Many marine species have widespread geographic ranges derived from their evolutionary and ecological history particularly their modes of dispersal. Seagrass (marine angiosperm) species have ranges that are unusually widespread, which is not unexpected following recent reviews of reproductive strategies demonstrating the potential for long-distance dispersal combined with longevity through clonality. An exemplar of these dual biological features is turtle grass (Thalassia testudinum) which is an ecologically important species throughout the tropical Atlantic region. Turtle grass has been documented to have long-distance dispersal via floating fruits and also extreme clonality and longevity. We hypothesize that across its range, Thalassia testudinum will have very limited regional population structure due to these characteristics and under typical models of population structure would expect to detect high levels of genetic connectivity. There are very few studies of range-wide genetic connectivity documented for seagrasses or other sessile marine species. This study presents a population genetic dataset that represents a geographic area exceeding 14,000 km2. Population genetic diversity was evaluated from 32 Thalassia testudinum populations sampled across the Caribbean and Gulf of Mexico. Genotypes were based on nine microsatellites, and haplotypes were based on chloroplast DNA se-quences. Very limited phylogeographic signal from cpDNA reduced the potential comparative analyses possible. Multiple analytical clustering approaches on population genetic data revealed two significant genetic partitions: (a) the Caribbean and (b) the Gulf of Mexico. Genetic diversity was high (HE = 0.641), and isolation by distance was significant; gene flow and migration estimates across the entire range were however modest, we suggest that the frequency of successful recruitment across the range is uncommon. Thalassia testudinum maintains genetic diversity across its entire distribution range. The genetic split may be explained by genetic drift during recolonization from refugia following relatively recent reduction in available habitat such as the last glacial maxima.
Tussenbroek, B.I. van
Seagrasses comprise 78 species and are rarely invasive. But the seagrass Halophila stipulacea, firstly recorded in the Caribbean in the year 2002, has spread quickly throughout the region. Previous works have described this species as invasive in the Caribbean, forming dense mats that exclude native seagrass species. During a reconnaissance field survey of Caribbean seagrass meadows at the islands of Bonaire and Sint Maarten in 2013, we observed that this species was only extremely dense at 5 out of 10 studied meadows. Compared to areas with sparse growth of H. stipulacea, these dense meadows showed consistently higher nutrient concentrations, as indicated by higher leaf tissue N contents of the seagrass Thalassia testudinum (dense when C:N < 22.5) and sediments (dense when %N > 11.3). Thus, the potential invasiveness of this non-native seagrass most likely depends on the environmental conditions, especially the nutrient concentrations.