Toxicity of malathion during Senegalese sole, Solea senegalensis larval development and metamorphosis: Histopathological disorders and effects on type B esterases and CYP1A enzymatic systems

The toxicity of malathion to Solea senegalensis was studied in a static renewal bioassay during its first month of larval life (between 4 and 30 dph). Through the use of different biomarkers and biochemical, cellular and molecular approaches (inhibition of cholinesterases [ChEs], changes in cytochrome P450-1A [CYP1A] and the study of histopathological alterations), the effects of three concentrations of malathion (1.56, 3.12, and 6.25 μg/L) have been analyzed. In subacute exposure, malathion inhibited cholinesterase activities (AChE, BChE, CbE) in a dose- and time-dependent manner, ranging the inhibition percentage from 20% to 90%. However, the expression levels of CYP1A and AChE transcripts or proteins were not modified. Additionally, exposure to malathion provoked histopathological alterations in several organ systems of Senegalese sole in a time- and dose dependent way, namely disruption of parenchymal architecture in the liver, epithelial desquamation, pyknotic nuclei and steatosis in the intestine, disorganization of supporting cartilage, and sings of hyperplasia and hypertrophy in the gills and degeneration of the epithelial cells from the renal tubules. Malathion exposure also provoked strong disorganization of cardiac fibers from the heart. The findings provide evidence that exposure to sublethal concentrations of malathion that provoked serious injury to the fish S. senegalensis, were below the expected environmental concentrations reported in many other ecosystems and different fish species,revealing a higher sensitivity for Solea senegalensis to malathion exposure, thus reinforcing its use as sentinel species for environmental pollution in coastal and estuarine environments.