Histopathological screening of Pontogammarus robustoides (Amphipoda), an invader on route to the United Kingdom

Biological invasions may act as conduits for pathogen introduction. To determine which invasive non-native species pose the biggest threat, we must first determine the symbionts (pathogens, parasites, commensals, mutualists) they carry, via pathological surveys that can be conducted in multiple ways (i.e., molecular, pathological, and histological). Whole animal histopathology allows for the observation of pathogenic agents (virus to Metazoa), based on their pathological effect upon host tissue. Where the technique cannot accurately predict pathogen taxonomy, it does highlight pathogen groups of importance. This study provides a histopathological survey of Pontogammarus robustoides (invasive amphipod in Europe) as a baseline for symbiont groups that may translocate to other areas/hosts in future invasions. Pontogammarus robustoides (n = 1,141) collected throughout Poland (seven sites), were noted to include a total of 13 symbiotic groups: a putative gut epithelia virus (overall prevalence = 0.6%), a putative hepatopancreatic cytoplasmic virus (1.4%), a hepatopancreatic bacilliform virus (15.7%), systemic bacteria (0.7%), fouling ciliates (62.0%), gut gregarines (39.5%), hepatopancreatic gregarines (0.4%), haplosporidians (0.4%), muscle infecting microsporidians (6.4%), digeneans (3.5%), external rotifers (3.0%), an endoparasitic arthropod (putatively: Isopoda) (0.1%), and Gregarines with putative microsporidian infections (1.4%). Parasite assemblages partially differed across collection sites. Co-infection patterns revealed strong positive and negative associations between five parasites. Microsporidians were common across sites and could easily spread to other areas following the invasion of P. robustoides. By providing this initial histopathological survey, we hope to provide a concise list of symbiont groups for risk-assessment in the case of a novel invasion by this highly invasive amphipod.

Podocotyle atomon (Trematoda: Digenea) impacts reproductive behaviour, survival and physiology in Gammarus zaddachi (Amphipoda)

The Trematoda are a group of phylogenetically diverse metazoan parasites that exhibit complex life cycles that often pass through invertebrate and vertebrate hosts. Some trematodes influence their host's behaviour to benefit transmission. Their parasitic influence may impact host population size by inhibiting an individual's reproductive capacity. We assessed the impact of infection by Podocotyle atomon on the reproductive behaviour and fecundity of its amphipod intermediate host, Gammarus zaddachi, using laboratory and field studies. Parasite prevalence was high in the field, with males more likely to be infected (prevalence in males 64%, in females 39%). Males also suffered a higher parasite burden than females. Infected females were less active, but we found no evidence for a reduction in female reproductive success. Infected females also had comparable pairing success to uninfected females. In males, infection reduced survival and fecundity, with mortality being highest, and sperm numbers lowest, in heavily infected individuals. Trematode parasites are sometimes associated with altered host fecundity, but studies often lack the relevant experimental data to explore the evolution of the trait. We discuss this among information specific to the effect of P. atomon infection in G. zaddachi.