Is salinity tolerance the key to success for the invasive water bug Trichocorixa verticalis?

Trichocorixa verticalis is a North American water bug (Heteroptera: Corixidae) that occurs in brackish and saline aquatic systems. Recently, it has been found invading three continents including Europe. Its invasive success has been attributed to the capacity of tolerating hypersalinity. We compared both the realized and standardized salinity niche of invasive T. verticalis and native Corixidae to verify if T. verticalis may fill in an unoccupied niche. We first established the field distribution of T. verticalis and native Corixidae along a salinity gradient. Second, we experimentally tested the salinity tolerance of T. verticalis and three common native Corixidae species. Of the seven Corixidae species found in the field study, three were positively related to the salinity gradient: S. selecta, S. stagnalis, and T. verticalis. T. verticalis showed the highest salinity optimum, however, after correcting for environmental background variation, salinity optima differed little among the three halophilic species. In the salinity tolerance experiment, S. selecta outperformed T. verticalis, which performed as well as S. stagnalis. Based on our experimental results, we cannot support the hypothesis that T. verticalis' invasion is mediated by a high salinity tolerance that allows this species to colonize habitats unoccupied by native Corixidae. Although these findings contrast with the field patterns in which T. verticalis showed the highest niche optimum and tolerance, these patterns may have been partly due to other environmental factors, particularly anthropogenic disturbance. Our comparative results are for adults only, and it remains possible that relative salinity tolerance patterns for juveniles differ from that for adults, which may add to the observed field pattern.status: publishe