Stress resistance in two colonial ascidians from the Irish Sea: The recent invader Didemnum vexillum is more tolerant to low salinity than the cosmopolitan Diplosoma listerianum

Abstract

It is widely believed that non-native species possess a high tolerance toward abiotic stress that allows them to survive transport and to colonize new habitats. However, to date empirical support for this concept is scarce. We therefore tested whether Didemnum vexillum, which recently invaded the Irish Sea, is more tolerant to fluctuations in salinity than the closely related Diplosoma listerianum, a cosmopolitan species of wide ranging distribution. The two fouling organisms exhibit similar habitat requirements and co-occur in many hard-bottom sites along the coasts of Ireland and Wales. We collected adult specimen of both taxa in Holyhead Marina, Wales, UK, and exposed them to different levels of reduced salinity ranging from 10 to 27 in short-term laboratory experiments. In separate trials, we mimicked chronic (2 weeks) as well as episodic (2 h) low-salinity stress to account for the natural temporal variability in osmotic fluctuations. We compared the ascidians' performances in the face of stress and found that the invasive D. vexillum showed higher growth and survival under low salinities than the cosmopolitan D. listerianum. This indicates that D. vexillum is generally better in adapting to fluctuating ion concentrations. This competitive advantage may facilitate the establishment and spread of D. vexillum, especially if osmotic conditions in coastal habitats will be altered by future climate-change induced shifts in local precipitation regimes.