Seagrasses are distributed in coastal areas of all continents and belong to nine species assemblages (floras), six of which are exclusively temperate (Duarte, 2001). Distributional ranges of single species can differ radically; ranging from species whose distribution is limited to a single flora (e.g. Posidonia oceanica in the Mediterranean) to widely distributed species such as Zostera marina. The present distribution of seagrass species is the result of processes occurring at a number of different temporal scales and may reflect events that occurred a very long time ago (tens of thousands of years or more). However, the actual distribution of populations is also the result of ongoing extant dynamic processes of colonization/extinction. When population distributions are viewed using a genetic perspective, higher genetic differentiation among populations reflects lower genetic exchange (gene flow) and indicates longer-term isolation. The application of molecular markers to study levels of population connectivity potentially can provide significant insights into the factors influencing present-day distribution of sea-grass species, particularly when applied across broad spatial scales.
Almost half of the roughly 65 papers published on seagrass population genetic studies attempt to quantify gene flow among distinct populations (Table 2). To date studies addressing the broad scale distribution genetic diversity has only been documented for a few species (Table 2). However, there are many studies in progress on different species with new and potentially more polymorphic molecular markers. When viewed across all the broader scale studies, levels of population connectivity are not always related to differences in dispersal mode or reproductive characteristics among species, and do not appear to be linked to their phylogenetic relatedness.
Only few species have been studied in detail: within the genus Posidonia, only the Australian endemic P. australis and the Mediterranean endemic
P. oceanica; within the genus Zostera, only Zostera marina from Europe and North America and in the genus Thalassia, only the Caribbean species, Tha-lassia testudinum (these species are the subject of individual chapters in this volume, see Chapters 16, 17 and 18).
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