Understanding the ecological and evolutionary dynamics of tropical seagrasses and the factors driving habitat distribution

Seagrasses occur worldwide, and are essential primary producers that uptake carbon dioxide, fix nutrients, stabilize sediments, prevent reef degradation, filter bacteria, provide food and nursery habitats to marine organisms. When seagrass meadows disappear, carbon is released back into the water column, sediments get stirred, water clarity decreases, and reefs become infected, with negative impacts on marine biodiversity and maritime economy. My thesis utilizes multidisciplinary ecology and evolutionary biology approaches to better understand the biology of seagrasses, particularly an invasive seagrass, to help improve management strategies for seagrass conservation. Seagrasses frequently display distinct depth distribution, although drivers of these patterns can be spatially and temporally variable. Chapter 1 examines the factors that influence the depth distribution of a circumtropical seagrass, Halophila decipiens. While H. decipiens can grow in waters as shallow as 1 m, in Moorea, French Polynesia we only found it in waters deeper than 6.4 m. To understand why H. decipiens did not grow in shallower habitats, we transplanted it into 3 habitats: the existing seagrass bed (control), just outside the seagrass bed, and shallower habitat adjacent to a fringing coral reef. Results showed that growth was not significantly different between the seagrass bed and just outside of the seagrass bed; however, its growth was significantly reduced when adjacent to the reef. We then transplanted seagrass into a shallower reef site with and without herbivore exclusion cages, and the results showed that H. decipiens grew best when herbivores were excluded, but lost growth when herbivores were allowed access. These results indicate that H. decipiens can grow in shallow habitats adjacent to reefs, but herbivory pressure from the reef limits its depth distribution. Seagrass meadows are in decline around the world. Biological invasions can magnify threats to seagrass ecosystems with detrimental consequences to seagrass biodiversity. In Chapter 2, I used mesocosm experiments to investigate the interactions between the invasive seagrass Halophila stipulacea and native seagrasses to determine whether species interactions can drive, prevent, or facilitate invasions in both the Mediterranean and Caribbean Sea. In the Caribbean, invasive H. stipulacea increased in growth when grown with the native Syringodium filiforme, and lost shoots when grown alone, while S. filiforme only increased in shoots when grown alone. This pattern was the same in the Mediterranean; when invasive H. stipulacea grew with the native Cymodocea nodosa, it gained more shoots than when grown alone, but C. nodosa only did better when grown alone. Results suggest that the invasive seagrass H. stipulacea can drive its own success by negatively affecting native seagrasses and benefiting from that negative interaction. This novel example of native species facilitating the success of an invasive provides one possible mechanism for the widespread success of this invasive species. Mechanisms that influence invasion success can further be understood by understanding how it was introduced to a specific region. In Chapter 3, I used genomic tools to reconstruct the origins of the globally invasive seagrass Halophila stipulacea in the Mediterranean and Caribbean Seas. While H. stipulacea almost certainly invaded the Mediterranean from native populations in the Red Sea through the Suez canal, it is unclear whether the Caribbean invasion represents stepping stone colonization from the Mediterranean, an independent introduction from the native range, or an admixture from multiple native/invasive populations. To test these hypotheses, we examined population genetic structure and genetic diversity from multiple locations spanning across the native, historic, and recent invasive ranges of H. stipulacea, including the Indian Ocean and Red Sea, Mediterranean Sea, and the Caribbean Sea, respectively. Data from 524 SNP loci and restrictive, 45 SNP loci at >10x coverage revealed significant genetic structure among all five regions. The analyses revealed that the widespread invasion of H. stipulacea into the Caribbean Sea came from multiple introductions originating from the Mediterranean. This work provides a baseline for the distribution of the invasive H. stipulacea in the Caribbean, and may help predict how to minimize detrimental impacts of a non-indigenous seagrass across its invaded ranges. Life history differences can provide a link in invasion potential and dispersal. In Chapter 4 I investigated the life history of seagrass Halophila stipulacea in the Caribbean. Reports of asexual and sexual reproduction are common in its native range, with sexual reproduction being less common in the Mediterranean Sea. Here we make the first report of H. stipulacea male flowers in the Caribbean and suggest that asexual fragmentation is the main strategy of expansion. These findings have important implications for the future dispersal, survival, and maintenance of the non-native populations in the Caribbean