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The term amphibian is derived from the ancient Greek term "amphíbios". It means both (amphi) and lives (bios) referring to how amphibians lives in both water and on land.
Amphibians are “cold-blooded” animals that lay their eggs in water and metamorphose from a juvenile water-breathing form in to an adult air breathing form. The class amphibia contains frogs, toads, newts, salamanders and ceacilians. It can sometimes be hard to see the differences between amphibians and reptiles; it can for instance be tricky to tell salamanders and lizards apart.
Amphibians evolved in the Devonian era from fish similar to lungfish and coelacanth. They were top predators during the Carboniferous and Permian periods, but their reign was ended by a mass extinction that made the way for the reptiles (including dinosaurs) to become dominant. The only group of amphibians survived the mass extinction and the early Jurassic era was the subclass Lissamphibia which contains all now living amphibians.
Amphibians are considered ecological indicators as they are sensitive to pollutions and environmental changes. The global population of amphibians has seen a dramatic decline with many species becoming critically endangered or even extinct. A lot of local populations have also vanished and the situation is similar all over the world. Scientists are not sure as to the reason for this dramatic decline, but according to the main hypothesis water pollution is the worse culprit. Other reasons for the dramatic decline in amphibian populations around the world are habitat loss, climate change, invasive species, over exploitation, disease and the destruction of the ozone layer. Amphibian skin is very sensitive to ultraviolet light. Attempts are being made to form and enact a global strategy for how global amphibian population should be saved.
Most amphibians need freshwater to propagate. There are no true saltwater species, but a few species can live in brackish water and will tolerate but not thrive in saltwater. There are several hundreds frog species that do not need water to breed; they reproduce via direct development which makes them independent of standing water. Most of these frog species live in the rainforest and hatch as miniature frogs. They still go through a tadpole phase but go through it before they hatch.
Amphibians that lay their eggs in water hatch as tadpoles that slowly develop into the form of the adult frog. They first get their hind legs, then their front legs. The tail then disappears in frogs and toads, while other types of amphibians keep the tail. Other changes include the fact that the gills are replaced with lungs, the skin changes to be suited to a life on land, eye lids grows and eyes are adapted to see on land, ear drums develop and so on. Most species leave the water when they have gone through this process (called metamorphosis), but some species stay and spend most of their life in water even though they occasional enters land.