Passage I

Frog eggs develop into tadpoles, which then metamorphose into adult frogs. Recently, reported observations of frogs with morphological abnormalities (usually a missing or malformed hind limb) have increased in many localities. Four hypotheses attempt to explain this increase.

Hypothesis 1

Tadpoles are commonly parasitized by trematodes, flatworms that burrow into the tadpoles, causing cysts to form. The presence of cysts during tadpole development can cause morphological abnormalities in the adult frog. Thus, increases in trematode populations most likely account for the recent increase in morphological abnormalities.

Hypothesis 2

Due to industrial pollution, Earth's ozone layer has thinned. This process has led to an increase in the amount of ultraviolet (UV) radiation reaching Earth's surface. UV radiation can directly cause abnormalities in frog eggs by producing mutations in the DNA that controls frog development. UV radiation can also indirectly cause abnormalities in frog eggs by chemically transforming certain pollutants into teratogens, chemicals that cause developmental abnormalities.

Hypothesis 3

Certain pesticides contain a class of compounds called retinoids. Frogs naturally produce retinoids, but at high concentrations, retinoids cause mutations in frog eggs. These mutations later result in morphological abnormalities. Many of these mutations affect genes that control hind limb development. Thus, the recent increase in morphological abnormalities is probably due to an increase in retinoids from pesticides.

Hypothesis 4

Tadpoles and frogs are prey for many aquatic predators, including fish, turtles, and wading birds such as herons. Predator attacks are not alwáys completely successful; many result in injured frogs and tadpoles. Permanent injuries in tadpoles and frogs could later appear to be developmental abnormalities. Thus, increases in predator populations most likely account for the. recent increase in morphological abnormalities.