Complex selection processes on invasive crayfish phenotype at the invasion front of the Zambezi floodplains ecoregion

1. Understanding the eco-evolutionary processes which drive phenotypic variability in non-native species is crucial to predicting impact and informing spatially explicit management models. Invasive redclaw crayfish, Cherax quadricarinatus, are spreading rapidly through the ecologically and economically important Upper and Middle Zambezi system in south-central Africa.

2. We tested whether there was an invasion front effect in the Barotse floodplain and disentangle whether these effects were related to environmental filtering, spatial sorting, or density-dependent processes along a nonlinear 200 km invasion gradient over seasonal hydrological periods.

3. Crayfish spread up- and down-stream indicated an expanding population compared to the former invasion front in 2019, facilitated by both sexes. Wet season detection probability and relative abundance were lower than in the dry season. No signals of environmental filtering were detected as a result of high connectivity of habitats in the wet season.

4. The invasion front population was characterised by lower relative abundance, lower body condition, larger overall body and chelae size, and longer front leg length. There was no effect on sex ratio. This indicated that spatial sorting, density-dependent processes and predation pressure are acting as selective forces on phenotypic variability across the invasion gradient.

5. Better understanding of the effects of flood regime and trophic interactions on phenotypic variability and dispersal is urgently needed. Population suppression to reduce spatial sorting and conspecific competition could be a viable management option at the invasion front.