Differences in antennal morphology and body size might offset increased overlap in food particle size among invasive Ponto-Caspian corophiids (Crustacea, Amphipoda)

After Chelicorophium curvispinum , C. sowinskyi , and C. robustum , recently the fourth Ponto-Caspian filter feeding corophiid ( C. maeoticum ) has colonized the lower section of the River Rhine in the Netherlands. To test for niche differentiation among the four species along the food particle size spectrum, we measured their filter mesh sizes (i.e., the distance between bristles on the filtering setae). For comparison, we also analyzed a historic sample from the River Tisza in Hungary in which C. maeoticum co-occurred only with C. sowinskyi . We also compared the relative lengths and widths of the second antennae among the species, considering the potential involvement of this appendage in interference competition and deposit feeding. Despite significant interspecific differences in filter mesh sizes in most comparisons, there was a partial overlap between C. maeoticum and C. sowinskyi in the River Rhine but not in the River Tisza. In addition, there was an almost complete overlap between C. curvispinum and C. robustum . Marked interspecific differences could be observed in the relative widths of the second antennae in both sexes, potentially related to performance in competition for microhabitats. Remarkably, the earlier colonists C. curvispinum and C. sowinskyi have thinner second antennae while C. robustum and C. maeoticum with more robust appendages could establish in habitats already occupied by their congeners. Our results suggest that competition for both food and space can shape the coexistence and invasion patterns of Ponto-Caspian corophiids. Nevertheless, more studies are needed to reveal the range of behaviors second antennae are involved in and how their morphological features determine their functionality. The observed differentiation in the measured traits along with differences in body sizes close to the 1.3 ratio are likely to allow the coexistence of the four species and portend the continued spread of C. maeoticum .