Differences in the sensitivity among cryptic lineages of the Gammarus fossarum complex

Lineages that are at least superficially morphologically identical but genetically distinct are usually misclassified as a single nominal species and, hence, belong to a cryptic species complex, as for example observed for Gammarus fossarum. Since genetic differentiation between cryptic lineages of the G. fossarum complex is comparable to an interspecific level, deviations in physiological and behavioral characteristics are conceivable. The present study investigates for the first time deviations in the sensitivity between two cryptic G. fossarum lineages — namely lineage A and lineage B. Two size classes of both cryptic lineages were exposed in six independent experiments to different concentrations of the fungicide tebuconazole or the insecticide thiacloprid (each n=20) for seven days. Feeding rate on leaf discs was used as a measure of sensitivity. By combining the outcome of all bioassays in a meta-analysis, cryptic lineage A showed – with an approximate 50% more pronounced decline in feeding – a significantly higher overall sensitivity compared to lineage B (n=17). This was also obvious, when each chemical stressor was considered individually; however, data were not significant for thiacloprid. Although other reasons, like land-use of habitats around sampling sites and physiological characteristics of organisms, could not be excluded as driving factors, the results indicate that affiliation to a cryptic lineage is a potential explanatory variable for the observed deviations in sensitivity. As the present study considered only one population for each cryptic lineage, a final conclusion cannot yet be drawn. Hence, it is recommended to investigate behavioral and physiological deviations between cryptic lineages of the G. fossarum complex in further studies.