Abstract: To explore the effects of nano-scale microplastics on the health status of Gosner stage 37 tadpoles of Dark-spotted frogs, Pelophylax nigromaculatus, the tadpoles were exposed to low, moderate, or high concentration of fluorescent polystyrene microspheres (FPMs, 50 nm in diameter) solution for 7 days (day 7), then they were transferred to clear water and housed for 7 days (day 14) or 14 days (day 21). Clear water group was used as the control. The variations of the ratio of body mass to total body length, the ratio of neutrophils to lymphocytes (N/L ratio), and the responsiveness to phytohemagglutinin (PHA-P) were measured in multiple sampling time points for each group. On day 14, the ratio of body mass to total body length was significantly lower in control than that of any other FPMs groups. On day 21, the N/L ratio was significantly lower in control than that of any other FPMs groups, which was greatly increased from day 7 to day 21 for low, moderate or high concentration groups. The peak of the responsiveness to PHA-P for different concentration groups of FPMs on day 7 was observed after injection 3 h or 4 h, and occurred forwards after injection 2 h or 3 h on day 14, then gradually returned to that after injection 3 h or 4 h on day 21. On day 21, the peak of the responsiveness to PHA-P in high concentration group was higher than that of low and moderate concentration groups. Exposure and primary clearance of FPMs significantly promoted the growth and development of tadpoles of Dark-spotted pond frog, but such effect disappeared with increasing duration of treatments. Meanwhile, the stress level of the tadpoles responding to external stimuli was significantly increased, but not at the expense of suppressed immune function.

Key words: tadpole, fluorescent polystyrene microsphere, phytohemagglutinin-P, stress.