Question

why cant the tadpoles grow well in water without iodine?

Answer 1

If Iodine is not present in the diet ,the thyroid may produce an inefficient amount of hormone which will adversely affect the metamorphosis . so tadpoles grow in water with iodine.

Answer 2

in frogs, metamorphosis is controlled by thyroxine produced by the thyroid gland, iodine is required for the proper production of thyroxine. This is why tadpoles that grow in water that id deficient in iodine fail to develop into adult frog.