Amphibian
- For other uses, see Amphibian (disambiguation).
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 White's Tree Frog (Litoria caerulea) | ||||||||
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Subclass Labyrinthodontia - extinct |
Amphibians (class Amphibia) are a taxon of cold-blooded animals that include all tetrapods (four-legged vertebrates) that do not have amniotic eggs. An amniotic sac is a tough but thin transparent pair of membranes, which hold a developing embryo (and later fetus) until shortly before birth.
Amphibians are so diverse in nature that there are few universal, defining characteristics that apply to all species. Amphibians have skin that is smooth and naked, without hair, feathers or true scales. Some possess dermal scales. Unlike fish, amphibians generally respire through the skin and lungs rather than gills, and have limbs instead of fins. (Some amphibians utilize gills as well.) If feet are present, they are webbed and the toes lack claws. Unlike reptiles, amphibians lack a scaly or armored outer covering and respire (oxygen exchange) and absorb water via the skin. The lower skin layer (dermis) of almost all amphibians have mucous glands to provide moisture, as well as poison glands that produce toxins, ranging from mildly noxious to deadly. Generally these are toxic to natural enemies, such as birds and mammals, but harmless to humans.
Most amphibians produce eggs without shells or membranes (amamniotic) that are deposited in water and rely on moisture from the surroundings. Adult amphibians have three chambered hearts (larvae have two-chambered hearts), and usually two lungs. Amphibians have two protusions on the back of the skull (occipital condyles) that articulate with the vertebra of the backbone, whereas reptiles have a single occipital condyle.
Most amphibians are biphasic, having an aquatic stage where they spend part of their time and a terrestrial stage as well. Many change from an aquatic larval stage, where they breathe water and lack limbs, to a terrestrial, air-breathing, four-legged adult form (metamorphosis); however, two-thirds do not incorporate this stratedy (Pough et. al 1998). The term amphibian itself comes from the Greek αμφις "both" and βιος "life," meaning double life. In general, amphibians lack the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). However, some are fully terrestrial, even being born on land, and other may require only a moist environment. Some are completely aquatic.
Being cold-blooded organisms, many amphibians enter a state of dormancy under unfavorable conditions, known as hibernation in the winter and estivation in the summer.
Unification aspects??
There are about 6,000 described, living species of amphibians. Examples include frogs, toads, salamanders, netws, mudpuppies, and caecilians. The study of amphibians and reptiles is known as herpetology.
Classification and diversity
All amphibians belong to the class Amphibia of the Subphylum Vertebrata, of the Phylum Chordata or Craniata. All extant (living) amphibians are placed in a single subclass, Lissamphibia. There are two ancient, extinct, subclasses:
- Subclass Labyrinthodontia
- Subclass Lepospondyli
Recently there has been a tendency to restrict the class Amphibia to the Lissamphibia, by excluding tetrapods that are not more closely related to modern forms than they are to living reptiles, birds, and mammals.
Three orders are recognized in the subclass Lissamphibia:
- Order Anura or Salientia (frogs and toads)
- Order Caudata or Urodela (salamanders, newts, waterdogs, mudpuppies, sirens, and amphiuma)
- Order Gymnophiona or Apoda (caecilians)
Taxonomists disagree on whether to consider Salientia a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. In effect, Salientia includes all the Anura plus a single, extinct Triassic proto-frog species, Triadobatrachus massinoti. Practical considerations seem to favor using the former arrangement now.
Frogs and toads belong to the order Anura or Salientia. These are divided into about 30 families and more than 5,000 species have been identified. Frogs and toads differ from the other Amphibian orders by the presence of larger hind limbs among the four limbs. Extant anurans lack tails.
Salamanders, newts, waterdogs, mudpuppies, sirens, and amphiuma are members of the order Caudata or Urodela. Over 500 species of caudates have been identified, and these are organized in about 10 families. All caudates have tails. Generally, caudates have similar-sized limbs, but Amphiuma has reduced lims, and the sirens lack hind limbs and possess reduced forelimbs. The largest amphibian in the world is a caudate, the Chinese Giant Salamander, Andrias davidanius, which can reach six feet long.
