Genetic conspecificity of the worldwide populations of <i>Didemnum vexillum</i> Kott, 2002

A colonial tunicate belonging to the genus Didemnum has recently been found in many temperate coastal regions throughout the world, as well as large areas of Georges Bank in the NW Atlantic. It continues to spread rapidly and compete aggressively with native, hard substrate species (e.g., mussels, barnacles, bryozoans, other ascidians). In addition, it can form dense mats on deepwater cobble-gravel substrates and influence the abundance and species composition of benthic epifauna and infauna. Thus, its ever-increasing presence is creating potentially severe detrimental economic and ecological impacts. This invasive species, referred to in recent publications as Didemnum sp. A, has been misidentified as five previously described species native to the regions where Didemnum sp. A has been discovered and has been described as two new species based solely on morphological characteristics. There are relatively few diagnostic characters and a great deal of variability in the relevant characters, making the task of identification very difficult. Adding to the confusion has been the widespread and apparently disjunct distribution of the species. Here, we present molecular data on both mitochondrial and nuclear genes from colonies sampled from Europe, east and west coasts of North America, Japan, and New Zealand. These data strongly indicate that Didemnum sp. A is a single species, possibly native to the northwestern Pacific Ocean, that has become established globally. Considering genetic and morphological evidence, the most appropriate name for this species is Didemnum vexillum Kott, 2002.