Topical Area: Seagrass Ecology

Jenny L. Cutler and David N. Kuhn, Department of Biological Sciences, Florida International University, University Park, Miami, FL; Daniel Childers, Department of Biological Sciences/Southeast Environmental Research Program, Florida International University, University Park, Miami, FL

Population structure within seagrass meadows is a topic that has historically been poorly understood. The clonal growth form of the plants complicates the problem by making it impossible to discern genetically distinct individuals or clones by sight. Because Thalassia testudinum Banks ex König is the dominant seagrass species within the Florida Bay estuary, an understanding of the population structure within this species may be important to understanding ecosystem level fluctuations within the Florida Bay system. It has been speculated that levels of genetic variation within this species would be extremely low due to its prolific clonal growth through rhizome extension and branching. Additionally, sexual reproduction within this species has been characterized as sporadic and the importance of sexual reproduction to community establishment and maintenance has not been demonstrated.

The purpose of this study was to search for clonal diversity within the large Thalassia testudinum meadow of Rabbit Key Basin. We sampled the T. testudinum populations of Rabbit Key basin with a hierarchical probability based sampling design using hexagonal tessellation. This method allowed us to quantify variation in T. testudinum over three different scales of spatial separation; 1) plants that were separated by less than 0.25 m; 2) plants separated by 1-16m; and 3) plants that were separated by 1 to several kilometers. We utilized a microsatellite based multilocus DNA fingerprinting technique known as inter-SSR to search for differences among plants. This technique was found to be both repeatable and informative in this species.

Our results indicate that the Thalassia testudinum meadows within Rabbit Key Basin are not monoclonal. Genetic variability was detected at all three spatial scales. The greatest degree of variation occurred at the largest spatial scale. This result suggested that clones were not so large that they stretched across the entire basin. Significant variability was also detected between plants that were separated by less than 0.25m. This, it appears that individuals of this species do not grow in monoclonal patches. In an analysis of genetic distance vs. geographic distance, we found that plants that were close to each other in space were more closely related than those that were separated by great distances. Thus, the T. testudinum meadows within Rabbit Key Basin do not appear to represent one, large, panmictic population.