Amphibians (class Amphibia, from Amphi- meaning on both sides and -bios meaning life) are a class of vertebrate animals, characterized as non-amniote ectothermic (or cold-blooded) tetrapods. Most amphibians undergo metamorphosis from a juvenile water-breathing form to an adult air-breathing form, but some are paedomorphs that retain the juvenile water-breathing form throughout life. Mudpuppies, for example, retain juvenile gills in adulthood.
The three modern orders of amphibians are
Figure 7. Atelopus certus calling male.
"Atelopus certus calling male" by brian gratwicke, Wikimedia Commons is licensed under CC BY 2.0
Figure 8. Shenandoah Salamander
" Shenandoah Salamander" by brian gratwicke, Wikimedia Commons is licensed under CC BY 2.0
Figure 9. Caecilians with its head in burried in the water.
"Caecilian" by Dawson, Wikimedia Commons is licensed under CC BY-SA 2.5
Many amphibians lay their eggs in water. Amphibians are superficially similar to reptiles, but reptiles, like mammals and birds, are amniotes.
Compared to those of amniotes, amphibians' lungs are primitive, possessing few internal septa, large alveoli, and a consequently slow diffusion rate of oxygen into the blood. Ventilation is accomplished by buccal pumping, though most amphibians are able to exchange gases with the water or air via their skin, cutaneous respiration. To enable cutaneous respiration and a sufficient diffusion rate of oxygen, the surface of amphibians' highly vascularized skin must remain moist. Because oxygen concentration in water increases at both low temperatures and high flow rates, aquatic amphibians like the Titicaca water frog or the hellbender salamander can rely primarily on cutaneous respiration. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange; these species include the plethodontid salamander, which has neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some, such as the axolotl, retaining gills as aquatic adults.
Most amphibians require freshwater environments in order to reproduce. A few can inhabit brackish water, and some can survive in seawater, but no true marine amphibians exist. Several hundred frog species in adaptive radiations, however, do not need any water for breeding in the wild. They reproduce via direct development, an ecological and evolutionary adaptation that has eliminated their dependency on free-standing water. Almost all of these frogs live in wet tropical rainforests and pass through the tadpole inside the egg. As a result, when these eggs hatch, they yield miniature versions of the adult frog. Reproductive success of many amphibians is dependent not only on the quantity but on the seasonal timing of rainfall.
Several species have also adapted to arid and semi-arid environments, but most still need water to lay their eggs. Symbiosis with single-celled algae that live in the jelly-like layer of the eggs has evolved several times. The larvae of frogs (i.e., tadpoles or polliwogs) initially breathe through exterior gills; later, a pouch forms and covers the gills and the front legs, and lungs form quite early. Newt larvae, which acquire their adult form at an early age, have large external gills that gradually disappear.
Frogs and toads have a distinct tadpole stage, in which they are grazing algae, ongrowth, or filtering plankton until reaching the size necessary for metamorphosis. This metamorphosis typically lasts 24 hours and consists of several changes:
Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades in locations around the world, and amphibian declines are regarded as a particularly critical threat to global biodiversity. The declines are traced to a number of causes, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, endocrine-disrupting pollutants, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the reasons for amphibian declines are still poorly understood and remain topics of ongoing discussion. A global strategy to stem the crisis has been released in the form of the Amphibian Conservation Action Plan.
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