High food quality increases infection of Gammarus pulex (Crustacea: Amphipoda) by the acanthocephalan parasite Pomphorhynchus laevis

Acanthocephalan parasites reflect ecological status of freshwater ecosystem

Acanthocephalans' position in food webs, in close interaction with free-living species, could provide valuable information about freshwater ecosystem health through the viability of the parasites' host populations. We explored Pomphorhynchus laevis cystacanths' and adults' intensities of infection, and the prevalence of infected hosts respectively in their Gammarus pulex intermediate hosts and Squalius cephalus definitive hosts in a Mediterranean river. First, we analysed the relationship between P. laevis intensity of infection, its two hosts populations and the other acanthocephalan species found (Pomphorhynchus tereticollis and Polymorphus minutus). Second, we characterised the influence of bacteriological, physicochemical and biological water parameters on these acanthocephalans, and their intermediate and definitive hosts. This research highlights that P. laevis infection was closely related to their two preferential hosts population in the river. Moreover, P. laevis intensity of infection was positively correlated with organic pollution in the river but negatively correlated with biodiversity and with ecological indexes of quality. Pomphorhynchus laevis could thus benefit from moderate freshwater pollution, which promotes their tolerant intermediate and definitive hosts.

Parasite infection influences the biomarker response and locomotor activity of Gammarus fossarum exposed to conventionally-treated wastewater

Modern wastewater treatment plants cannot completely remove pollutants. Often, effluents entering the aquatic environment still contain micropollutants such as pharmaceuticals or pesticides, which may impose adverse effects on aquatic biota. At the same time, a large proportion of free-living aquatic species are known to be infected with parasites, which raises the question of interactions between environmental stressors (such as micropollutants) and parasite infection. We chose the freshwater amphipod Gammarus fossarum (Koch, 1835) as a test organism to investigate potential pollutant-parasite interactions. This gammarid is frequently used in ecotoxicological tests and is also commonly infected with larvae of the acanthocephalan parasite species Polymorphus minutus (Zeder, 1800) Lühe, 1911. We exposed infected and uninfected specimens of G. fossarum to conventionally-treated wastewater and river water in a 22-day flow channel experiment. The test organisms' response was measured as mortality rates, concentrations or activities of five biomarkers, and overall locomotor activity. No significant differences were found between mortality rates of different exposure conditions. Contrastingly, three biomarkers (phenoloxidase activity, glycogen, and lipid concentrations) showed a significant increase in infected gammarids, while the effect of the water type was insignificant. Infected gammarids also showed a significantly higher locomotor activity in both water types. Our results suggest that the response of G. fossarum during the exposure experiments was mainly driven by parasite infection. This implies that parasites may act as additional biotic stressors in multiple stressor scenarios, and therefore, might play an important role when measuring the response of organisms to chemical stressors. Future ecotoxicological studies and assessments thus should consider parasite infection as an additional test parameter.

Continental-scale patterns of hyper-cryptic diversity within the freshwater model taxon Gammarus fossarum (Crustacea, Amphipoda)

Traditional morphological diagnoses of taxonomic status remain widely used while an increasing number of studies show that one morphospecies might hide cryptic diversity, i.e. lineages with unexpectedly high molecular divergence. This hidden diversity can reach even tens of lineages, i.e. hyper cryptic diversity. Even well-studied model-organisms may exhibit overlooked cryptic diversity. Such is the case of the freshwater crustacean amphipod model taxon Gammarus fossarum. It is extensively used in both applied and basic types of research, including biodiversity assessments, ecotoxicology and evolutionary ecology. Based on COI barcodes of 4926 individuals from 498 sampling sites in 19 European countries, the present paper shows (1) hyper cryptic diversity, ranging from 84 to 152 Molecular Operational Taxonomic Units, (2) ancient diversification starting already 26 Mya in the Oligocene, and (3) high level of lineage syntopy. Even if hyper cryptic diversity was already documented in G. fossarum, the present study increases its extent fourfold, providing a first continental-scale insight into its geographical distribution and establishes several diversification hotspots, notably south-eastern and central Europe. The challenges of recording hyper cryptic diversity in the future are also discussed.