Amphibian - Wikipedia, The Free Encyclopedia20150425221145

Amphibians

Temporal range: Late Devonian–present

Clockwise from top right: Seymouria,

Mexican burrowing caecilian, eastern newt

and leaf green tree frog

Scientific classification

Kingdom: Animalia

Phylum: Chordata

Superclass: Tetrapoda

Class: Amphibia

Gray 1825[1]

Subclasses and Orders

†Subclass Labyrinthodontia

†Order Temnospondyli

†Subclass Lepospondyli

Subclass Lissamphibia

Order Anura

Order Caudata

Order Gymnophiona

†Order Allocaudata

AmphibianFrom Wikipedia, the free encyclopedia

Amphibians are ectothermic, tetrapod vertebrates of the classAmphibia . Modern amphibians are all Lissamphibia. Theyinhabit a wide variety of habitats with most species living withinterrestrial, fossorial, arboreal or freshwater aquatic ecosystems.Amphibians typically start out as larvae living in water, but somespecies have developed behavioural adaptations to bypass this.The young generally undergo metamorphosis from larva withgills to an adult air-breathing form with lungs. Amphibians usetheir skin as a secondary respiratory surface and some smallterrestrial salamanders and frogs lack lungs and rely entirely ontheir skin. They are superficially similar to reptiles but, alongwith mammals and birds, reptiles are amniotes and do notrequire water bodies in which to breed. With their complexreproductive needs and permeable skins, amphibians are oftenecological indicators and in recent decades there has been adramatic decline in amphibian populations for many speciesaround the globe.

The earliest amphibians evolved in the Devonian period fromsarcopterygian fish with lungs and bony-limbed fins, featuresthat were helpful in adapting to dry land. They diversified andbecame dominant during the Carboniferous and Permianperiods, but were later displaced by reptiles and othervertebrates. Over time, amphibians shrank in size and decreasedin diversity, leaving only the modern subclass Lissamphibia. Thethree modern orders of amphibians are Anura (the frogs andtoads), Caudata/Urodela (the salamanders), andGymnophiona/Apoda (the caecilians). The number of knownamphibian species is approximately 7,000, of which nearly 90%are frogs. The smallest amphibian (and vertebrate) in the worldis a frog from New Guinea (Paedophryne amauensis) with alength of just 7.7 mm (0.30 in). The largest living amphibian isthe 1.8 m (5 ft 11 in) Chinese giant salamander (Andriasdavidianus), but this is dwarfed by the extinct 9 m (30 ft)Prionosuchus from the middle Permian of Brazil. The study ofamphibians is called batrachology, while the study of bothreptiles and amphibians is called herpetology.

Contents

1 Classification

2 Evolutionary history

3 Characteristics

3.1 Anura

Pre O S D C P T J K PgN

Amphibian - Wikipedia, the free encyclopedia https://en.wikipedia.org/w/index.php?title=Amphibian&printable=yes

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3.2 Caudata

3.3 Gymnophiona

4 Anatomy and physiology

4.1 Skin

4.2 Skeletal system and locomotion

4.3 Circulatory system

4.4 Nervous and sensory systems

4.5 Digestive and excretory systems

4.6 Respiratory system

5 Reproduction

6 Life cycle

6.1 Eggs

6.2 Larvae

6.2.1 Frogs

6.2.2 Salamanders

6.2.3 Caecilians

6.3 Parental care

7 Feeding and diet

8 Vocalization

9 Territorial behaviour

10 Defence mechanisms

11 Conservation

12 See also

13 References

13.1 Cited texts

14 Further reading

15 External links

Classification

The word "amphibian" is derived from the Ancient Greek term ἀµφίβιος (amphíbios), which means "bothkinds of life", ἀµφί meaning "of both kinds" and βιος meaning "life". The term was initially used as a generaladjective for animals that could live on land or in water, including seals and otters.[2] Traditionally, the classAmphibia includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest sense (sensu lato)was divided into three subclasses, two of which are extinct:[3]

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 (all modern amphibians, including frogs, toads, salamanders, newts and


