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19.3: Amphibians

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Lesson Objectives

  • Describe structure and function in amphibians.
  • Outline the reproduction and development of amphibians.
  • Identify the three living amphibian orders.
  • Describe how amphibians evolved.
  • State where amphibians live and how they obtain food.

Vocabulary

amphibian
ectothermic, tetrapod vertebrate that may live on land but must return to water in order to reproduce
cloaca
body cavity with a single opening in amphibians, reptiles, and monotreme mammals that collects and excretes wastes from the digestive and excretory systems and gametes from the reproductive system
keratin
tough, fibrous protein in skin, nails, and hair
tetrapod
vertebrate with four legs (amphibian, reptile, bird, or mammal)

Introduction

Amphibians are vertebrates that exist in two worlds. They divide their time between freshwater and terrestrial habitats. They share a number of features with air-breathing lungfish, but they also differ from lungfish in many ways. One way they differ is their appendages. Amphibians are the first true tetrapods, or vertebrates with four limbs. Modern amphibians include frogs, salamanders, and caecilians, as shown Figure below.

Examples of Living Amphibians. In what ways do these three amphibians appear to be similar? In what ways do they appear to be different?

Structure and Function in Amphibians

Amphibians have less variation in size than fish, ranging in length from 1 centimeter (2.5 inches) to 1.5 meters (about 5 feet). They generally have moist skin without scales. Their skin contains keratin, a tough, fibrous protein found in the skin, scales, feathers, hair, and nails of tetrapod vertebrates, from amphibians to humans. Some forms of keratin are tougher than others. The form in amphibian skin is not very tough, and it allows gases and water to pass through their skin.

Amphibian Ectothermy

Amphibians are ectothermic, so their internal body temperature is generally about the same as the temperature of their environment. When it’s cold outside, their body temperature drops, and they become very sluggish. When the outside temperature rises, so does their body temperature, and they are much more active. What do you think might be some of the pros and cons of ectothermy in amphibians?

Amphibian Organ Systems

All amphibians have digestive, excretory, and reproductive systems. All three systems share a body cavity called the cloaca. Wastes enter the cloaca from the digestive and excretory systems, and gametes enter the cloaca from the reproductive system. An opening in the cloaca allows the wastes and gametes to leave the body.

Amphibians have a relatively complex circulatory system with a three-chambered heart. Their nervous system is also rather complex, allowing them to interact with each other and their environment. Amphibians have sense organs to smell and taste chemicals. Other sense organs include eyes and ears. Of all amphibians, frogs generally have the best vision and hearing. Frogs also have a larynx, or voice box, to make sounds.

Most amphibians breathe with gills as larvae and with lungs as adults. Additional oxygen is absorbed through the skin in most species. The skin is kept moist by mucus, which is secreted by mucous glands. In some species, mucous glands also produce toxins, which help protect the amphibians from predators. The golden frog shown in Figure below is an example of a toxic amphibian.

Toxic Frog. This golden frog is only about 5 centimeters (2 inches) long, but it’s the most poisonous vertebrate on Earth. One dose of its toxin can kill up to 20 humans!

Amphibian Reproduction and Development

Amphibians reproduce sexually with either external or internal fertilization. They attract mates in a variety of ways. For example, the loud croaking of frogs is their mating call. Each frog species has its own distinctive call that other members of the species recognize as their own. Most salamanders use their sense of smell to find a mate. The males produce a chemical odor that attracts females of the species.

Amphibian Eggs

Unlike other tetrapod vertebrates (reptiles, birds, and mammals), amphibians do not produce amniotic eggs. Therefore, they must lay their eggs in water so they won’t dry out. Their eggs are usually covered in a jelly-like substance, like the frog eggs shown in Figure below. The jelly helps keep the eggs moist and offers some protection from predators.

Frog Eggs. Frog eggs are surrounded by “jelly.” What is its function?

Amphibians generally lay large number of eggs. Often, many adults lay eggs in the same place at the same time. This helps to ensure that eggs will be fertilized and at least some of the embryos will survive. Once eggs have been laid, most amphibians are done with their parenting.

Amphibian Larvae

The majority of amphibian species go through a larval stage that is very different from the adult form, as you can see from the frog in Figure below. The early larval, or tadpole, stage resembles a fish. It lacks legs and has a long tail, which it uses to swim. The tadpole also has gills to absorb oxygen from water. As the larva undergoes metamorphosis, it grows legs, loses its tail, and develops lungs. These changes prepare it for life on land as an adult frog.

Frog Development: From Tadpole to Adult. A frog larva (tadpole) goes through many changes by adulthood. How do these changes prepare it for life as an adult frog?

Classification of Amphibians

There are about 6,200 known species of living amphibians. They are placed in three different orders:

  1. Frogs and toads
  2. Salamanders and newts
  3. Caecilians

Frogs and Toads

One feature that distinguishes frogs and toads from other amphibians is lack of a tail in adulthood. Frogs and toads also have much longer back legs than other amphibians. Their back legs are modified for jumping. Frogs can jump up to 20 times their own body length. That’s the same as you jumping at least 100 feet, or more than the length of a basketball court. Think how fast you could move if you could travel that far on one jump!

