Acute toxicity of trichlorfon and histological changes in the gills of Arapaima gigas, a neotropical fish from Amazon

Trichlorfon is a widely used drug to control ectoparasites in fish farming and is effective in treating protozoa, helminths, and crustaceans. However, prior to a concentration recommendation made for therapeutic purposes in aquaculture, it is necessary to establish its toxicity, behavioural and histological effects, and determine a safe use dose for each target species. Here we investigated the acute toxicity (LC₅₀-₁₂ ₕ) of trichlorfon and its behavioural and histological effects on arapaima (Arapaima gigas) larvae and juveniles. For the acute toxicity tests (LC₅₀), arapaima larvae and juveniles were exposed at 0, 50, 250, 500, 750, 1.000 and 1.250 mg L⁻¹ of trichlorfon for 12 h. During this period, behavioural changes, mortality, and the water physico-chemical parameters were recorded. Immediately after a toxicity test, gills were collected for the histological analysis. The LC₅₀–₁₂ ₕ for the arapaima larvae and juveniles were 411.8 mg L⁻¹ and 587.3 mg L⁻¹, respectively. The fish exposed for different times to the 250, 500, 750, 1.000 and 1.250 mg L⁻¹ concentrations showed a dose-dependent effect with behavioural changes, such as erratic swimming, loss of balance, lethargy, gasping at the water surface and muscle spasms. Histological changes, such as lamellar hyperplasia and cell hypertrophy, total lamellar fusion, dilation, and vascular congestion in the central filament vessel, interlamellar hyperplasia, capillary congestion, proliferation of mucous cells, dilation of the venous sinus and capillaries, leukocyte infiltrate and haemorrhage, were observed and ranged from mild to diffuse lesions. However, branchial necrosis was presented only in the fish exposed to ≥ 500 mg L⁻¹ of trichlorfon. In conclusion, sublethal trichlorfon concentrations (≤ 250 mg L⁻¹) and short exposure times (up to 180 min) can be safely used in arapaima larvae and juveniles because they do not alter their normal behaviour nor induce mortalities and/or irreversible injuries to their gills.