General features ( Amphibian )
Size range and diversity of structure
The three living groups of amphibians vary greatly in size and structure. Long tails and two pairs of limbs of about equal size distinguish the salamander, although members of the family Sirenidae have no hindlimbs. Their total length varies greatly; a diminutive terrestrial species in Mexico measures 27 millimetres (1.06 inches) long, and the giant aquatic salamander in China reaches a length of more than 1.5 metres (5 feet). Frogs and toads (anurans) have long hindlimbs and large eyes by which they can be easily identified. They lack tails and have only five to nine presacral vertebrae. The West African goliath frog, which is 300 millimetres from snout to vent and weighs 3.3 kilograms (7.28 pounds), is the largest anuran; the smallest is the Brazilian brachycephalid, which reaches a snout-to-vent length of only 9.8 millimetres. The long, slender, limbless caecilian is a burrowing animal. Adaptations for this lifestyle include a body segmented by annular grooves and a short and pointed tail that is usually present. Caecilians can attain lengths of more than one metre Caecilia thompsoni reaches a length of 1,520 millimetres; the smallest species, Grandisonia brevis, is only 112 millimetres.
Distribution and abundance
Found in almost every type of environment except the high latitudes in the Arctic, Antarctica, various oceanic islands, and some extremel xeric (dry) deserts, frogs and toads show the greatest diversity in humid tropical environments. Salamanders primarily inhabit the Northern Hemisphere and are most abundant in cool, moist, montane forests; however, members of the family Plethodontidae are diverse in the humid tropical areas of Mexico, Central America, and northwestern South America. Caecilians are found throughout the tropics except for Madagascar.
For many years habitat destruction has had a severe impact on the distribution and abundance of numerous amphibian species. Since the 1980s a severe decline in the population of many species of frog has been observed. Although acid rain, global warming, and ozone depletion may be contributing factors to these reductions, the reasons for their decline have not been fully determined.
Economic importance
Amphibians, especially anurans, are economically useful in reducing the number of insects that destroy crops or transmit diseases. Frogs are commercially exploited as food, notably by Europeans; the French are the principal consumers, and they import thousands of tons of frog legs annually. The skin secretions of various tropical anurans are known to have hallucinogenic effects as well as effects on the central nervous and respiratory systems in humans. Some secretions have been found to contain magainin, a substance that provides a natural antibiotic effect. Biochemists are currently investigating these substances for possible uses.
[tags]amphibian, size, diversity, animals, habitat[/tags]
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