Amphibians, in the simplest definition, are animals that live in both water and on land. The word ‘Amphibia’ means dual lives. Amphibians are cold-blooded vertebrates that include the well-known frogs and toads. Being cold-blooded means that they depend on environmental sources of heat to regulate their body heat and temperature. The class of Amphibia is made up of more than 3,500 species which include the various order of amphibians. Most amphibians begin their lives in water and eventually adapt to life on land by developing lungs and limbs that allow them to move on land. The larvae mature while in the water. At this young stage, the offspring breathe through the gills and after some time they develop lungs through a process known as metamorphosis. The class of Amphibia is made of three orders namely; Anura (toads and frogs), Urodela or Caudata (newts and salamanders), and Apoda or Gymnophiona (caecilians).
How Did Amphibians Evolve?
About 400 million years ago in the Devonian era, amphibians evolved from fish. The main reason for this evolutionary process was because of the rapid increase in the spread of dry land on earth. As a result, certain fish adapted to the changing conditions by developing limbs to crawl on land and lungs to breathe while out of water. The evolutionary process due to environmental change led to the rise of amphibians, the ‘double-life’ organisms. Amphibians also developed a backbone and became the first vertebrates to live on land. They returned to water for breeding purposes while feeding mostly on land. Between 340-230 million years ago, the planet experienced alternating periods of wet and dry conditions which allowed for the occurrence of the largest variety of amphibians. However, only a few groups of amphibians survived until the present age, which can be traced back to no further than 200 million years ago.
Major Characteristics of Amphibians
Amphibians have characteristics that cross-over between fish and reptiles. At the youngster age, most of them function like fish while as adults, they have different characteristics that allow them to live on land. They are cold-blooded animals which regulate their body heat and temperature depending on the external environment.
Amphibians have scale-less skin that is very delicate and moist. They live close to water sources in order to dampen their skin. The skin greatly helps in regulating the body temperature but also makes them vulnerable to dehydration. In high temperatures, dehydration will lead to death. This is the reason why amphibians live close to marshes, swamps and ponds and other freshwater bodies.
They breathe oxygen through the skin. The skin plays an important role in gas exchange and in absorption of water. This is despite them having lungs which function rather poorly under certain conditions. The skin, therefore, plays a double role of protecting and absorbing the water and oxygen.
Certain frogs such as the brightly colored poison-dart frog, have skin that contains poisonous glands. The poison is used as a defense mechanism which can easily kill any predator or prey. The poison from frogs has been used by Native American Indians hunters to coat the tips of their spears and arrows.
The Three Orders of Amphibians
All amphibians are classified according to bodily characteristics of their legs and tails.
Anura is the largest order of living amphibians with over 3,000 different varieties. Toads and frogs fall under the order of Anura. This group lacks a tail and are characterized by long hind limbs that are adapted for swimming and leaping. Anura amphibians live in freshwater regions although some may be found in drier habitats. Frogs and toads are different in their body characteristic. Toads usually have shorter hind limbs and drier skin that appears warty, while frogs have a thin smooth skin and long hinder limbs. Anura amphibians feed on a variety of invertebrates such as insects. They can also feed on small mammals, birds, and fish.
Newts and salamanders fall under this category. The largest amphibian, the Japanese salamander, measures up to 1.5 meters while the smallest member of this order measures 10 centimeters in length. In this order, the tail is more pronounced than the limbs which are usually underdeveloped. Their preferred habitat is near water bodies and under moist soil and rocks. They mostly feed on insects and worms. Some species live in water, such as the genus Siren, while others burrow in the mud. They have lungs and external gills to aid in breathing.
Apoda consists of about 205 species. They are shaped like worms, legless, and blind. They can be found in mud where they live, especially in the tropical soils of Africa and South America. They measure between 10 centimeters and 1 meter in length.
Amphibian Life Cycle
The life of amphibians begins in water where the female lays eggs that are externally fertilized. After the eggs hatch into tadpoles, they breathe through external gills. Tadpoles have flat tails that are used for swimming, and feed on aquatic vegetation. Eventually, through metamorphosis, they experience physical changes that make them adults. This includes the developing of lungs and elaborate limbs that aid them in movement on land.
Important Roles of Amphibians
Amphibians such as frogs are vital to the balance of ecosystem in which they inhabit, both as predators or prey. They feed on pests and insects thereby reducing the spread of diseases to agricultural plants. This indirectly benefits agriculture. In certain cultures around the world, frogs are viewed as a source of luck and are cherished as important symbols in society. In medical research, the skin of amphibians is being studied due to their ability to resist virus infections. This could eventually provide an advance in the treatment of virus diseases such as AIDS.
Main Threats to the Existence of Amphibians
Today, the number of amphibian species has continued to decline due to a variety of reasons. This includes the pollution of freshwater ecosystems which provide habitat for most of the species. Ultraviolet radiation has also affected the thriving of amphibians due to their fragile skin. Additionally, diseases such as the Chytrid fungus have depopulated many amphibian habitats. Many have been wiped out at a rapid rate to the extent that they are not even noticeable. The loss of amphibians affects the balance of the ecosystem which in turn affects other animal and plant species on the planet.