 There is considerable evidence that parasitic trematode platyhelminths ( a type of fluke ) have contributed to developmental abnormalities and population declines of amphibians in some regions . These trematodes of the genus Ribeiroia have a complex life cycle with three host species . The first host includes a number of species of aquatic snails . The early larval stages of the trematodes then are transmitted into aquatic tadpoles , where the <unk> ( larvae ) <unk> in developing limb buds . These encysted life stages produce developmental abnormalities in post @-@ metamorphic frogs , including additional or missing limbs . These abnormalities increase frog predation by aquatic birds , the final host of the trematode . 
