Life history attributes of a global invader: factors contributing to the invasion potential of Didemnum vexillum

The colonial ascidian Didemnum vexillum has dispersed from its native Japan to many temperate coastal regions of the world, yet many features of its life history remain poorly studied. Using D. vexillum populations in the northeastern USA, several features of growth and sexual reproduction, as well as asexual reproduction via the unusual feature of detachment of larva-laden tendrils were examined from 2008 to 2011. Recruits of D. vexillum grown on PVC panels in Groton, CT, between July and September 2011 reached sexual maturity in 50 to 62 d. A natural, overwintering D. vexillum population in Noank, CT, sampled weekly from 12 May to 14 July 2008 first had reproductive structures on 19 May; 7 wk later the first recruits were found on recruitment panels at the same dock. Examination of paired samples of encrusting and tendril growth forms collected on 22 July 2010 in Newport, RI, revealed no significant differences in densities of reproductive structures between growth forms. Fifty-nine percent of tagged tendrils in Westport, MA, detached in 2 wk. One hundred percent of tendrils reattached within 48 h to PVC panels in the laboratory, while only 1 in 80 tendrils tethered to bare substrates (rock or concrete) at nearby field sites reattached to the substrate. Tendrils tethered to natural rock were rapidly consumed by predatory snails. Further laboratory experiments found a minimum of 8 h of undisturbed contact with the substrate was necessary for D. vexillum tendrils to reattach. Overall, we found that the tendril growth form is an important factor in the population biology of D. vexillum because it increases surface area for feeding and reproducing zooids in a space-limited environment.