Frogs and toads are closely related, but they differ in several ways. Generally, frogs spend more time in water, and toads spend more time on land. As you can see from Figure below, frogs also have smoother, moister skin than toads, as well as longer hind legs.

Frog and Toad. Frogs (a) and toads (b) are placed in the same amphibian order. What traits do they share?

Salamanders and Newts

Unlike frogs and toads, salamanders and newts keep their tails as adults (see Figure below). They also have a long body with short legs, and all their legs are about the same length. This is because they are adapted for walking and swimming rather than jumping. An unusual characteristic of salamanders is their ability to regenerate, or regrow, legs that have been lost to predators.

Salamander and Newt. Salamanders and newts can walk or swim. Salamander on a leaf (left), newt swimming in the water (right).

Caecilians

Caecilians are most closely related to salamanders. As you can see from Figure below, they have a long, worm-like body without legs. Caecilians evolved from a tetrapod ancestor, but they lost their legs during the course of their evolution.

Swimming Caecilian. Caecilians are the only amphibians without legs.

Evolution of Amphibians

Fossil evidence shows that amphibians evolved about 365 million years ago from a lobe-finned lungfish ancestor. As the earliest land vertebrates, they were highly successful. Some of them were much larger than today’s amphibians. For more than 100 million years, amphibians remained the dominant land vertebrates. Then some of them evolved into reptiles. Once reptiles appeared, with their amniotic eggs, they replaced amphibians as the dominant land vertebrates.

Ecology of Amphibians

Amphibians can be found in freshwater and moist terrestrial habitats throughout the world. The only continent without amphibians is Antarctica. Amphibians are especially numerous in temperate lakes and ponds and in tropical rainforests.

Amphibians as Prey and Predators

Amphibians are an important food source for animals such as birds, snakes, raccoons, and fish. Amphibians are also important predators. As larvae, they feed mainly on small aquatic animals such as water insects. They may also feed on algae. As adults, amphibians are completely carnivorous. They may catch and eat worms, snails, and insects, as the frog in Figure below is doing. Unlike other amphibians, caecilians are burrowers. They use their head to dig in the soil, and they feed on earthworms and other annelids. Caecilians can be found in moist soil near rivers and streams in tropical regions.

Frog Predator. A frog eating its insect prey.

The Threat of Amphibian Extinction

Currently, almost one third of all amphibian species face the threat of extinction. The reasons include habitat loss, pollution, climate change, and the introduction of non-native species. Most of these problems are the result of human actions.

Amphibians have permeable skin that easily absorbs substances from the environment. This may explain why they seem to be especially sensitive to pollution. Monitoring the health and survival of amphibians may help people detect pollution early, before other organisms are affected.

Lesson Summary

  • Amphibians are ectothermic vertebrates that divide their time between freshwater and terrestrial habitats. They are the first true tetrapods, or vertebrates with four limbs. Amphibians breathe with gills as larvae and with lungs as adults. They have a three-chambered heart and relatively complex nervous system.
  • Amphibians reproduce sexually with either external or internal fertilization. They may attract mates with calls or scents. They do not produce amniotic eggs, so they must reproduce in water. Their larvae go through metamorphosis to change into the adult form.
  • There are about 6,200 known species of living amphibians that are placed in three orders: frogs and toads, salamanders and newts, and caecilians. Frogs and toads are adapted for jumping. Salamanders and newts may walk or swim. Caecilians live in the water or soil and are the only amphibians without legs.
  • Amphibians evolved about 365 million years ago from a lobe-finned fish ancestor. As the earliest land vertebrates, amphibians were highly successful for more than 100 million years until reptiles took over as the dominant land vertebrates.
  • Amphibians are found throughout the world except in Antarctica and Greenland. They are important prey for animals such as birds, snakes, and raccoons. They are important predators of insects, worms, and other invertebrates. Up to one third of all amphibian species are at risk of extinction because of human actions, such as habitat destruction, climate change, and pollution.

Lesson Review Questions

Recall

1. What is a tetrapod?

2. How does the temperature of the environment affect the level of activity of an amphibian?

3. What is the cloaca? What functions does it serve in amphibians?

4. Describe three different ways that amphibians may absorb oxygen.

5. Outline the life cycle of frogs.

Apply Concepts

6. Assume that a certain species of toad appears to be dying out in a given ecosystem, perhaps because of pollution. Many people think that the toad problem is unimportant because “it’s just a toad.” Write a letter to a hypothetical newspaper editor in which you explain why the health and survival of amphibians such as this toad species are important to all living things in an ecosystem.

Think Critically

7. Compare and contrast the three orders of living amphibians.

8. Explain why amphibians were able to become the dominant land vertebrates for millions of years.

Points to Consider

Amphibians gave rise to reptiles, which replaced them as the dominant land vertebrates.

  • Besides amniotic eggs, can you think of other ways that reptiles differ from amphibians?
  • What other adaptations might reptiles have evolved that contributed to their success on land?

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