Characterizing the spread and impacts of the invasive colonial tunicate Didemnum vexillum on Georges Bank

Since the discovery of the invasive tunicate Didemnum vexillum on Georges Bank in 2002, scientists have been investigating its spread and potential impacts on the benthic community. Previous research on the invasion of Didemnum vexillum on Georges Bank found that since its introduction, it has colonized at least 230 km2 of pebble gravel habitat in two adjacent areas with contrasting levels of bottom fishing disturbance, Area 18 (open to fishing) and Area 19 (closed to fishing). The aim of the present study is to better understand the impacts of the colonization of Didemnum vexillum to the benthic community on Georges Bank, and to investigate the potential role of bottom fishing disturbance. To accomplish this, two types of sampling were conducted: still photographs to quantify attached epifauna, including Didemnum vexillum, and Naturalist dredge samples to quantify free-living epifaunal taxa. The USGS SEAbed Observation and Sampling System (SEABOSS) was used on annual research cruises to take still photographs of the ocean bottom on Georges Bank in Area 18 from 1994-2000 and 2003-2004, and in both Areas 18 and 19 from 2006-2007. Bottom photos were analyzed with either a grid cell method or with a Matlab random point program. Naturalist dredge samples were collected from Area 18 from 1996-2008 and from Area 19 from 2005-2008. Analyses to investigate the long-term effects of the invasion of Didemnum vexillum in Area 18 revealed a significant increase in the percent cover of Didemnum vexillum after the infestation (2002-2008) versus before the infestation (1994-2001). A significant negative relationship was found to exist between the frequency of free-living macrofauna and the percent cover of Didemnum vexillum; as the percent cover of Didemnum vexillum increases, the frequency of macrofauna decreases. Naturalist dredge abundance data revealed a distinct difference in the species composition before the infestation compared to after the infestation. The significant increase in the abundance of two polychaete species, Nereis zonata and Harmothoe extenuata was found to be responsible for this change. Analyses used to investigate the potential role of bottom fishing disturbance revealed significant differences in the percent cover of colonial epifauna in Area 19 compared with Area 18. Didemnum vexillum and Filograna implexa both had a higher percent cover in Area 19 while hydroid and bushy bryozoans had a higher percent cover in Area 18. A significant higher abundance of free-living macrofauna was observed in Area 18 compared to Area 19. Analysis of naturalist dredge samples confirmed that there was a significant difference in species composition in Area 18 compared to Area 19, and the two species that were identified for being largely responsible for this change were Nereis zonata and Urticina felina. The results of this study show that the invasion of Didemnum vexillum has had significant impacts on the benthic community of Georges Bank. While the tunicate appears to be negatively impacting free-living macrofauna, it may be positively impacting two polychaete species, Nereis zonata and Harmothoe extenuata by offering them protection from predation by bottom feeders. Additionally, bottom fishing disturbance in Area 18, also appears to be significantly impacting the benthic community with the fragile and structurally complex polychaete Filograna implexa, the most negatively impacted