Abstract: It is well known that chitosan has no toxicity to animals. Surprisingly, however, chitosan has an acute lethal effect on loach (Misgurnus anguillicaudatus). The underlying mechanism deserves to be explored. In this study, we evaluated the effect of different types and concentrations of chitosan on the death of loach, and this study also observed symptoms during the dying process and measured half lethal concentration (LC50), respiration rate, cell membrane permeability and erythrocyte cell nucleus abnormality. Results showed that all types of chitosan caused loach to rapidly die. Higher chitosan concentration or higher temperature accelerated the dying process. The LC50 of chitosan for loach was determined as 16 mg/L. In the presence of chitosan, large quantities of mucilage fell off the surface of loach skin, thus making the water appear turbid. Moreover, chitosan destroyed the structure of loach skin, affected gill respiration, stimulated loach to rapidly swim at the early stage of the experiment, and significantly increase respiration rate, resulting in a dissolved oxygen concentration below the safe limit. After using up all the glycogen, loach could not come up to the water surface and have to respire through intestine at the later stage of the experiment. Chitosan also led to exudation of large amounts of chlorine, sodium, potassium ion and ammonium from loach and destroyed cell functions. In conclusion, chitosan exerts its lethal effect on loach mainly by destroying therespiratory system. Since chitosan normally does not enter the human respiratory system, it can be used safely as a food additive.

Key words: chitosan, loach, death, toxicity