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Effect of sublethal levels of malathion on the gills of Heteropneustes fossilis: scanning electron microscopic study.

Scanning electron microscopic studies of the gills of catfish, Heteropneustes fossilis, exposed to sublethal malathion (4 mg/L and 6 mg/L) revealed that 24-hour exposure to 4 mg/L had a mild effect. However, severe damage was found after 48- and 72-hour exposures. After a 24-hour exposure to a 6 mg/L concentration, more severe damage ensued. The microridged epithelial cells of the gill arch became perforated and the central portion of the filament appeared elevated. Numerous mucous gland openings also became visible. After 48- and 72-hour exposures, the damage and structural changes were more pronounced when compared with the 4 mg/L exposure. Enlarged mucous gland openings were found on the gill arch. The lamellar surface had many crevices, elevations and depressions. Broken microridges in the gill arch surface were visible at a 72-hour exposure. At 96 hours of exposure, structural recovery occurred to some extent in both the 4- and 6-mg/L exposures. Corrugation and dissociated epithelium along with some interlamellar bridges were evident after 72 hours of exposure to both concentrations. Such deleterious effects cause reduction in available water supply as well as available respiratory area that may result in decreased oxygen uptake. Consequently, fish may fail to get sufficient oxygen and asphyxiate.