- Describe amphibian traits.
- List the features of salamanders.
- Compare and contrast frogs and toads with other amphibians.
- Describe the roles of amphibians.
Check Your Understanding
- What are some adaptations that amphibians, like fish, have for living in the water?
- What are the characteristics that amphibians share with all other vertebrates?
- convergent adaptation
- hyoid bone
- valarian respiration
Characteristics of Amphibians
What group of animals begins its life in the water, but then spends most of its life on land? Amphibians! Amphibians are a group of vertebrates that has adapted to live in both water and on land. Their ancestors evolved from living in the sea to living on land. There are approximately 6,000 species of amphibians, of many different body types, physiologies, and habitats, ranging from tropical to subarctic regions. Frogs, toads, salamanders, newts, and caecilians are all types of amphibians (Figure below).
One of the many species of amphibian is this dusky salamander.
Like fish, amphibians are ectothermic vertebrates. They belong to the class Amphibia. There are three orders:
- Urodela, containing salamanders and newts.
- Anura, containing frogs and toads.
- Apoda, containing caecilians.
Amphibian larvae are born and live in water, and they breathe using gills. The adults live on land for part of the time, and breathe both through their skin and using lungs.
Where do Amphibians Live?
Most amphibians live in fresh water, not salt water. Although there are no true saltwater amphibians, a few can live in brackish (slightly salty) water. Some species do not need any water at all, and several species have also adapted to live in drier environments. Most amphibians still need water to lay their eggs.
How do Amphibians Reproduce?
Amphibians reproduce sexually. The life cycle of amphibians happens in the following stages:
- Egg Stage: Amphibian eggs are fertilized in a number of ways. External fertilization, employed by most frogs and toads, involves a male gripping a female across her back, almost as if he is squeezing the eggs out of her. The male releases sperm over the female's eggs as they are laid. Another method is used by salamanders, whereby the male deposits a packet of sperm onto the ground. The female then pulls it into her cloaca, where fertilization occurs internally. By contrast, caecilians and tailed frogs use internal fertilization, just like reptiles, birds and mammals. The male deposits sperm directly into the female's cloaca.
- Larval stage: When the egg hatches, the organism is legless, lives in water and breathes with gills.
- During the larval stage, the amphibian slowly transforms into an adult by losing its gills and growing four legs. Once development is complete, it can live on land.
How did Amphibians Adapt to Living on Land?
In order to live on land, amphibians replaced gills with another respiratory organ, the lungs.
Other adaptations include:
- A glandular skin that prevents loss of water.
- Eyelids that allow them to adapt to vision outside of the water.
- An eardrum developed to separate the external ear from the middle ear.
- In frogs and toads, the tail disappears in adulthood.
Salamanders belong to a group of approximately 500 species of amphibians. The order Urodela, containing salamanders and newts, is divided into three suborders:
- Giant salamanders (including the hellbender and Asiatic salamanders).
- Advanced salamanders (including lungless salamanders, mudpuppies, and newts).
Salamanders are characterized by slender bodies, short legs, and long tails. They are most closely related to the caecilians, little-known legless amphibians (Figure below). Since they have moist skin, salamanders live in or near water or on moist ground, often in a swamp. Some species live in water most of their life, some live their entire adult life on land, and some live in both habitats. Salamanders are carnivorous, eating only other animals, not plants. They will eat almost any smaller animal. Finally, salamanders have the ability to grow back lost limbs, as well as other body parts. This process is known as ecdysis.
Left: The marbled salamander shows the typical salamander body plan: slender body, short legs, long tail and moist skin. Right: A species of South American caecilian, Siphonops Annulatuss, a legless amphibian most closely related to salamanders.
How Do Salamanders Breathe?
Different salamanders breathe in different ways. In those that have gills, breathing occurs through the gills as water passes over the gill slits. Species that live on land have lungs that are used in breathing, much like breathing in mammals. Other land-living salamanders do not have lungs or gills. Instead, they "breathe", or exchange gases, through their skin. This is known as valarian respiration, and requires blood vessels that exchange gases to be spread throughout the skin.
How Big Are Salamanders?
Salamanders are found in most moist or arid habitats in the northern hemisphere. They are generally small, but some can reach a foot or more, as in the mudpuppy of North America. In Japan and China, the giant salamander reaches 6 feet and weighs up to 66 pounds (Figure below).
The Pacific giant salamander can reach up to 6 feet in length and weigh up to 66 pounds.
Frogs and Toads
Frogs and toads (Figure below) are amphibians in the order Anura. In terms of classification, there is actually not a big difference between frogs and toads. Some amphibians that are called "toads" have leathery, brown colored, wart covered skin, but they are still in the same order as frogs.
