Amphibians (class Amphibia, from Amphi- meaning "on both sides" and -bios meaning "life") are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are 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 Anura (frogs and toads), Caudata (salamanders and newts), and Gymnophiona (caecilians, limbless amphibians that resemble snakes), and in total they number approximately 6,500 species.[1] Many amphibians lay their eggs in water. Amphibians are superficially similar to reptiles, but reptiles are amniotes, along with mammals and birds. Amphibians are ecological indicators,[2] and in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct. The study of amphibians is called batrachology.
The earliest amphibians evolved in the Devonian period from lobe-finned fish that used their strong, bony fins to venture onto dry land.[3] They were the top predators in the Carboniferous and Permian periods,[4] but they later faced competition from their descendants, the reptiles, and many lineages were wiped out during the Permian–Triassic extinction. One group, the metoposaurs, remained important predators during the Triassic, but as the world became drier during the Early Jurassic they died out, leaving a handful of relict temnospondyls like Koolasuchus and the modern orders of Lissamphibia.
Etymology
Amphibian is derived from the Ancient Greek term ἀμφίβιος amphíbios, which means both kinds of life, amphi meaning “both” and bio meaning life. The term was initially used for all kinds of combined natures. Eventually it was used to refer to animals that live both in the water and on land.[5]
Evolution
Possible paths of Lissamphibia evolution.
The first major groups of amphibians developed in the Devonian period from lobe-finned fish similar to the modern coelacanth and lungfish,[3] which had evolved multi-jointed leg-like fins that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were lacking in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances required it. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. Amphibians evolved adaptations which allowed them to stay out for longer periods. However, they never developed the ability to live their entire lives on land, having a fully aquatic tadpole stage and still needing to return to water to lay their shell-less eggs.
The first true amphibians appeared in the Carboniferous Period, by which time they were already moving up the food chain and occupying the ecological position currently claimed by such animals as crocodiles. Amphibians were once the top land predators, sometimes reaching several meters in length, preying on the large insects on land and many types of fish in the water. During the Triassic Period, the better-adapted reptiles began to compete with amphibians, leading to the reduction of their size and importance in the biosphere. Lissamphibia, which includes all modern amphibians and is the only surviving lineage of amphibians left, could have branched off from the extinct groups Temnospondyli and Lepospondyli anytime between the mid-Permian to the early Triassic, but the relative scarcity of fossil evidence does not permit an exact date.[4]
Classification
Traditionally, amphibians have included all tetrapod vertebrates that are not amniotes. They are divided into three subclasses, of which two are only known as extinct subclasses:
- Subclass Labyrinthodontia† (diverse Paleozoic and early Mesozoic group)
- Subclass Lepospondyli† (small Paleozoic group, sometimes included in the Labyrinthodontia, which may actually be more closely related to amniotes than Lissamphibia)
- Subclass Lissamphibia (frogs, toads, salamanders, newts, etc.)
Of these only the last subclass includes recent species.
With the phylogenetic classification Labyrinthodontia has been discarded as it is a paraphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author, whether they use a stem-based or node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called Lissamphibia. The phylogeny of Paleozoic amphibians is by no means satisfactory understood, and lissamphibia may possibly include extinct groups like the temnospondyls (traditionally placed in the subclass “Labyrinthodontia”), and the Lepospondyls, and in some analysis even the amniotes. This means that phylogenetic nomenclature list a large number of basal Devonian and Carboniferous tetrapod groups, undoubtedly were “amphibians” in biology, that are formally placed in Amphibia in Linnaean taxonomy, but not in cladistic taxonomy.
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 that salamanders arose separately from a temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniots.[6][7]
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 favor 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.
Class Amphibia
- Subclass Lissamphibia
-
- Superorder Salientia
- Genus Triadobatrachus — Triassic (extinct) — A stem Anuran
- Order Anura (frogs and toads): Jurassic to recent — 5,602 recent species in 48 families
- Order Caudata or Urodela (salamanders, newts): Jurassic to recent — 571 recent species in 9 families
- Order Gymnophiona or Apoda (caecilians): Jurassic to recent — 174 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.[8] The numbers of species cited above follow Frost.
