Overgrowth of eelgrass by the invasive colonial tunicate Didemnum vexillum: Consequences for tunicate and eelgrass growth and epifauna abundance

The invasive tunicate Didemnum vexillum has become widely distributed in estuaries in western North America over the past decade. It has recently invaded eelgrass meadows in a central California estuary, growing directly on shoots and rhizomes. This invasion is of concern due to the important role of eelgrass as a foundation species in coastal communities and the known detrimental effects of this tunicate on sessile organisms in other habitats. The effects of tunicate overgrowth on eelgrass growth and morphology were studied using surveys, field experiments, and outdoor mesocosm experiments. In field and mesocosm experiments, there were decreases in eelgrass aboveground growth relative to unfouled shoots. Comparison of D. vexillum growth rates on eelgrass vs. hard substrate in mesocosms showed that growth was higher on hard substrate. The effects of D. vexillum overgrowth on the eelgrass epifaunal community were also characterized; overall invertebrate densities, and specifically tanaid and polychaete densities, were higher on shoots fouled with D. vexillum. Overall, the mean cover of D. vexillum on eelgrass shoots in the field was approximately 15-20% of shoot length. As the eelgrass in the mesocosms had less D. vexillum coverage than in the field, these results are conservative and suggest that relatively low D. vexillum coverage of individual eelgrass shoots may result in modest impacts on eelgrass.