surface when salinity drops (<10), becoming more likely to be dragged to further downstream areas, where salinities are higher (Figure 5). As such, it can be viewed as an avoidance mechanism allowing organisms to avoid impacts associated with severe salinity declines. Dispersion of adult cockles has been previously observed in a field experiment study, with individuals being dragged \"upslope\" by the tide over distances up to 200 m [51]. Figure 5: Illustration of _C.