Anatomy and physiology
Integumentary system
The
fire salamander has brightly colored yellow spots, indicating that it secretes toxins.
Amphibian skin is permeable to water and contains many mucous glands which keep the skin from drying out. To compensate for their thin and delicate skin, all amphibians have evolved poison glands as a defense mechanism, although toxicity varies by species. Some amphibian toxins can be lethal to humans while others have no effect at all.[9] The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a molting process controlled by the pituitary and thyroid glands. Local thickenings (often called warts) are common, such as those found on toads.
The skin color of amphibians is produced by three layers of pigment cells called chromatophores. These three cell layers correspond to the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing many granules, producing a blue-green color) and lipophores (yellow, the most superficial layer). The color change experienced by many species is caused by secretions from the pituitary gland. Unlike bony fish, there is no direct control by the nervous system of the pigment cells. Therefore, the color change is slower. Amphibians are predominantly green. Bright colors usually indicate that the species produces an exceptionally toxic poison.
Skeletal system
The skeletal system of amphibians is structurally similar to other tetrapods, though with a number of variations. They possess a cranium, spine, rib cage, long bones such as the humerus and femur, and short bones such as the phalanges, metacarpals, and metatarsals. Bones in most amphibians are hollow and lightweight.
Circulatory and nervous systems
Digestive and excretory systems
Amphibians swallow their prey whole, with some chewing done in the oral cavities of some species, so they possess voluminous stomachs. Sphincters separate the esophagus from both the oral cavity and the stomach. The relatively short esophagus is lined with cilia that help transport food and secretions into the stomach. Mucous and pepsin, a digestive enzyme, are secreted by glands lining the esophagus. The stomach is separated from the intestine by a pyloric sphincter. The duodenum controls the transport of food into the intestine from the stomach.
Amphibians possess a pancreas, liver and gall bladder. Like mammals, the liver functions as the central metabolic organ that regulates blood sugar, and also produces the final metabolic products and transports them through the vascular system to the kidneys, and finally to excretion. The liver in most amphibians is large with two lobes. The size of the liver is determined by its vital function as a glycogen and fat storage unit, and may change proportionally with the seasons with increasing or decreasing activity. In aquatic amphibians, the liver plays only a small role in processing nitrogen for excretion, and ammonia is diffused mainly through the skin and excretion. The liver of terrestrial amphibians converts ammonia to urea, a less toxic, water soluble nitrogenous compound, as a means of water conservation. In some species, urea is further converted into uric acid. The liver secretions from the liver collect in the gall bladder, and flow into the small intestine. Salamanders lack a valve separating the small intestine from the large intestine. In the small intestine, enzymes digest carbohydrates, fats, and proteins. Salt and water absorption occur in the large intestine, as well as mucous secretion to aid in the transport of fecal matter, which is excreted through the cloaca. Amphibians have two kidneys located dorsally, near the roof of the body cavity, and in pairs. Their jobs are to filter the blood of waste and transport it to the gall bladder.
Respiratory system
The lungs in amphibians are primitive compared to that of the amniotes, possessing few internal septa, large alveoli and therefore a slow diffusion rate of oxygen into the blood. Ventilation is accomplished by buccal pumping. However, most amphibians are able to exchange gasses with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularized skin must remain moist in order for the oxygen to diffuse at a sufficient rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the Titicaca water frog and hellbender salamanders. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders, which have 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.
Reproduction
For the purpose of reproduction most amphibians require fresh water. A few (e.g. Fejervarya raja) can inhabit brackish water and even survive (though not thrive) in seawater, but there are no true marine amphibians. However, there are reports of particular amphibian populations invading marine waters where their species is normally unable to survive. Such is the case[10] with the Black Sea invasion of the natural hybrid Pelophylax esculentus reported in 2010.
