First occurrence of the invasive tunicate Didemnum vexillum in eelgrass habitat

During the late 20th century, several species of alien tunicates invaded New England marine coastal waters. In Autumn 2008, we surveyed for tunicates in Lake Tashmoo, a protected marine pond with shellfish aquaculture operations and restored bay scallop Argopecten irradians irradians habitat on Martha's Vineyard, Massachusetts. We found the invasive tunicates Ascidiella aspersa, Botrylloides violaceus, Botryllus schlosseri, Didemnum vexillum, Diplosoma listerianum, Styela clava and native tunicate Molgula manhattensis attached to artificial substrates throughout Lake Tashmoo and B. violaceus, B. schlosseri, D. vexillum, D. listerianum and M. manhattensis attached to eelgrass Zostera marina in the middle of Lake Tashmoo. Tunicates were growing on the stalk and blade of in situ eelgrass, floating pieces of eelgrass (a transport and dispersal mechanism), and pieces of eelgrass in fouling communities on boat hulls and aquaculture floats. Botrylloides violaceus, B. schlosseri, D. listerianum and M. manhattensis have been previously recorded as utilizing eelgrass as substrate, but this is the first recorded occurrence of D. vexillum to utilize eelgrass as substrate. Perhaps because of lack of substrate space in Lake Tashmoo, D. vexillum spread to eelgrass. Eelgrass serves as a juvenile shellfish and fish habitat and threats to it are of concern by coastal managers and the fishing industry.