Histomorphological responses of red shiner (Cyprinella lutrensis) to atrazine, terbufos, and their mixture

Alterations of normal histomorphology of fish tissues, behavior, and thermal tolerance are recognized by environmental toxicologists and fish biologists as powerful tools indicating diverse biochemical and physiological changes. Toxicity of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), terbufos (S-(((1,1-dimethyl-ethyl)thio)methyl)O,O-diethyl phosphorodithioate), and their mixture to red shiner (Cyprinella lutrensis) at 23$\sp\circ$C and 30$\sp\circ$C was investigated in 14-d bioassays after 14-d acclimation. During the bioassays, fish behavior, indications of toxicosis, and external anomalies were observed. After the bioassays, the critical thermal maximum (CTM) was determined. Fish also were preserved for examinations of gill, liver, and kidney tissues using scanning electron microscopy (SEM) and light microscopy (LM). SEM and LM revealed exacerbated gill lesions in pesticide-exposed fish, distinguishable from the pre-existing mild lesions in the controls. Gill abnormalities included curling, clubbing, fusion, hyperplasia causing fusion, shortening of lamellae, epithelial swelling, hemorrhagic globes, respiratory epithelial uplifting, alteration of mucous cells, coagulation of mucus, and reduction or loss of pavement cell microridges. Gill lesions in fish exposed to the pesticide mixture were similar to those from exposure to each chemical alone, however some effects were more severe, suggesting additive or synergistic effects. Liver cytopathology were more prominent at 100 $\mu$g terbufos/L and 1000 $\mu$g atrazine/L + 100 $\mu$g terbufos/L than at 1000 $\mu$g atrazine/L. Kidneys of fish exposed to terbufos and the mixture exhibited flower-like appearance of Bowman capsules and proliferation in number and size, while atrazine had no effect. Overall survival was lowest for the pesticide mixture (92% at 23$\sp\circ$C or 48% at 30$\sp\circ$C), in comparison to atrazine only at 30$\sp\circ$C (54%), terbufos (97% at 23$\sp\circ$C and 91% at 30$\sp\circ$C), or the controls (100% survival) at both temperatures. Altered fish behavior (e.g., hyperactivity, loss of schooling, over-schooling, hypoactivity, inhibition of feeding, loss of equilibrium) and anomalies (e.g., light coloration, softness of muscle tissues, scoliosis) were evident after exposure. CTM decreased significantly after exposure to the pesticides at both temperatures. This combination of histomorphological, behavioral, and thermal tolerance changes demonstrated how atrazine and terbufos can affect C. lutrensis in nature, rendering them more susceptible to predation, competition, or disease