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 for breeding in the wild. 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. Reproductive success of many amphibians is dependent not only on the quantity of rainfall, but the seasonal timing.[11]
Many amphibians exhibit different kinds of parenting behaviour. After their hatching, the tadpoles of different species of poison dart frogs (family Dendrobatidae) are carried by the adults to a suitable place where they can pass metamorphosis. Such places are the rosettes of many bromeliads in which water is gathered and used by the plant. The Surinam toad raises its young in pores at its back and after enough time they appear out of these pores fully developed. The ringed caecilian (Siphonops annulatus) has developed a unique adaptation for the purposes of reproduction. The progeny feeds on a skin layer that is specially developed by the adult. This phenomenon is known as maternal dermatophagy.
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 of frogs (tadpoles or polliwogs) breathe with exterior gills at the start, but soon a pouch is formed that covers the gills and the front legs. Lungs are also formed quite early to assist in breathing. Newt larvae have large external gills that gradually disappear and the larvae of newts are quite similar to the adult form from early age on.
Frogs and toads however have a tadpole stage, which is a totally different organism that is a grazing algae or ongrowth or filtering plankton until a certain size has been reached, where metamorphosis sets in. This metamorphosis typically lasts only 24 hours and consists of:
- The disappearance of the gill pouch, making the front legs visible.
- The transformation of the jaws into the big jaws of predatory frogs (most tadpoles are scraping of algae or are filter feeders)
- The transformation of the digestive system: the long spiral gut of the larva is being replaced by the typical short gut of a predator.
- An adaptation of the nervous system for stereoscopic vision, locomotion and feeding
- A quick growth and movement of the eyes to higher up the skull and the formation of eyelids.
- Formation of skin glands, thickening of the skin and loss of the lateral line system
- An eardrum is developed to lock the middle ear.
The disappearance of the tail is somewhat later (occurs at higher thyroxin levels) and after the tail has been resorbed the animals are ready to leave the water. The material of the tail is being used for a quick growth of the legs. The disappearance of the larval structures is a regulated process called apoptosis.
The transformation of newts when leaving the water is reversible except for the loss of the external gills. When the animals enter the water again for reproduction changes are driven by prolactin, when they return to the land phase by thyroxin
Growth and development
Most amphibians go through metamorphosis, a process of significant morphological change after birth. In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the egg as larvae with external gills. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.
Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers, and fins. They also make use of the lateral line organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called apoptosis. The amount of adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.
Frogs and toads
With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral‐shaped gut to digest that diet.
Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis. It isn't until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.
Newts
In newts, there is no true metamorphosis because newt larvae already feed as predators and continue doing so as adults. Newts' gills are never covered by a gill sac and will be resorbed only just before the animal leaves the water. Just as in tadpoles, their lungs are functional early, but newts don't make as much use of them as tadpoles do. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
Diet
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, 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 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 http://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. Amphibian Ark is an organization that was formed to implement the ex-situ conservation recommendations of this plan, and they have been working with zoos and aquaria around the world encouraging them to create assurance colonies of threatened amphibians. One such project is the Panama Amphibian Rescue and Conservation Project that built on existing conservation efforts in Panama to create a country-wide response to the threat of chytridiomycosis rapidly spreading into eastern Panama.[12]
On January 21, 2008, Evolutionarily Distinct and Globally Endangered (EDGE), as given by 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.[13][14][15][16]
See also
References
- ^ Amphibian diversity and life history.
- ^ Waddle, J, USE OF AMPHIBIANS AS ECOSYSTEM INDICATOR SPECIES
- ^ a b Waikato - Evolution of amphibians
- ^ a b About.com - Prehistoric amphibians
- ^ "Amphibious definition". Dictionary.reference.com. http://dictionary.reference.com/search?q=amphibious&db=luna. Retrieved 2009-04-07.
