Pathogens of Dikerogammarus haemobaphes regulate host activity and survival, but also threaten native amphipod populations in the UK

ABSTRACT: Dikerogammarus haemobaphes is a non-native amphipod in UK freshwaters. Studies
have identified this species as a low-impact invader in the UK, relative to its cousin Dikero gammarus
villosus. It has been suggested that regulation by symbionts (such as Microsporidia) could explain
this difference in impact. The effect of parasitism on D. haemobaphes is largely unknown. This
was explored herein using 2 behavioural assays measuring activity and aggregation. First, D.
haemobaphes were screened histologically post-assay, identifying 2 novel viruses (D. haemo baphes
bi-facies-like virus [DhbflV], D. haemobaphes bacilliform virus [DhBV]), Cucumispora ornata
(Micro sporidia), Apicomplexa, and Digenea, which could alter host behaviour. DhBV infection
burden increased host activity, and C. ornata infection reduced host activity. Second, native invertebrates
were collected from the invasion site at Carlton Brook, UK, and tested for the presence of
C. ornata. PCR screening identified that Gammarus pulex and other native invertebrates were
positive for C. ornata. The host range of this parasite, and its impact on host survival, was additionally
explored using D. haemobaphes, D. villosus, and G. pulex in a laboratory trial. D. haemo baphes
and G. pulex became infected by C. ornata, which also lowered survival rate. D. villosus did not
become infected. A PCR protocol for DhbflV was also applied to D. haemobaphes after the survival
trial, associating this virus with decreased host survival. In conclusion, D. haemobaphes has a
complex relationship with parasites in the UK environment. C. ornata likely regulates populations
by decreasing host