The presence of the ctenophore Mnemiopsis leidyi in the Oslofjorden and considerations on the initial invasion pathways to the North and Baltic Seas

The ctenophore Mnemiopsis leidyi is one of the most cited invasive species due to its ecological and economical impact that occurred in the Black and Caspian Seas in previous years. Recent reports on the species presence in the Baltic and North Seas alarmed the scientific community. While the earliest report of M. leidyi in the Baltic was spring 2006, this paper indicates of the presence of M. leidyi in the Oslofjorden as early as autumn 2005, and discusses the possible invasion routes from its native range along the eastern coast of the Americas.