17.3 Amphibians

1. 17.3 Amphibians

2. Objectives • Describe the origin of amphibians. • Describe the characteristics and adaptations of amphibians. • Explain the process of metamorphosis in frogs.

3. Movement onto Land • Imagine seeing a fish crawl across the highway. • In some places, such as parts of Florida, this sight is not uncommon. • When ponds dry up or food sources become scarce, fishes called walking catfish drag themselves across the ground. • They "walk" to new water holes.

4. Millions of years ago, this kind of ability to move over land for short periods of time led to the evolution of amphibians. • Amphibians evolved from a group of lobe-finned fishes that are now extinct.

5. The lobed fins of these fishes were strong and bony. • As they evolved, the fins became stronger and more supportive. • They made possible both swimming and a kind of crawling on land. • Fossil records suggest that these fins evolved into the two pairs of legs that allow modern amphibians to live both on land and water.

6. Characteristics of Amphibians • As amphibians evolved, these four-legged vertebrates continued to adapt to a land environment. • The gills and simple lungs that developed in their fishlike ancestors evolved into lungs better able to exchange gases with the atmosphere.

7. Amphibians also began to use their skin for respiration. • No longer covered by bony scales, the skin could be an extra site for gas exchange. • In fact, some amphibians today lack lungs and gills and use the moist surface of their skin and mouth to meet all their respiratory needs.

8. Although most amphibians are adapted to life on land, they remain dependent upon water. • Except for some species, such as desert toads, amphibians require moist environments. • Most live in or near bodies of water, or spend much of their time in moist burrows.

9. Amphibians also depend on water for reproduction. • Their eggs lack a protective, waterproof shell. • Since the eggs can easily dry out, most amphibians must return to the water to reproduce.

10. In most species, the female lays eggs in a pond, swamp, or stream, and they are fertilized externally by the male. • A few species have evolved interesting ways of adapting to living where water is scarce. • Some desert toads, for example, produce a moist foam in which they lay their eggs.

11. The eggs of many amphibians develop into a fishlike larval stage called a tadpole. • In amphibian species where the adult remains a water-dweller, the changes from the tadpole stage to the adult stage are minor. • In amphibians such as frogs and toads, the tadpole changes completely as it becomes an adult.

12. Frogs and Toads • Amphibians are divided into several groups. • One group includes frogs and toads, the two kinds of amphibians that do not have tails as adults. • Most frogs spend their lives in and around water.

13. When frogs reproduce, the female lays hundreds • Of jellylike eggs. • As the eggs pass out of her body, the make fertilizes them. • Within several weeks, the eggs hatch into tadpoles.

14. Tadpoles are a water-dwelling stage in amphibian development. • Like fishes, tadpoles have gills, fins, tails, and a streamlined body form.

15. As a tadpole develops, it undergoes great changes in • appearance. • These changes during development make up the process called metamorphosis.

16. When metamorphosis is complete, the adult frog emerges from the water as a land-dwelling vertebrate. • Most toads have a similar metamorphosis. • Not all frogs and toads have a tadpole stage, however. • In some species, a tiny frog or toad emerges directly from the egg.

17. Salamanders and Newts • Amphibians with tails make up another group that includes the salamanders and newts. • Like frogs, salamanders and newts have a smooth, moist skin. • Some live in relatively dry habitats similar to where toads live. • Others prefer very moist places, and a few even live in water their entire lives.

18. End