Interactive Effects of Two Global Contaminants on Behavioral Variation in Brine Shrimp

Polycyclic aromatic hydrocarbons and chlorophenols are common aquatic pollutants from industrial and agricultural runoff that have adverse effects on living organisms and aquatic ecosystems in general. While aquatic wildlife is often exposed to these contaminants simultaneously, little is known about their combined effects. To fill this gap, we tested whether and how exposure to relevant concentrations of phenanthrene (Phe), 2,4-dichlorophenol (2,4-DCP), or a mixture of both altered key behaviors in brine shrimp (Artemia parthenogenetica), an emerging model in behavioral ecotoxicology. We exposed newborns to one of four treatments until adulthood: control, Phe (400 ng/L), 2,4-DCP (400 ng/L), and the combination of both Phe and 2,4-DCP (400 ng/L each). We repeatedly measured individual activity levels at adulthood in both the x-y plane (distance moved) and in three dimensions (mobility), as suggested for planktonic animals. We found that Phe, but not 2,4-DCP, significantly reduced average activity levels that were better captured by looking at activity in three than two dimensions. When combined, the pollutants appeared to exhibit an antagonistic interaction, with 2,4-DCP mitigating phenanthrene's negative impact on average mobility levels. Moving beyond average effects, we tested whether effects also appeared at the individual level. Our analysis revealed that within-individual variation (behavioral plasticity) in mobility decreased substantially in all exposure treatments compared to unexposed animals, whereas between-individual variation (behavioral individuality) did not vary. Our findings highlight the importance of considering interactive and individual-level effects of pollutants in ecological risk assessment, as effects may differ from predictions based on single contaminants and average responses alone.