Annual Research & Review in Biology

Heavy metal pollution in aquatic environments remains a persistent concern because metals accumulate in fish tissues and interfere with essential physiological functions. In euryhaline fishes, metal toxicity is closely linked to salinity, which influences metal uptake, speciation, bioavailability, ion competition, membrane permeability and the energetic cost of osmoregulation. Under low-salinity conditions, fish generally absorb higher metal loads through the gills, digestive tract and, in early life stages, the skin. In contrast, higher salinity often reduces the activity of free metal ions through chloride binding and competition with other ions. However, these effects vary according to metal type, fish species, life stage, exposure duration and water chemistry. The gills, liver and kidneys are the main organs involved in metal uptake, detoxification, ion regulation and excretion, making them particularly vulnerable to toxic injury. At the cellular level, heavy metals can induce oxidative stress, lipid and DNA damage, mitochondrial dysfunction, altered gene expression and cell death. Salinity may intensify or reduce these effects by changing metal absorption and the energetic demands of salt balance. Fish respond through antioxidant enzymes, metallothioneins, ion transporters, heat-shock proteins and mitochondrial regulation. This review synthesises current evidence on salinity-modulated heavy metal toxicity in euryhaline fishes at physiological, cellular and molecular levels, identifies key knowledge gaps and outlines future research needs for ecological risk assessment, environmental monitoring and fish health.