Metamorphosis: Amphibians aren’t always amphibious

The word ‘metamorphosis’ comes from the Greek words ‘meta’ meaning change, ‘morph’ which refers to shape, and ‘osis’ meaning a process. Metamorphosis is the process of changing the shape of the body. The word ‘amphibian’ comes from the Greek word ‘amphibious’, which loosely translates to ‘living a double life’. This relates to amphibian’s aquatic and terrestrial life.

Frogs are amphibians and they lay eggs in water. Eggs hatch and tadpoles emerge. Tadpoles have gills, a tail fin, and no arms or legs. They are amphibians, but they’re not amphibious. They can’t survive on land until they undergo metamorphosis.

This tadpole was discovered in ECHO's mossy frog exhibit on Aug. 3, 2020 at ECHO.

The tadpole’s small circular mouth is well-suited to munch on aquatic plant material and dead insects. As they go through metamorphosis, they develop a jaw, a wider mouth opening, and a tongue capable of catching insects. A tadpole’s long spiral shaped intestine is appropriate for the digestion of aquatic plant material. As tadpoles change into frogs, their digestive tract shortens and develops folds to better digest insects. Perfect for a hungry frog.

The external gills, essential for the tadpole’s aquatic life, are eventually covered by a gill sac, and lungs quickly form. Front legs form under the gill sac and soon after, hindlegs develop. The

tail shrinks, is reabsorbed, and provides nutrition for the evolving tadpole. The skin changes to prevent dehydration, a new threat for an animal about to hop up onto land.

Tadpoles eyes, on the sides of their head, provide little overlap in each eye’s field of view. This creates a wide field of view, which helps elude predators but is little help in capturing fast moving food like insects. As the tadpole’s skull changes, their eyes move forward on the skull and the field of view of the eyes overlap. Overlapping or stereoscopic vision improves the frog’s ability to determine how far away things are, which increases the possibility of successfully capturing a meal. Try this to appreciate stereoscopic vision: close one eye and reach out to touch something in front of you. Then try with both eyes open. It’s easier to determine how far away something is with both eyes open, with overlapping fields of view – stereoscopic vision.

In just a few weeks, an aquatic tadpole goes through metamorphosis and becomes an amphibious frog, capable of living in water and on land. Can you think of other animals that go through metamorphosis? Here’s a hint: pollinators.

Animal Care Staff coordinate environmental programs at ECHO, Leahy Center for Lake Champlain. The center, which works to engage families in the wonder of nature, and care of Lake Champlain., partnered with the Burlington Free Press to publish this feature.