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 information 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 gradients using seawater (SW) and the chloride salts MgCl ₂ , KCl, and CaCl ₂ . Salinity gradients were established from conductivities known to cause 50% egg mortality in a 96-h exposure (LC _{50,96h,embryo} ). The triggering of acclimation 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 (AC _50,12h ), and the Population Immediate Decline (PID) were computed. All non-pre-exposed larvae were able to detect and flee from conductivities corresponding to the LC _{50,96h,embryo} , selecting compartments with lower conductivities, except for KCl. The AC _50,12h and LC _50,96h overlapped for MgCl ₂ and CaCl ₂ , though the former is considered as more sensitive as it was obtained in 12 h of exposure. The AC _50,12h for SW was 1.83-fold lower than the LC _50,96h , thus, reinforcing the higher sensitivity of the parameter AC _x 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 MgCl ₂ . 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 salinization events organisms may acclimate, remaining in altered habitats.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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