Physiological tolerance as a tool to support invasion risk assessment of tropical ascidians

Physiological tolerance is a trait that may increase the invasion potential of transported species. A review of current literature, in which most species tested were from temperate or subtropical regions, shows that invasive ascidians can indeed tolerate a large range of salinities and temperatures. In this study, we used 4 tropical ascidians from Caribbean Panama as models to test survival of adults and their ability to maintain ionic gradients between blood and seawater in different salinities (10 to 45 ppt) and temperatures (15 and 29 degrees C); we also measured early development success. We used these physiological data along with environmental information collected from ports to estimate the colonization pressure of these species in ports with shipping connections to Panama. Adults were more tolerant than gametes to both increased and decreased salinities; order of tolerance was Ascidia sydneiensis > A. curvata > Phallusia nigra > A. panamensis. All species were able to regulate ions (Cl-, Na+, Mg++, K+) when tested at different salinities, indicating a mechanism for tolerance to varying environmental conditions. Preliminary colonization pressure analysis indicated that 31% of the ports we evaluated are at risk of colonization by the 3 most tolerant species, with a high of 78% risk by A. sydneiensis; only 22% of the ports studied were determined to be not at risk. We predict that A. sydneiensis will spread in the East Pacific and the expansion of the Panama Canal will increase opportunities for A. curvata (high proba bility) and P. nigra (lower probability) to be transported to the Pacific coast of America.