The effects of Didemnum vexillum overgrowth on Mytilus edulis biology and ecology

Didemnum vexillum is an invasive colonial ascidian in the Gulf of Maine that readily colonizes hard substrates. These substrates include hard-shelled organisms, such as the common blue mussel Mytilus edulis. Preliminary observations and short-term studies showed potential effects of epibiosis on M. edulis growth, specifically lip thickness and tissue index. This dissertation study further examined the effects of D. vexillum on growth and reproduction of, and predation on, M. edulis. Shell thickness index, tissue index, shell mass to tissue mass ratio, lip thickness and mussel length were measured throughout a 12-month period in control and overgrown mussels. Additionally, histological preparations of the mussels were used to determine reproductive condition of the mussels in each of these treatments. These variables were measured every three months (November 2008, February 2009, May 2009, and August 2009). Laboratory choice and consumption experiments examined the effects of overgrowth of the ascidian on predation by Carcinus maenas. Finally, mussel primary settlement was measured from summer 2008 through summer 2009 and compared to historical data. This settlement was also correlated with the abundance of neighboring D. vexillum. Overgrowth had a negative impact on mussel growth. Tissue index and lip thickness were negatively affected as the mussel lip margin was overgrown. Overall mussel growth was significantly higher in control mussels by the end of the experiment. The pattern of spawning and gonad development was reversed in overgrown male mussels from the control mussels. Predation studies showed a potential positive effect for the mussel, as crabs consumed more control mussels than overgrown mussels in both a choice study and a consumption study. Finally, winter mussel settlement in 2008-2009 was lower than historical 1980-1981 settlement, and there was a decrease in mussel plantigrades with D. vexillum recruits. These studies show tradeoffs in the effects of overgrowth by D. vexillum; growth and reproduction are inhibited, while predation is decreased. As mussels are an important source of food and habitat for other Gulf of Maine organisms, overgrowth by D. vexillum has the potential to change ecosystem dynamics