2 od 31 25.4.2015 22:11

The world's smallest known

vertebrate, Paedophryne

amauensis, sitting on a U.S.

dime, which is 17.9 mm in

diameter, for scale

Triadobatrachus massinoti, a

proto-frog from the Early

Triassic of Madagascar

caecilians)

Order Anura (frogs and toads): Jurassic to present—6,200

current species in 53 families

Order Caudata or Urodela (salamanders, newts): Jurassic to

present—652 current species in 9 families

Order Gymnophiona or Apoda (caecilians): Jurassic to

present—192 current species in 10 families

The actual number of species in each group depends on the taxonomicclassification followed. The two most common systems are theclassification adopted by the website AmphibiaWeb, University ofCalifornia, Berkeley and the classification by herpetologist Darrel Frost andthe American Museum of Natural History, available as the online referencedatabase "Amphibian Species of the World".[4] The numbers of speciescited above follow Frost and the total number of known amphibian speciesis over 7,000, of which nearly 90% are frogs.[5]

With the phylogenetic classification, the taxon Labyrinthodontia has beendiscarded as it is a paraphyletic group without unique defining featuresapart from shared primitive characteristics. Classification varies accordingto the preferred phylogeny of the author and whether they use a stem-basedor a node-based classification. Traditionally, amphibians as a class aredefined as all tetrapods with a larval stage, while the group that includes thecommon ancestors of all living amphibians (frogs, salamanders andcaecilians) and all their descendants is called Lissamphibia. The phylogenyof Paleozoic amphibians is uncertain, and Lissamphibia may possibly fallwithin extinct groups, like the Temnospondyli (traditionally placed in thesubclass Labyrinthodontia) or the Lepospondyli, and in some analyses evenin the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number ofbasal Devonian and Carboniferous amphibian-type tetrapod groups that were formerly placed in Amphibiain Linnaean taxonomy, and included them elsewhere under cladistic taxonomy.[1] If the common ancestor ofamphibians and amniotes is included in Amphibia, it becomes a paraphyletic group.[6]

All modern amphibians are included in the subclass Lissamphibia, which is usually considered a clade, agroup of species that have evolved from a common ancestor. The three modern orders are Anura (the frogsand toads), Caudata (or Urodela, the salamanders), and Gymnophiona (or Apoda, the caecilians).[7] It hasbeen suggested that salamanders arose separately from a Temnospondyl-like ancestor, and even thatcaecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes.[8] Althoughthe fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" isProsalirus bitis, from the Early Jurassic Kayenta Formation of Arizona. It is anatomically very similar tomodern frogs.[9] The oldest known caecilian is another Early Jurassic species, Eocaecilia micropodia and isalso from Arizona.[10] The earliest salamander is Beiyanerpeton jianpingensis from the Late Jurassic ofnortheastern China.[11]

Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anurais a sub-order of the order Salientia. The Lissamphibia are traditionally divided into three orders, but anextinct salamander-like family, the Albanerpetontidae, is now considered part of Lissamphibia alongside thesuperorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog,Triadobatrachus.[12]


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Top: Restoration of Eusthenopteron,

a fully aquatic lobe-finned fish

Bottom: Restoration of Tiktaalik, an

advanced tetrapodomorph fish

The Permian

lepospondyl

Diplocaulus was largely

aquatic

Evolutionary history

The first major groups of amphibians developed in the Devonianperiod, around 370 million years ago, from lobe-finned fish whichwere similar to the modern coelacanth and lungfish.[13] These ancientlobe-finned fish had evolved multi-jointed leg-like fins with digitsthat enabled them to crawl along the sea bottom. Some fish haddeveloped primitive lungs to help them breathe air when the stagnantpools of the Devonian swamps were low in oxygen. They could alsouse their strong fins to hoist themselves out of the water and onto dryland if circumstances so required. Eventually, their bony fins wouldevolve into limbs and they would become the ancestors to alltetrapods, including modern amphibians, reptiles, birds, andmammals. Despite being able to crawl on land, many of theseprehistoric tetrapodomorph fish still spent most of their time in thewater. They had started to develop lungs, but still breathedpredominantly with gills.[14]

