Pre-exposure to seawater or chloride salts influences the avoidance-selection behavior of zebrafish larvae in a conductivity gradient

The risk assessment of freshwater salinization is constructed around standard assays and using sodium chloride (NaCl), neglecting that the stressor is most likely a complex mixture of ions and the possibility of prior contact with it, triggering acclimation mechanisms in the freshwater biota. To date, as far as we are aware of, no in-formation has been generated integrating both acclimation and avoidance behavior in the context of salinization, that may allow these risk assessments upgrading. Accordingly, 6-days-old Danio rerio larvae were selected to perform 12-h avoidance assays in a non-confined 6-compartment linear system to simulate conductivity gradi-ents using seawater (SW) and the chloride salts MgCl2, KCl, and CaCl2. Salinity gradients were established from conductivities known to cause 50% egg mortality in a 96-h exposure (LC50,96h,embryo). The triggering of accli-mation processes, which could influence organisms' avoidance-selection under the conductivity gradients, was also studied using larvae pre-exposed to lethal levels of each salt or SW. Median avoidance conductivities after a 12-h of exposure (AC50,12h), and the Population Immediate Decline (PID) were computed.All non-pre-exposed larvae were able to detect and flee from conductivities corresponding to the LC50,96h, embryo, selecting compartments with lower conductivities, except for KCl. The AC50,12h and LC50,96h overlapped for MgCl2 and CaCl2, though the former is considered as more sensitive as it was obtained in 12 h of exposure. The AC50,12h for SW was 1.83-fold lower than the LC50,96h, thus, reinforcing the higher sensitivity of the parameter ACx and its adequacy for risk assessment frameworks. The PID, at low conductivities, was solely explained by the avoidance behavior of non-pre-exposed larvae. Larvae pre-exposed to lethal levels of salt or SW were found to select higher conductivities, except for MgCl2. Results indicated that avoidance-selection assays are ecologically relevant and sensitive tools to be used in risk assessment processes. Stressor pre-exposure influenced organisms' avoidance-selection behavior under conductivity gradients, suggesting that under salini-zation events organisms may acclimate, remaining in altered habitats.

Automated summary

The risk assessment of freshwater salinization is often based on standard assays using sodium chloride, but neglects the fact that salinization is a complex mixture of ions and can trigger acclimation mechanisms in freshwater organisms. This study aimed to integrate acclimation and avoidance behavior in salinization risk assessments by conducting avoidance assays using different salts and seawater in a linear system. The results showed that avoidance-selection assays are ecologically relevant and sensitive tools for risk assessment, and pre-exposure to stressors influenced organisms' avoidance-selection behavior under conductivity gradients, suggesting that organisms may acclimate to salinization events.