Caecilians belong to the order Gymnophiona (or Apoda), and are elongated, segmented amphibians, looking almost wormlike. Caecilians lack external limbs, but like snakes are still considered tetrapods because the lack of limbs is considered a derived, secondary characteristic since it is assumed they evolved from forms that did have them. Caecilian heads are adapted for burrowing, being strong with highly ossified skulls. Caecilians are also the only amphibians with dermal scales, but these scale-like structures differ from reptile scales. Reptile scales are keratinized folds of skin, whereas caecilian scales are layers of fibers topped by mineralized nodules. Caecilian have a unique sense organ, a retractable tentacle found between the nostril and eye that acts as a chemical sensor. Their name means "blind," but most have small eyes. Adapted for life underground, caecilians are poorly known, and many do not even have common names. There are about 200 known caecilian species.
Amphibians range in size from the tiny Brachycephalus didactylus (Brazilian Gold Frog) and Eleutherodactylus iberia from Cuba, with a total length of 9.6-9.8 millimeters (0.4 inches), to the Chinese Giant Salamander mentioned above. Amphibians have mastered almost every climate on earth from the hottest deserts to the frozen arctic.
Evolution of amphibians
Amphibians are generally considered the first terrestrial vertebrates. The first record of amphibian-like animals in the fossil record is 360 to 390 million years ago, during the Devonian period. Amphibians are considered to be the first four-legged animals to have lungs and limbs. It was during the following Carboniferous period that amphibians were considered to have the ability to walk on land, which could have allowed them to avoid aquatic competition and predation while allowing them to travel from water source to water source. As a group, they maintained the status of the dominant animal for nearly 75 million years. The ancient amphibians were considered to be typically larger than modern amphibians, with massive teeth,a nd some with scaled skin. The earliers fossils considered to be salamanders, caecilians, and frogs date to the Juraissic (190 to 160 million years ago) (Zardoya and Meyer 2001).
The specific relationship between the three orders of extant amphibians (anurans, caudates, and urodeles) represent one of the great controversies in vertebrate evolution. There is no generally accepted consensus regarding the phylogenetic relationships between the three orders (Zardoya and Meyer 2001). One hypothesis is that salamanders are the closest living relatives of frog, and these are less related to caecilians. This is supported by morphological and paleontological studies of living and fossil specimens and some phylogenetic analysis of mitrochondrial rRNA data. The second hypothesis is that salamanders are the sister group of caecilians and these are less related to frogs. This is supported by molecular studies and some morphological evidence. Zardoya and Meyer (2001) analyzed the complete mitrochondrial genomes of a salamander and a caecilian and compared to that of a frog, and found support for the view of a sister relationship between salamanders and frogs.
Reproduction
For the purpose of reproduction most amphibians are bound to fresh water. A few tolerate brackish water, but there are no true sea water 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, bypassing the tadpole stage entirely. 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 get eyelids and adapt to vision outside the water
- An eardrum is developed to lock the middle ear
- In frogs and toads, the tail disappears
Amphibian importance and conservation
importance of amphibians??
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, and disease. However, many of the causes of amphibian declines are still poorly understood, and amphibian declines are currently a topic of much ongoing research.
See also
- Frog zoology
- Prehistoric amphibian
- Tetrapod
ReferencesISBN links support NWE through referral fees
- Duellman/Trueb, Biology of Amphibians
- Pounds, J. Alan and Martín R. Bustamante, Luis A. Coloma, Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster, Enrique La Marca, Karen L. Masters, Andrés Merino-Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo Sánchez-Azofeifa, Christopher J. Still and Bruce E. Young (January 2006). Widespread amphibian extinctions from epidemic disease driven by global warming. Nature 439: 161-167. Digital object identifier (DOI): 10.1038/nature04246.
- Solomon Berg Martin, Biology
- Stuart, Simon N. and Janice S. Chanson, Neil A. Cox, Bruce E. Young, Ana S. L. Rodrigues, Debra L. Fischman, Robert W. Waller (December 2004). Status and trends of amphibian declines and extinctions worldwide. Science 306 (5702): 1783-1786. Digital object identifier (DOI): 10.1126/science.1103538.
Pough, Harvey F., R.M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky & K. D. Wells. 1998: Herpetology. Prentice-Hall, Inc. Upper Saddle River, NJ
External links
- American Museum of Natural History: Department of herpetology
- The Global Amphibian Assessment
- AmphibiaWeb
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