Assessing Lethal Dissolved Oxygen Tolerance for Invasive Tunicate Ciona savignyi in Puget Sound

Ciona savignyi is a solitary tunicate (Phylum Chordata, Class Ascidiacea) native to Japan that has invaded coastal habitats in the north-east Pacific and New Zealand. In the Puget Sound of Washington, USA, we examined the ability of C. savignyi to survive in artificially created hypoxic environments to determine if reduced dissolved oxygen (DO) treatments could be a viable control method. In laboratory bioassays, treatment groups that were immersed in DO concentrations ranging from completely hypoxic (1 mg/L) to low DO (5 mg/L) had zero survivorship of individually isolated tunicates after 14 to 22 days of exposure, respectively. Additionally, hypoxic conditions (approximately 1.5 mg/L) were created in the field using polyethylene tarp wraps applied around dock surfaces fouled with C. savignyi in a Puget Sound marina. To estimate mortality rates underneath the tarp wraps, dock units with clusters of C. savignyi remained wrapped for 10, 14 and 18 days and displayed decreasing survivorship with increased wrap time (76%, 51% and 33%, respectively). Our laboratory and field experiments indicate that wrapping docks fouled by C. savignyi with polyethylene tarps may be an effective management option to locally-control and reduce the spread of this tunicate species from marina habitats, which serve as hubs for non-native species transport via hull fouling. These results inform the development of a rapid response plan for C. savignyi in the state of Washington and may be a viable control method for other high priority non-native tunicates pending further work on species-specific tolerances to low DO.