Distribution and host range of a highly invasive parasitic copepod

The parasitic copepod Neoergasilus japonicus (Ergasilidae), native to East Asia, has spread rapidly around the world over the past half-century and now represents a clear example of successful parasitic invader. The species is now found in western Asia, Europe, America and Africa, with aquaculture and fish introductions identified as the primary vectors of dispersal. Regional field investigations have revealed surprisingly high number of affected localities, indicating potentially wider distribution than currently recognised. Neoergasilus japonicus exhibits low host specificity, parasitising a diverse range of freshwater fishes. This study updates the global fish host species list to 132, spanning 27 families across 15 orders, with Cypriniformes identified as the most susceptible host species. Under experimental conditions in this study, however, N. japonicus avoided its natural host, topmouth gudgeon Pseudorasbora parva, suggesting a level of resistance to its native parasite. Piscivorous and demersal fish were less infected by copepods than planktivorous, benthivorous, pelagic and benthopelagic species, reflecting the ecology of both the parasite and its hosts. An ability to re-attach to another host, though limited, was confirmed under experimental conditions. Recognising the ecological impacts and potential consequences associated with the introduction of non-native parasites emphasises the need for continuous monitoring and research globally.