- ^ Carroll, 2007
- ^ Anderson J. S., Reisz R. R., Scott D., Fröbisch N. B., & Sumida S. S. (2008): A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature No. 453, pp. 515–518 Anderson, J. S.; Reisz, R. R.; Scott, D.; Fröbisch, N. B.; Sumida, S. S. (2008). "A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders". Nature 453 (7194): 515–518. doi:10.1038/nature06865. PMID 18497824. edit
- ^ Amphibian Species of the World The online database by Darrel Frost and The American Museum of Natural History
- ^ "Amphibian Facts - About.com". http://animals.about.com/od/amphibians/a/amphibian-facts.htm. Retrieved February 5, 2012.
- ^ [1].
- ^ C.Michael Hogan. 2010. Abiotic factor. Encyclopedia of Earth. eds Emily Monosson and C. Cleveland. National Council for Science and the Environment. Washington DC
- ^ Panama Amphibian Rescue and Conservation Project http://amphibianrescue.org/?page_id=91
- ^ Lovell, Jeremy (2008-01-20). "Reuters, Giant newt, tiny frog identified as most at risk". Reuters.com. http://www.reuters.com/article/latestCrisis/idUSL2038808. Retrieved 2009-04-07.
- ^ Sample, Ian (2008-01-20). "Drive to save weird and endangered amphibians". The Guardian (London). http://www.guardian.co.uk/environment/2008/jan/21/conservation. Retrieved 2009-04-07.
- ^ "/environment, images of the species". London: Guardian. 2008-01-18. http://www.guardian.co.uk/environment/gallery/2008/jan/21/wildlife.conservation?picture=332110244. Retrieved 2009-04-07.
- ^ "/environment, Gallery: the world's strangest amphibians". London: Guardian. 2008-01-18. http://www.guardian.co.uk/environment/gallery/2008/jan/21/wildlife.conservation. Retrieved 2009-04-07.
Further reading
- Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York: W.H. Freeman & Co..
- Carroll, Robert L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. Baltimore: The Johns Hopkins University Press. ISBN 978-0-8018-9140-3.
- Duellman, William E.; Linda Trueb (1994). Biology of Amphibians. Johns Hopkins University Press. ISBN 978-0801847806.
- Frost, Darrel R.; Taran Grant, Julián Faivovich, Raoul H. Bain, Alexander Haas, Célio F.B. Haddad, Rafael O. De Sá, Alan Channing, Mark Wilkinson, Stephen C. Donnellan, Christopher J. Raxworthy, Jonathan A. Campbell, Boris L. Blotto, Paul Moler, Robert C. Drewes, Ronald A. Nussbaum, John D. Lynch, David M. Green, Ward C. Wheeler (March 2006). "The Amphibian Tree of Life". Bulletin of the American Museum of Natural History 297: 1–291. doi:10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2. hdl:2246/5781.
- Pounds, J. Alan; 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 (7073): 161–167. doi:10.1038/nature04246. PMID 16407945. http://www.nature.com/nature/journal/v439/n7073/full/nature04246.html.
- San Mauro, Diego; Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer (May 2005). "Initial diversification of living amphibians predated the breakup of Pangaea". American Naturalist 165 (5): 590–599. doi:10.1086/429523. PMID 15795855.
- San Mauro, Diego (2010). "A multilocus timescale for the origin of extant amphibians". Molecular Phylogenetics and Evolution 56 (2): 554–561. doi:10.1016/j.ympev.2010.04.019. PMID 20399871.
- Solomon Berg Martin, Biology
- Stuart, Simon N.; 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. doi:10.1126/science.1103538. PMID 15486254. http://www.sciencemag.org/cgi/content/full/306/5702/1783.
- S.N.Stuart, M.Hoffmann, J.S.Chanson, N.A.Cox, R.J.Berridge, P.Ramani, B.E. Young (editors), Collective work (September 2008). Threatened Amphibians of the World. Published by Lynx Edicions, in association with IUCN-The World Conservation Union, Conservation International and NatureServe.. ISBN 978-84-96553-41-5. http://www.hbw.com/lynx/en/lynx-edicions/portada-lynx/MON0017-threatened-amphibians-world.html. 776 pages
External links
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