Linking Morphological and Genetic Differentiation Among Populations in Seagrass Species With Contrasting Dispersal Strategies

ABSTRACT

Long‐distance dispersal sustains genetic diversity across populations and facilitates recovery from disturbance, and dispersal also may shape how ecologically important morphological traits shift in response to environmental change. We examined links among morphology, genetic differentiation, and dispersal through surveys of traits and genetics for Thalassia testudinum and Halodule wrightii distributed across a naturally occurring gradient in concentrations of total phosphorus in the water column along Florida's Gulf Coast (USA). We expected the contrasting dispersal strategies of the two seagrasses would drive patterns in genetic differentiation: T. testudinum produces floating fruits that can disperse over longer distances than the negatively buoyant seeds of H. wrightii . Genetic differentiation among populations was higher for H. wrightii ( G' _ST  = 0.02–0.15) than for T. testudinum ( G' _ST  = 0.00–0.07), supporting the expectation that limited dispersal leads to more genetic structure. Morphology for both species varied along the phosphorus gradient, where leaves were generally shorter in the estuarine system with the lowest phosphorus concentrations, with more pronounced differences in leaf length observed for T. testudinum . The role of the environment in driving leaf length was tested in a common garden experiment with plants of both species collected from three estuarine systems along the phosphorus gradient. Initially, leaf lengths of both species differed across plants originating from different systems. Leaf lengths became more similar throughout the experiment, with leaves of H. wrightii from the northernmost system beginning to lengthen after 11 months. Together, our results suggest that responses to environmental change can differ across seagrass species, where the species capable of long‐distance dispersal exhibited greater variability in a morphological trait along an environmental gradient despite less genetic differentiation than the species with more limited dispersal.