Cold – blooded animals, amphibians are vertebrates (animals with a backbone) that evolved from fish. They are adapted for life on land, but most must return to water in some form to breed. They undergo a process known as Metamorphosis in their development from larvae to adult, hence the Greek origin of the name: Amphi meaning double and Bios meaning life. There are three groups and more than 3,000 species.
Apart from the caecilians and few species of salamander, adult amphibians have four legs, each with four or five digits. Most species take to the water to mate and produce their eggs, but some make nests on land occasionally in burrows in the ground or in moss.
There are three groups of amphibians: the worm – like caecilians; the tailed amphibians, including newts and salamanders and the tailless frogs and toads, probably the most diverse group.
Distribution of Amphibians:
Amphibians live everywhere. Desert species survive the driest season by staying underground inside a membranous sac which they secrete themselves. Some temperate species hibernate in pond mud in the winter.
Caecilians are legless, carnivorous amphibians most of which live in the tropics. Some species burrow in the ground. Others are aquatic. They have small eyes and ears and sensory tentacles on the head.
Frogs and Toads:
In temperate regions, frog are more aquatic than toads, have slimier skin and longer legs. In the tropics, some species of frog and toad are fully aquatic and live in trees or underground.
Newts and Salamanders:
The tailed amphibian’s newts, salamanders and the eel – like sirens of North America – live in tropical forests, temperate woods, mountain streams and lakes. Some have very specialized lifestyles. A few even live in the total darkness of caves.
Amphibian skin is thin and scaleless. It is usually kept moist with mucus to increase its ability to allow oxygen through for skin breathing. Skin can be smooth or rough. In secretes certain chemicals. Pheromones can attack potential mates while poisons deter predators. As they grow, amphibians shed the top layer of skin.
Amphibians may have skin colours that absorb or reflect heat. Colour also varies with temperature, becoming pale when warm and darker if cold and damp.
Many frog and toads are camouflaged to avoid detection by predators. Most have to combination of forest colors and disruptive patterning. Some rainforest species are shaped to look like dead leaves.
The bright colours of Colombian poison–draft frogs warn predators of their highly toxic skin. The tadpoles develop their skin poisons as their colours develop. Marine toads secrete a strong toxin through large glands behind the head.
Many frogs and toads have smooth skin covered by mucus. Other amphibians, such as the mandarin salamander and many dry–skinned toads, have raised nodules.
The development from an aquatic larva that breathes through gills or spiracles to an air-breathing adult is called metamorphosis. It involves the growth of legs and the loss of the tail in frogs and toads.
Amphibian eggs are laid singly in clumps or insuring of clear Jelly called spawn. They have to shell and require a moist environment to survive.
Larvae or tadpoles hatch from the eggs. Salamander tadpoles have limbs but frogs and toads develop these during metamorphosis. Salamander larvae are carnivorous, but most frog and toad tadpoles are herbivorous.