Salinity affects the dispersal pattern of the freshwater invasive snail Physa acuta Draparnaud, 1805 (Gastropoda: Physidae) in an anthropogenically-affected river-estuary catchment

In the context of biological invasions, the alien freshwater snail Physa acuta is recognised as one of the most effective and spectacular invaders among the macrozoobenthos assemblages, although its preferential ecological pathways for dispersion remain poorly understood, yet little is known about its biological response and tolerance under saline environments. The present study investigates the influence of salinity on the distribution and density of P. acuta in 45 sites selected in three water body types (streams, perennial gutters, and estuary) across the Wouri river-estuary in Douala. Each site was sampled on six occasions on a seasonal base (three campaigns during rainy and dry season each) to collect snail's specimens and measure key water salinity-related variables. Hierarchical cluster analysis defined four salinity gradients with mean values of 0.16, 2.88, 8.71, and 12.32 psμ, respectively. The density of P. acuta declined significantly across the salinity gradient with mean values of 69.77, 7.94, 5.17, and 0 ind./m², respectively. Results showed a general salinity threshold of 8.5 psμ beyond which no living individual of P. acuta was found. The Self Organizing Map predicted a distribution probability profile of P. acuta with very low density in all the estuary sites, and no individuals in sites with salinity above 8.5 psμ. Densities also decreased significantly during dry season regardless the salinity gradient. The Spearman correlation and binary logistic regression analyses revealed that snail occurrence and density were negatively related to salinity. This study is consistent with previous studies which also indicated that increasing salinity has a significant threat on this freshwater acute bladder snail. However, further studies are required to address the expansion range of P. acuta in relation to climate change and raising sea level with salinisation of coastal water bodies, which may reduce the colonisation ability of P. acuta.