Many examples of species showing transitional features have been discovered. Ichthyostega was one of thefirst primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail withfins and a skull very similar to that of the lobe-finned fish, Eusthenopteron.[13] Amphibians evolvedadaptations that allowed them to stay out of the water for longer periods. Their lungs improved and theirskeletons became heavier and stronger, better able to support the weight of their bodies on land.. Theydeveloped "hands" and "feet" with five or more digits;[15] the skin became more capable of retaining bodyfluids and resisting desiccation.[14] The fish's hyomandibula bone in the hyoid region behind the gillsdiminished in size and became the stapes of the amphibian ear, an adaptation necessary for hearing on dryland.[16] An affinity between the amphibians and the teleost fish is the multi-folded structure of the teeth andthe paired supra-occipital bones at the back of the head, neither of these features being found elsewhere inthe animal kingdom.[17]

At the end of the Devonian period (360 million years ago), the seas, rivers andlakes were teeming with life while the land was the realm of early plants anddevoid of vertebrates,[17] though some, such as Ichthyostega, may havesometimes hauled themselves out of the water. It is thought they may havepropelled themselves with their forelimbs, dragging their hindquarters in a similarmanner to that used by the elephant seal.[15] In the early Carboniferous (360 to345 million years ago), the climate became wet and warm. Extensive swampsdeveloped with mosses, ferns, horsetails and calamites. Air-breathing arthropodsevolved and invaded the land where they provided food for the carnivorousamphibians that began to adapt to the terrestrial environment. There were noother tetrapods on the land and the amphibians were at the top of the food chain,occupying the ecological position currently held by the crocodile. Thoughequipped with limbs and the ability to breathe air, most still had a long taperingbody and strong tail.[17] They were the top land predators, sometimes reachingseveral metres in length, preying on the large insects of the period and the manytypes of fish in the water. They still needed to return to water to lay their

shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fishancestors. It was the development of the amniotic egg, which prevents the developing embryo from dryingout, that enabled the reptiles to reproduce on land and which led to their dominance in the period thatfollowed.[13]

After the Carboniferous rainforest collapse amphibian dominance gave way to reptiles,[18] and amphibians


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The temnospondyl Eryops had

sturdy limbs to support its body

on land

were further devastated by the Permian–Triassic extinction event.[19] During the Triassic Period (250 to 200million years ago), the reptiles continued to out-compete the amphibians, leading to a reduction in both theamphibians' size and their importance in the biosphere. According to the fossil record, Lissamphibia, whichincludes all modern amphibians and is the only surviving lineage, may have branched off from the extinctgroups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the EarlyTriassic. The relative scarcity of fossil evidence precludes precise dating,[14] but the most recent molecularstudy, based on multilocus sequence typing, suggests a Late Carboniferous/Early Permian origin for extantamphibians.[20]

The origins and evolutionary relationships between the three main groupsof amphibians is a matter of debate. A 2005 molecular phylogeny, based onrDNA analysis, suggests that salamanders and caecilians are more closelyrelated to each other than they are to frogs. It also appears that thedivergence of the three groups took place in the Paleozoic or earlyMesozoic (around 250 million years ago), before the breakup of thesupercontinent Pangaea and soon after their divergence from thelobe-finned fish. The briefness of this period, and the swiftness with whichradiation took place, would help account for the relative scarcity ofprimitive amphibian fossils.[21] There are large gaps in the fossil record, butthe discovery of a proto-frog from the Early Permian in Texas in 2008 provided a missing link with many ofthe characteristics of modern frogs. Molecular analysis suggests that the frog–salamander divergence tookplace considerably earlier than the palaeontological evidence indicates.[8]

As they evolved from lunged fish, amphibians had to make certain adaptations for living on land includingthe need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelledthem forward, but on land, quite different mechanisms were required. Their vertebral columns, limbs, limbgirdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding.Terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli viathe medium of the air. They needed to develop new methods to regulate their body heat to cope withfluctuations in ambient temperature. They developed behaviours suitable for reproduction in a terrestrialenvironment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by thewater. The skin changed to become more protective and prevent excessive water loss.[22]

