Infochemicals Influence Neonicotinoid Toxicity-Impact in Leaf Consumption, Growth, and Predation of the Amphipod Gammarus fossarum

One like all? Behavioral response range of native and invasive amphipods to neonicotinoid exposure

Native and invasive species often occupy similar ecological niches and environments where they face comparable risks from chemical exposure. Sometimes, invasive species are phylogenetically related to native species, e.g. they may come from the same family and have potentially similar sensitivities to environmental stressors due to phylogenetic conservatism and ecological similarity. However, empirical studies that aim to understand the nuanced impacts of chemicals on the full range of closely related species are rare, yet they would help to comprehend patterns of current biodiversity loss and species turnover. Behavioral sublethal endpoints are of increasing ecotoxicological interest. Therefore, we investigated behavioral responses (i.e., change in movement behavior) of the four dominant amphipod species in the Rhine-Main area (central Germany) when exposed to the neonicotinoid thiacloprid. Moreover, beyond species-specific behavioral responses, ecological interactions (e.g. parasitation with Acanthocephala) play a crucial role in shaping behavior, and we have considered these infections in our analysis. Our findings revealed distinct baseline behaviors and species-specific responses to thiacloprid exposure. Notably, Gammarus fossarum exhibited biphasic behavioral changes with hyperactivity at low concentrations that decreased at higher concentrations. Whereas Gammarus pulex, Gammarus roeselii and the invasive species Dikerogammarus villosus, showed no or weaker behavioral responses. This may partly explain why G. fossarum disappears in chemically polluted regions while the other species persist there to a certain degree. But it also shows that potential pre-exposure in the habitat may influence behavioral responses of the other amphipod species, because habituation occurs, and potential hyperactivity would be harmful to individuals in the habitat. The observed responses were further influenced by acanthocephalan parasites, which altered baseline behavior in G. roeselii and enhanced the behavioral response to thiacloprid exposure. Our results underscore the intricate and diverse nature of responses among closely related amphipod species, highlighting their unique vulnerabilities in anthropogenically impacted freshwater ecosystems.

Selected neonicotinoids and associated risk for aquatic organisms

Neonicotinoids are one of the newest groups of systemic pesticides, effective on a wide range of invertebrate pests. The success of neonicotinoids can be assessed according to the amount used, for example, in the Czech Republic, which now accounts for 1/3 of the insecticide market. The European Union (EU) has a relatively interesting attitude towards neonicotinoids. Three neonicotinoid substances (imidacloprid, clothianidin and thiamethoxam) were severely restricted in 2013. In 2019, imidacloprid and clothianidin were banned, while thiamethoxam and thiacloprid were banned in 2020. In 2022, another substance, sulfoxaflor, was banned. Therefore, only two neonicotinoid substances (acetamiprid and flupyradifurone) are approved for outdoor use in the EU. Neonicotinoids enter aquatic ecosystems in many ways. In European rivers, neonicotinoids usually occur in nanograms per litre. Due to the low toxicity of neonicotinoids to standard test species, they were not expected to significantly impact the aquatic ecosystem until later studies showed that aquatic invertebrates, especially insects, are much more sensitive to neonicotinoids. In addition to the lethal effects, many studies point to sublethal impacts - reduced reproductive capacity, initiation of downstream drift of organisms, reduced ability to eat, or a change in feeding strategies. Neonicotinoids can affect individuals, populations, and entire ecosystems.

Natural variation in social conditions affects male mate choosiness in the amphipod Gammarus roeselii

The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.