Invasions mediated by humans have been reported from around the world, and ships’ ballast water has been recognized as the main source of marine invaders worldwide. Some invasions have dramatic economic and ecological consequences. On the other hand, many invasions especially in the marine realm, can go unnoticed. Here we identify a human mediated, worldwide introduction of the hydrozoan species Turritopsis dohrnii. The normal life cycle of hydrozoans involves the asexual budding of medusae from colonial polyps. Medusae of Turritopsis, however, when starved or damaged, are able to revert their life cycle, going back to the polyp stage through a process called transdifferentiation. They can thus easily survive through long journeys in cargo ships and ballast waters. We have identified a clade of the mitochondrial 16S gene in Turritopsis which contains individuals collected from Japan, the Pacific and Atlantic coasts of Panama, Florida, Spain, and Italy differing from each other in only an average of 0.31% of their base-pairs. Fifteen individuals from Japan, Atlantic Panama, Spain, and Italy shared the same haplotype. Turritopsis dohrnii medusae, despite the lack of genetic differences, are morphologically different between the tropical and temperate locations we sampled, attesting to a process of phenotypic response to local conditions that contributes to making this grand scale invasion a silent one.
Invasive species Morphological response Hydrozoa TurritopsisMedusa
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We thank the staff of the Smithsonian Marine Stations of Naos, Bocas del Toro, Galeta and Fort Pierce for logistical support. We also thank S. Piraino for sharing information, A. Faucci and M. Rossi for discussion and suggestions on the manuscript, A. Driskell for DNA extraction and sequencing of some of the specimens, and C.S. Dugas for collecting some of the specimens from Bocas del Toro. This work was funded by a Smithsonian Marine Science Network postdoctoral fellowship to M.P.M.
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