Characteristics

The superclass Tetrapoda is divided into four classes of vertebrate animals with four limbs.[23] Reptiles, birdsand mammals are amniotes, the eggs of which are either laid or carried by the female and are surrounded byseveral membranes, some of which are impervious.[24] Lacking these membranes, amphibians require waterbodies for reproduction, although some species have developed various strategies for protecting or bypassingthe vulnerable aquatic larval stage.[22] They are not found in the sea with the exception of one or two frogsthat live in brackish water in mangrove swamps.[25] On land, amphibians are restricted to moist habitatsbecause of the need to keep their skin damp.[22]

The smallest amphibian (and vertebrate) in the world is a microhylid frog from New Guinea (Paedophryneamauensis) first discovered in 2012. It has an average length of 7.7 mm (0.30 in) and is part of a genus thatcontains four of the world's ten smallest frog species.[26] The largest living amphibian is the 1.8 m (5 ft 11 in)Chinese giant salamander (Andrias davidianus)[27] but this is a great deal smaller than the largest amphibianthat ever existed—the extinct 9 m (30 ft) Prionosuchus, a crocodile-like temnospondyl dating to 270 millionyears ago from the middle Permian of Brazil.[28] The largest frog is the African Goliath frog (Conrauagoliath), which can reach 32 cm (13 in) and weigh 3 kg (6.6 lb).[27]

Amphibians are ectothermic (cold-blooded) vertebrates that do not maintain their body temperature through


5 od 31 25.4.2015 22:11

Red-eyed tree frog (Agalychnis

callidryas) with limbs and feet

specialised for climbing

internal physiological processes. Their metabolic rate is low and as a result, their food and energyrequirements are limited. In the adult state, they have tear ducts and movable eyelids, and most species haveears that can detect airborne or ground vibrations. They have muscular tongues, which in many species canbe protruded. Modern amphibians have fully ossified vertebrae with articular processes. Their ribs areusually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are oftenincompletely ossified. Their skin contains little keratin and lacks scales, apart from a few fish-like scales incertain caecilians. The skin contains many mucous glands and in some species, poison glands. The hearts ofamphibians have three chambers, two atria and one ventricle. They have a urinary bladder and nitrogenouswaste products are excreted primarily as urea. Most amphibians lay their eggs in water and have aquaticlarvae that undergo metamorphosis to become terrestrial adults. Amphibians breathe by means of a pumpaction in which air is first drawn into the buccopharyngeal region through the nostrils. These are then closedand the air is forced into the lungs by contraction of the throat.[29] They supplement this with gas exchangethrough the skin.[22]

Anura

The order Anura (from the Ancient Greek a(n)- meaning "without" andoura meaning "tail") comprises the frogs and toads. They usually have longhind limbs that fold underneath them, shorter forelimbs, webbed toes withno claws, no tails, large eyes and glandular moist skin.[7] Members of thisorder with smooth skins are commonly referred to as frogs, while thosewith warty skins are known as toads. The difference is not a formal onetaxonomically and there are numerous exceptions to this rule. Members ofthe family Bufonidae are known as the "true toads".[30] Frogs range in sizefrom the 30 centimetres (12 in) goliath frog (Conraua goliath) of WestAfrica[31] to the 7.7 millimetres (0.30 in) Paedophryne amauensis, firstdescribed in Papua New Guinea in 2012, which is also the smallest knownvertebrate.[32] Although most species are associated with water and damphabitats, some are specialised to live in trees or in deserts. They are found worldwide except for polarareas.[33]

