Differential copper toxicity in invasive and native ascidians of New England provides support for enhanced invader tolerance

Colonial ascidians are a group of prolific invaders found in urban marine environments. Despite often high levels of pollution in these areas, some species of ascidians not only tolerate but thrive in these conditions, while native species decline. The role of pollution tolerance in establishment success is often overlooked, and therefore the mechanisms employed by invaders are poorly understood. This research explores both acute and chronic copper toxicity of Botrylloides violaceus, a prominent invader in the Gulf of Maine, and a native counterpart, Aplidium glabrum. Acute toxicity tests revealed that B. violaceus larvae are significantly more tolerant to exposure than A. glabrum larvae (median effective concentration for settlement inhibition [EC50] values = 107.9 and 46.7 mu g l(-)(1), respectively). Further, the results indicate that B. violaceus is more tolerant than members of several other native benthic phyla using a species sensitivity distribution approach. Chronic toxicity tests determined that EC50 values impacting the number of zooids per colony and colony area for A. glabrum were achieved at environmentally relevant concentrations of copper, while B. violaceus values were over twice the concentration (approximately 8 and 20 mu g l(-1), respectively). These findings support the hypothesis that invader pollution tolerance is indeed greater than that of native species, both within and beyond the same phyla, and likely plays a role in invader establishment and spread as described in the invader tolerance model.