A toad, showing typical characteristics of leathery and warty skin, and brown coloration.
Frogs are found in many areas of the world, from the tropics to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species (about 88% of amphibian species are frogs), they are among the most diverse groups of vertebrates. Frogs range in size from less than 0.5 inches in species in Brazil and Cuba to 1-foot goliath frog of Cameroon.
Characteristics of Frogs
Adult frogs are characterized by long hind legs, a short body, webbed finger-like parts, and the lack of a tail (Figure below). They also have a three-chambered heart, as do all tetrapods except birds and mammals. Most frogs live part of the time in water and part of the time on land, and move easily on land by jumping or climbing. To become great jumpers, frogs evolved long hind legs and long ankle bones. They also have a short backbone with only 10 vertebrae. Frog and toad skin hangs loosely on the body, and skin texture can be smooth, warty, or folded.
In order to live on land and in water, frogs have three eyelid membranes: one is see-through to protect the eyes underwater, and two other ones let them see on land. Frogs also have a tympanum, which acts like a simple ear. They are found on each side of the head. In some species, the tympanum is covered by skin.
A tree frog. Notice the powerful muscles in the limbs and the coverings around the eyes.
Frogs typically lay their eggs in puddles, ponds or lakes. Their larvae, or tadpoles, have gills, and the frogs develop in water. You may hear males "ribbiting," producing a mating call used to attract females to the bodies of water best for mating and breeding. Frogs calls can occur during the day or night.
Listen here http://www.youtube.com/watch?v=mISMwN-0ggE for these distinctive sounds (3:30).
Adult frogs are meat-eaters and eat mostly arthropods, annelids, and gastropods. Frogs do not have teeth on their lower jaw, so they usually swallow their food whole, using the teeth they do have to hold the prey in place. Other frogs do not have any teeth, so they must swallow their prey whole.
Roles of Amphibians
Amphibians as Foods
Frogs are raised as a food source. Frog legs are a delicacy in China, France, the Philippines, northern Greece and the American south, especially Louisiana.
Amphibians in Research
Amphibians are used in cloning research and other branches of embryology, because their eggs lack shells, so it is easy to watch their development.
The African clawed frog, Xenopus laevis, is a species that is studied to understand certain biological phenomena in developmental biology, because it is easy to raise in a lab and has a large and easy to study embryo. Many Xenopus genes have been identified and cloned, especially those involved in development.
Many environmental scientists believe that amphibians, including frogs, indicate when an environment is damaged. Since they live on land and water, when species of frogs begin to decline, it often indicates that there is a bigger problem with the ecosystem.
Amphibians in Popular Culture
Amphibians can be found in folklore, fairy tales and popular culture. Numerous legends have developed over the centuries around the salamander (its name originates from the Persian, for “fire” and “within") , many related to fire. This connection likely originates from the tendency of many salamanders to live inside rotting logs. When placed into the fire, salamanders would escape from the logs, lending to the belief that the salamander was created from flames.
- Amphibians have adaptations for both aquatic (gills), and terrestrial (lungs and moist skin) lifestyles.
- Most amphibians must reproduce in water.
- Development includes a shell-less egg, larval stage, and adult stage.
- Salamanders have some unique features, including the use of the hyoid bone in hunting prey, and the process of ecdysis.
- Adult frogs and toads have features for living in the water (such as webbed finger-like parts) and for living on the land (such as long hind legs for jumping).
- Frogs are well known for their mating calls, which are used to attract females to aquatic breeding grounds.
- Amphibians play a role as a food source, are used in various types of biological research, can serve as indicators of ecosystem health, and are found in folklore and popular culture.
1. Describe three general traits of amphibians.
2. Describe the life cycle of amphibians from egg stage to adult stage.
3. What are two ways that amphibians have adapted to living on land?
4. Why would a scientist want to use a frog for research?
5. Name one way amphibians have evolved to avoid predation.
6. A frog’s skin must remain moist at all times in order for oxygen to pass through the skin and into the blood. Why does this fact make frogs susceptible to many toxins in the environment?
Further Reading / Supplemental Links
Points to Consider
Future studies of molecular genetics should soon provide further insights to the evolutionary relationships among frog families. These studies will also clarify relationships among families belonging to the rest of vertebrates. We discuss reptiles next.
- Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian species care for their young, and that there is a great diversity of parental behaviors. As you begin to examine the reproductive system of reptiles in the next lesson, think about what kinds of parental behaviors reptiles might have and how they compare to that of amphibians.