Anura is divided into three suborders that are broadly accepted by the scientific community, but therelationships between some families remain unclear. Future molecular studies should provide further insightsinto their evolutionary relationships.[34] The suborder Archaeobatrachia contains four families of primitivefrogs. These are Ascaphidae, Bombinatoridae, Discoglossidae and Leiopelmatidae which have few derivedfeatures and are probably paraphyletic with regard to other frog lineages.[35] The six families in the moreevolutionarily advanced suborder Mesobatrachia are the fossorial Megophryidae, Pelobatidae, Pelodytidae,Scaphiopodidae and Rhinophrynidae and the obligatorily aquatic Pipidae. These have certain characteristicsthat are intermediate between the two other suborders.[35] Neobatrachia is by far the largest suborder andincludes the remaining families of modern frogs, including most common species. 96% of the over 5,000extant species of frog are neobatrachians.[36]

Caudata

The order Caudata (from the Latin cauda meaning "tail") consists of the salamanders—elongated, low-slunganimals that mostly resemble lizards in form. This is a symplesiomorphic trait and they are no more closelyrelated to lizards than they are to mammals.[37] Salamanders lack claws, have scale-free skins, either smoothor covered with tubercles, and tails that are usually flattened from side to side and often finned. They rangein size from the Chinese giant salamander (Andrias davidianus), which has been reported to grow to a lengthof 1.8 metres (5 ft 11 in),[38] to the diminutive Thorius pennatulus from Mexico which seldom exceeds20 mm (0.8 in) in length.[39] Salamanders have a mostly Laurasian distribution, being present in much of theHolarctic region of the northern hemisphere. The family Plethodontidae is also found in Central America and


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Japanese giant

salamander

(Andrias japonicus), a

primitive salamander

Danube crested newt

(Triturus dobrogicus), an

advanced salamander

The limbless South American

caecilian Siphonops paulensis

South America north of the Amazon basin;[33] South America was apparentlyinvaded from Central America by about the start of the Miocene, 23 million yearsago.[40] Urodela is a name sometimes used for all the extant species ofsalamanders.[41] Members of several families of salamanders have becomepaedomorphic and either fail to complete their metamorphosis or retain somelarval characteristics as adults.[42] Most salamanders are under 15 cm (6 in) long.They may be terrestrial or aquatic and many spend part of the year in eachhabitat. When on land, they mostly spend the day hidden under stones or logs orin dense vegetation, emerging in the evening and night to forage for worms,insects and other invertebrates.[33]

The suborder Cryptobranchoidea contains the primitive salamanders. Anumber of fossil cryptobranchids have been found, but there are only threeliving species, the Chinese giant salamander (Andrias davidianus), theJapanese giant salamander (Andrias japonicus) and the hellbender(Cryptobranchus alleganiensis) from North America. These largeamphibians retain several larval characteristics in their adult state; gills slitsare present and the eyes are unlidded. A unique feature is their ability tofeed by suction, depressing either the left side of their lower jaw or theright.[43] The males excavate nests, persuade females to lay their egg stringsinside them, and guard them. As well as breathing with lungs, they respirethrough the many folds in their thin skin, which has capillaries close to thesurface.[44]

The suborder Salamandroidea contains the advanced salamanders. They differ from the cryptobranchids byhaving fused prearticular bones in the lower jaw, and by using internal fertilisation. In salamandrids, the maledeposits a bundle of sperm, the spermatophore, and the female picks it up and inserts it into her cloacawhere the sperm is stored until the eggs are laid.[45] The largest family in this group is Plethodontidae, thelungless salamanders, which includes 60% of all salamander species. The family Salamandridae includes thetrue salamanders and the name "newt" is given to members of its subfamily Pleurodelinae.[7]

The third suborder, Sirenoidea, contains the four species of sirens, which are in a single family, Sirenidae.Members of this order are eel-like aquatic salamanders with much reduced forelimbs and no hind limbs.Some of their features are primitive while others are derived.[46] Fertilisation is likely to be external assirenids lack the cloacal glands used by male salamandrids to produce spermatophores and the females lackspermathecae for sperm storage. Despite this, the eggs are laid singly, a behaviour not conducive for externalfertilisation.[45]

