Exposure of Xenopus laevis Tadpoles to Cadmium Reveals Concentration-dependent Bimodal Effects on Growth and Monotonic Effects on Development and Thyroid Gland Activity

Xenopus laevis were exposed to 0-855 microg cadmium (Cd)/l (measured concentrations) in FETAX medium from fertilization to 47 days postfertilization. Measurements included embryonic survival and, at 47 days, tadpole survival, snout-vent length, tail length, total length, hindlimb length, weight, Nieuwkoop-Faber (NF) stage of development, initiation of metamorphic climax (> or = NF 58), and thyroid follicle cell height. Embryonic and larval survival were unaffected by Cd. Relative to control tadpoles, reduced tail and total length were observed at 0.1- 8 and at 855 microg Cd/l; and reduced snout-vent length, hindlimb length, and weight were observed at 0.1-1 and at 855 microg Cd/l. Mean stage of development and rate of initiation of climax were unaffected by Cd at 0-84 microg/l; however, none of the tadpoles exposed to 855 microg Cd/l progressed beyond mid-premetamorphosis (NF 51). Thyroid glands with fully formed follicles were observed in all tadpoles > or = NF 49 examined. Follicle cell height was unaffected by Cd at 0-84 microg/l but it was reduced at 855 microg/l; in the latter, cell height was reduced even when compared with NF 49-51 tadpoles pooled from the 0 to 84 microg Cd/l groups. In conclusion, (1) Cd affected tadpole growth in a bimodal pattern with the first and second inhibitory modes at concentrations below and above 84 microg Cd/l, respectively; (2) exposure to high Cd concentrations (855 microg/l) reduced thyroid activity and arrested tadpole development at mid-premetamorphosis; and (3) unlike its effect on growth, Cd inhibited tadpole development and thyroid function in a seemingly monotonic pattern.