Use of environmental DNA in early detection of Mnemiopsis leidyi in UK coastal waters

Since 2005, the ctenophore Mnemiopsis leidyi has been reported as present in northern Europe. Although the impacts of this voracious predator on North Sea fisheries remain uncertain, it is in direct competition with fish larvae for food availability and is considered a threat to the structure and function of the marine ecosystem. M. leidyi has never been officially recorded in UK coastal waters but it has been listed among the most likely species to arrive within the next 10 years by a panel of experts. The threat is confirmed by recurrent visual observations of M. leidyi in the North Sea, as well as the identification of suitable areas to establish a population in the same area. In this study, we developed and validated an environmental DNA (eDNA) protocol to detect the presence of M. leidyi in the water of an economically-important shellfishery. This technique is based on the detection of DNA from cells secreted by an organism into its environment and is particularly suitable to detect a low number of individuals. A set of specific primers were used to target a region in the mitochondrial COI locus. Conventional PCR and SYBR® Green quantitative PCR assays were optimised and compared for sensitivity during laboratory experiments describing the kinetic of DNA released by M. leidyi. Natural water samples were collected in The Wash on the North Norfolk coast between 2010 and 2014 and tested. Samples taken only in 2014 were positive for the presence of M. leidyi, although no visual observation of individuals was reported to confirm the establishment of a population. This study showed how powerful eDNA analyses could be used to investigate the pathway and early invasion stages of alien invasive species such as M. leidyi by complementing the traditional monitoring techniques.