What were the first land vertebrates?
Amphibians! In order for water-dwelling animals to adapt to life on land, many new adaptations had to take place. First, they needed to be able to breathe air instead of obtaining oxygen from water. And fins don't work well as legs! They needed to be able to move around well on land.
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. 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 with their lungs as their lungs are not sufficient to provide the necessary amount of oxygen.
There are approximately 6,000 species of amphibians. They have many different body types, physiologies, and habitats, ranging from tropical to subarctic regions. Frogs, toads, salamanders (Figure below), newts, and caecilians are all types of amphibians.
One of the many species of amphibian is this dusky salamander.
How did Amphibians Adapt to Living on Land?
Transition to life on land meant significant changes to both external and internal features. In order to live on land, amphibians replaced gills with another respiratory organ, the lungs. Other adaptations include:
- 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.
- A tail that disappears in adulthood (in frogs and toads).
Classification of the Amphibians
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.
Where do Amphibians Live?
Most amphibians live in fresh water, not salt water. Their habitats can include areas close to springs, streams, rivers, lakes, swamps and ponds. They can be found in moist areas in forests, meadows and marshes. Amphibians can be found almost anywhere there is a source of fresh 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, a single opening for her internal organ systems. Therefore, 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, resembling their evolutionary ancestors (fish).
- 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.
- Amphibians live in both water and on land; 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 with their lungs.
- Adaptations for land in amphibians include protective skin and eyelids that allow them to adapt to vision outside of the water.
Use the resources below to answer the questions that follow.
Explore More I
- San Diego Zoo Amphibians at http://kids.sandiegozoo.org/animals/amphibians
- Describe the skin of an amphibian.
- What is metamorphosis?
- What is meant by "ectothermic"? How does this affect an animals behavior?
Explore More II
- Respiratory Organs In Amphibians at http://www.youtube.com/watch?v=Nfojq4ikHH0 (3:29)
- How do frogs fill their lungs?
- Why do amphibians need less oxygen than birds or mammals?
- How do frogs breathe when they are underwater?
- List three adaptions amphibians have for life on land?
- List four examples of amphibians.
- How do amphibians reproduce?
- Describe the amphibian larval stage. What changes occur during this stage?