Differences in Salinity Performance Curves between Two Closely Related Poecilia Species: Implications for Species Turnover along a Salinity Gradient.

AbstractMany locomotor traits exhibit predictable variation along environmental gradients, which in turn influence the lower-level cellular and physiological processes that determine whole-organism performance (i.e., performance curves). While the impact of salinity on locomotor performance has been previously studied, few have investigated the ecological consequences of salinity-induced changes in performance. On the island of Trinidad, Poecilia reticulata are confined to freshwater, while the closely related Poecilia picta primarily inhabit lowland brackish water and saltwater. However, both species coexist in a narrow freshwater band at the brackish water interface. Because both species are prey for larger fish and their escape performance is highly correlated with survival, we hypothesized that the different distributional limits could be influenced by the ability to maintain escape performance across a range of salinities. To test this, we assessed how increasing salinity affects escape performance by generating salinity performance curves for these euryhaline species. We subjected both species to a predatory stimulus to elicit an escape response using serial acclimation to increasing levels of salinity (0-18 practical salinity units). We measured (1) latency of response initiation, (2) distance traveled, (3) velocity, and (4) acceleration. We found that the transition from freshwater to brackish water (6 practical salinity units) caused a decline in escape performance for both species. However, P. picta acclimated more rapidly and recovered performance at higher salinities. These results suggest that P. reticulata's slower acclimation to brackish conditions may contribute to its confinement to freshwater. Moreover, this study emphasizes the value of salinity performance curves for understanding how aquatic organisms may respond to changes in salinity gradients.