Cadmium interference with visual performance in Danio rerio

Sight is the sense mainly used by teleosts to hunt for food and escape predators. As a consequence, the eyes are large and a complex visual behavior has been developed. This consist in a series of ‘instinctive’ responses that the animal puts into action when exposed to a stimulation of visual nature. These mechanisms are potentially threatened by the fact that the eye is an easy target for many xenobiotics: in fact, it is directly exposed to the environment and, also, contains a large vitreal chamber that facilitates contaminants distribution to all its compartments. In the present work, we verified whether cadmium, at environmental concentrations, interferes with the visually mediated behaviors in adult Danio rerio,a model system for behavioral screens since 1970’s. Re-illumination tests were carried out with white and colored lights (red, yellow, green and blue), to test color sensitivity in fish contaminated for 30 days. Results reveal that cadmium exposure induces a marked escape response to all light wavelengths. Parallel light and electron microscope investigations demonstrate that cadmium alters retinal organization: the ion, in particular, induces occasional retinal folding, a marked swelling, especially in the ganglion cell and, most important, induces degeneration among ganglion and inner nuclear layer cells. In conclusion, results indicate that cadmium has detrimental effects on visual behavior and that this probably depends on an altered signals transmission from the photoreceptors to the central nervous system. Further studies are required to fully understand the mechanisms underlying cadmium response in the retina; however, the evidences collected so far clearly indicate that animals living in contaminated sites have a reduced fitness. The implications at the ecological level are obvious.