Tracking Cell Movement in Two-Dimensional, Fragmented Microcosms Reveals Dispersal Syndromes and Strategies in...

A major challenge in dispersal ecology consists of testing whether distinct sets of phenotypic traits are associated with the three main phases of dispersal, requiring direct observations of disperser movements during emigration, transience, and immigration. Although freshwater ciliates have been used as a model in artificial dispersal landscapes for over 15 years, most studies would relate dispersal propensity to phenotypic traits measured at the end of dispersal assays. Using 'two-dimensional' fragmented microcosms, abundance, movement and morphology data of Tetrahymena thermophila were collected at numerous time points throughout 6.5 h-long dispersal assays. Data were compared across distinct zones ('Start' and 'Target' patches, connected by a 'Corridor') to identify shifts in the mean value and distribution of dispersal-related traits. Inference on the existence of dispersal decisions was obtained by comparing these results to similar outputs generated by a 'null' movement model (without decision rules). Five genotypes were used, among which two strategies were identified: swimming speed and linearity either increased ('hump') or decreased ('slope') during transience, while both traits generally decreased at immigration. Doubling the length of corridors (10 mm vs. 20 mm) modified dispersal timing, but did not affect emigration rates. Simulated data predicted a shift towards increased velocity at immigration; however, the opposite was found in most strains, suggesting a plastic inducement of typical foraging movements after settling in the 'Target' patch. Since a 'snapshot' approach was used (capturing sparse movement sequences throughout the dispersal process instead of prolonged tracking), phenotypic plasticity could not be confirmed with certainty; however, the hypothesis of strict spatial sorting was insufficient to explain movement patterns. Overall, our results hint at the plastic and reversible nature of dispersal syndromes displayed by T. thermophila across fragmented landscapes, which bears significance in the context of habitat loss and the maintenance of metapopulation stability.