AskDefine | Define amphibia

Dictionary Definition

Amphibia n : frogs; toads; newts; salamanders; caecilians [syn: class Amphibia]

User Contributed Dictionary

English

Noun

amphibia (plural)
  1. Plural of amphibian

Extensive Definition

see also Prehistoric amphibian Amphibians (class Amphibia), such as frogs and salamanders, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form. Typically, amphibians have four limbs. Unlike other land animals (amniotes), amphibians lay eggs in water, as their fish ancestors did. Amphibians are superficially similar to lizards.
Amphibian populations around the globe are threatened or extinct, and scientists do not agree on the reason.
Amphibians evolved in the Devonian Period (c 400 million years ago). They were a top predator in the Carboniferous Period, but proto-crocodiles evolved and took over that niche.

Taxonomy

Traditionally, amphibians have included all tetrapods that are not amniotes . They are divided into three subclasses, of which two are only known as extinct subclasses:
Of these only the last subclass includes recent species.
With the cladistic revolution, this classification has been modified, or changed, and the Labyrinthodontia discarded as being a paraphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author, and whether they use a stem-based or node-based classification. Generally amphibians are defined as the group that includes the common ancestors of all living amphibians (frogs, salamanders, etc) and all their descendants. This may also include extinct groups like the temnospondyls (traditionally placed in the disbanded subclass "labyrinthodontia"), and the Lepospondyls. This means that there are a now large number of basal Devonian and Carboniferous tetrapod groups, described as "amphibians" in earlier books, that are no longer placed in the formal Amphibia.
All recent amphibians are included in the subclass Lissamphibia, superorder Salientia, which is usually considered a clade (which means that it is thought that they evolved from a common ancestor apart from other extinct groups), although it has also been suggested also that salamanders arose separately from a temnospondyl-like ancestor (Carroll, 2007).
Authorities also disagree on whether Salientia is a Superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. Practical considerations seem to favour using the former arrangement now.
The Lissamphibia, superorder Salientia, are traditionally divided into three orders, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of the Lissamphibia, besides the superorder Salientia. Furthermore, Salientia includes all three recent orders plus a single Triassic proto-frog, Triadobatrachus.
  • Subclass Lissamphibia
    • Superorder Salientia
      • Genus Triadobatrachus - Triassic (extinct)
      • Order Anura (frogs and toads): Jurassic to recent - 5,453 recent species in 45 families
      • Order Caudata or Urodela (salamanders, newts): Jurassic to recent - 560 recent species in 9 families
      • Order Gymnophiona or Apoda (caecilians): Jurassic to recent - 171 recent species in 3 families
The actual number of species partly also depends on the taxonomic classification followed, the two most common classifications being the classification of the website AmphibiaWeb, University of California (Berkeley) and the classification by herpetologist Darrel Frost and The American Museum of Natural History, available as the online reference database Amphibian Species of the World (see external links below). The numbers of species cited above follow Frost.

Systems

Reproductive

For the purpose of reproduction most amphibians are bound to have fresh water. A few tolerate brackish water, but there are no true seawater amphibians. Several hundred frog species in adaptive radiations (e.g., Eleutherodactylus, the Pacific Platymantines, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water whatsoever. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that lives in the jelly-like layer of the eggs has evolved several times. The larvae (tadpoles or polliwogs) breathe with exterior gills. After hatching, they start to transform gradually into the adult's appearance. This process is called metamorphosis. Typically, the animals then leave the water and become terrestrial adults, but there are many interesting exceptions to this general way of reproduction.
The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on land. But there are several other changes:
  • The gills are replaced by other respiratory organs, i.e., lungs.
  • The skin changes and develops glands to avoid dehydration.
  • The eyes develop eyelids and adapt to vision outside the water.
  • An eardrum is developed to lock the middle ear.
  • In frogs and toads, the tail disappears.

Conservation

Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades from locations all over the world, and amphibian declines are thus perceived as one of the most critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, 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 causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion. A global strategy to stem the crisis has been released in the form of the Amphibian Conservation Action Plan (available at www.amphibians.org). Developed by over 80 leading experts in the field, this call to action details what would be required to curtail amphibian declines and extinctions over the next 5 years - and how much this would cost. The Amphibian Specialist Group of the World Conservation Union (IUCN) is spearheading efforts to implement a comprehensive global strategy for amphibian conservation.
On January 21, 2008, Evolutionarily Distinct and Globally Endangered (EDGE), per chief Helen Meredith identified nature's most endangered species: "The EDGE amphibians are amongst the most remarkable and unusual species on the planet and yet an alarming 85% of the top 100 are receiving little or no conservation attention." The top 10 endangered species (in the List of endangered animal species) include: the Chinese giant salamander, a distant relative of the newt, the tiny Gardiner's Seychelles, the limbless Sagalla caecilian, South African ghost frogs, lungless Mexican salamanders, the Malagasy rainbow frog, Chile's Darwin frog (Rhinoderma rufum) and the Betic Midwife Toad.

