Intraspecific variability in the filter mesh size of suspension feeding organisms: the case of invasive Ponto-Caspian corophiids (Crustacea: Amphipoda).

Suspension feeders play pivotal roles in the nutrient cycling of almost all aquatic ecosystems. Since sufficiently large differences in the filter mesh size (FMS) can lead to different food web positions, the inter- and intraspecific variability of this trait might be of community-level importance. The aim of this study was to quantify the range of FMS variation within the three invasive Ponto-Caspian species based on a large material representing various conditions (1,224 specimens from 40 samples across Central Europe), characterize the components of variation within populations, identify the main factors determining intraspecific differences, and reveal how intraspecific variation affects the FMS overlaps among species. The FMS of the most widespread invader, , varied within the broadest range (between 2.34-8.28 μm, compared to 2.51-5.97 μm in and 1.08-3.23 μm in ); nevertheless, the contribution of intraspecific plasticity to the invasion success of the species is not evident based on the present study. The within-individual variability of FMS increased with the individual mean of the trait and decreased with body size; however, it showed little differences among samples. The among-individual variation within samples could be partitioned into components related to body size (ontogenetic niche shift/differences among cohorts) and sex (ecological sexual dimorphism) as well as a seemingly random component (individual specialization), varying widely in extent and relative contributions. The FMS of was significantly larger in the presence of than in allopatry, likely reflecting character displacement; however, it did not show further increase when was also present. Similar differences could not be observed in . The FMS ranges of and never overlapped with that of in co-occurrence despite the considerable intraspecific differences among sites, suggesting that their interaction can be seen as a clear case of niche differentiation by food particle size. On the contrary, the strong overlaps observed between and indicate that other factors might play the primary role in their coexistence. The studied species appear to be suitable model organisms for identifying the drivers and mechanisms of FMS variability.