Evolutionary history

The first major groups of amphibians developed in the Devonian Period from fish similar to the modern coelacanth where the fins had evolved into legs. These amphibians were around five meters long. The land was safe as the giant fish and sharks in the ocean could not come onto land. However, there were two problems with living out their entire lives on land. Primarily, the food that these amphibians consumed was in the water, but also at this point the skin on most of these amphibians was not water-tight.
In the Carboniferous Period, the amphibians moved up in the food chain and began to occupy the ecological position where we now find crocodiles. These amphibians were notable for eating the mega-insects on land and many types of fishes in the water. Towards the end of the Permian Period and the Triassic Period, the amphibians started having competition with proto-crocodiles which led to their drop in size in the temperate zones or leaving for the poles. (Amphibians were able to hibernate during the winter whereas crocodiles could not, allowing the amphibians in higher latitudes protection from the reptiles.)
Paleontologists once believed that the kind of lifestyle and adaptations that proto-amphibians was similar to the modern mudskipper. (Mudskippers are not closely related to coelocanths.)

References

Further reading

  • Solomon Berg Martin, Biology
i f ing love frogs.com al the info you need for frogs
amphibia in Afrikaans: Amfibie
amphibia in Arabic: برمائيات
amphibia in Bengali: উভচর প্রাণী
amphibia in Min Nan: Lióng-chhe-lūi
amphibia in Bosnian: Vodozemci
amphibia in Bulgarian: Земноводни
amphibia in Catalan: Amfibi
amphibia in Czech: Obojživelníci
amphibia in Welsh: Amffibiad
amphibia in Danish: Padde
amphibia in German: Amphibien
amphibia in Estonian: Kahepaiksed
amphibia in Spanish: Amphibia
amphibia in Esperanto: Amfibioj
amphibia in Basque: Anfibio
amphibia in Persian: دوزیستان
amphibia in French: Amphibia
amphibia in Irish: Amfaibiach
amphibia in Scottish Gaelic: Dà-bheathach
amphibia in Galician: Anfibio
amphibia in Korean: 양서류
amphibia in Croatian: Vodozemci
amphibia in Ido: Amfibia
amphibia in Indonesian: Amfibia
amphibia in Icelandic: Froskdýr
amphibia in Italian: Amphibia
amphibia in Hebrew: דו-חיים
amphibia in Javanese: Amfibi
amphibia in Pampanga: Amphibian
amphibia in Georgian: ამფიბიები
amphibia in Latin: Amphibia
amphibia in Latvian: Abinieki
amphibia in Luxembourgish: Lurchen
amphibia in Lithuanian: Varliagyviai
amphibia in Limburgan: Amfibieë
amphibia in Lojban: banfi
amphibia in Hungarian: Kétéltűek
amphibia in Macedonian: Водоземец
amphibia in Malayalam: ഉഭയജീവി
amphibia in Malay (macrolanguage): Amfibia
amphibia in Dutch: Amfibieën
amphibia in Japanese: 両生類
amphibia in Norwegian: Amfibier
amphibia in Norwegian Nynorsk: Amfibium
amphibia in Occitan (post 1500): Amphibia
amphibia in Low German: Amphibien
amphibia in Polish: Płazy
amphibia in Portuguese: Anfíbios
amphibia in Romanian: Amfibian
amphibia in Romansh: Amphibia
amphibia in Quechua: Allpa yaku kawsaq
amphibia in Russian: Земноводные
amphibia in Sicilian: Anfibbiu
amphibia in Simple English: Amphibian
amphibia in Slovak: Obojživelníky
amphibia in Slovenian: Dvoživke
amphibia in Serbian: Водоземци
amphibia in Serbo-Croatian: Vodozemac
amphibia in Sundanese: Ampibi
amphibia in Finnish: Sammakkoeläimet
amphibia in Swedish: Groddjur
amphibia in Tamil: நிலநீர் வாழிகள்
amphibia in Telugu: ఉభయచరము
amphibia in Thai: สัตว์สะเทินน้ำสะเทินบก
amphibia in Tonga (Tonga Islands): Monumanu nofo ʻuta mo vai
amphibia in Turkish: Amfibyumlar
amphibia in Ukrainian: Земноводні
amphibia in Urdu: جل تھلیل
amphibia in Walloon: Glumiantès biesses
amphibia in Zeeuws: Amfibieën
amphibia in Chinese: 两栖动物
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