Gymnophiona

The order Gymnophiona (from the Greek gymnos meaning "naked" andophis meaning "serpent") or Apoda (from the Latin an- meaning "without"and the Greek poda meaning "legs") comprises the caecilians. These arelong, cylindrical, limbless animals with a snake- or worm-like form. Theadults vary in length from 8 to 75 centimetres (3 to 30 inches) with theexception of Thomson's caecilian (Caecilia thompsoni), which can reach150 centimetres (4.9 feet). A caecilian's skin has a large number oftransverse folds and in some species contains tiny embedded dermal scales.It has rudimentary eyes covered in skin, which are probably limited todiscerning differences in light intensity. It also has a pair of short tentaclesnear the eye that can be extended and which have tactile and olfactoryfunctions. Most caecilians live underground in burrows in damp soil, in rotten wood and under plant debris,


7 od 31 25.4.2015 22:11

The bright colours of

the common reed frog

(Hyperolius

viridiflavus) are typical

of a toxic species

Cross section of frog skin. A: Mucus gland, B:

Chromatophore, C: Granular poison gland, D:

Connective tissue, E: Stratum corneum, F: Transition

zone, G: Epidermis, H: Dermis

but some are aquatic.[47] Most species lay their eggs underground and when the larvae hatch, they maketheir way to adjacent bodies of water. Others brood their eggs and the larvae undergo metamorphosis beforethe eggs hatch. A few species give birth to live young, nourishing them with glandular secretions while theyare in the oviduct.[48] Caecilians have a mostly Gondwanan distribution, being found in tropical Africa, Asiaand Central and South America.[49]

Anatomy and physiology

Skin

The integumentary structure contains some typical characteristics common toterrestrial vertebrates, such as the presence of highly cornified outer layers,renewed periodically through a moulting process controlled by the pituitary andthyroid glands. Local thickenings (often called warts) are common, such as thosefound on toads. The outside of the skin is shed periodically more or less in onepiece, in contrast to mammals and birds where it is shed in flakes. Amphibiansoften eat the sloughed skin.[33] Caecilians are unique among amphibians in havingmineralized dermal scales embedded in the dermis between the furrows in theskin. The similarity of these to the scales of bony fish is largely superficial.Lizards and some frogs have somewhat similar osteoderms forming bony depositsin the dermis, but this is an example of convergent evolution with similarstructures having arisen independently in diverse vertebrate lineages.[50]

Amphibian skin is permeableto water. Gas exchange cantake place through the skin(cutaneous respiration) and

this allows adult amphibians to respire without rising tothe surface of water and to hibernate at the bottom ofponds.[33] To compensate for their thin and delicate skin,amphibians have evolved mucous glands, principally ontheir heads, backs and tails. The secretions produced bythese help keep the skin moist. In addition, most speciesof amphibian have granular glands that secretedistasteful or poisonous substances. Some amphibiantoxins can be lethal to humans while others have littleeffect.[51] The main poison-producing glands, the paratoids, produce the neurotoxin bufotoxin and arelocated behind the ears of toads, along the backs of frogs, behind the eyes of salamanders and on the uppersurface of caecilians.[52]

The skin colour of amphibians is produced by three layers of pigment cells called chromatophores. Thesethree cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming anintermediate layer and containing many granules, producing a blue-green colour) and the lipophores (yellow,the most superficial layer). The colour change displayed by many species is initiated by hormones secretedby the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system,and this results in the colour change taking place more slowly than happens in fish. A vividly coloured skinusually indicates that the species is toxic and is a warning sign to predators.[53]

Skeletal system and locomotion

Amphibians have a skeletal system that is structurally homologous to other tetrapods, though with a numberof variations. They all have four limbs except for the legless caecilians and a few species of salamander with


8 od 31 25.4.2015 22:11

Skeleton of Surinam horned

frog

(Ceratophrys cornuta)

reduced or no limbs. The bones are hollow and lightweight. The musculoskeletal system is strong to enable itto support the head and body. The bones are fully ossified and the vertebrae interlock with each other bymeans of overlapping processes. The pectoral girdle is supported by muscle, and the well-developed pelvicgirdle is attached to the backbone by a pair of sacral ribs. The ilium slopes forward and the body is heldcloser to the ground than is the case in mammals.[54]

In most amphibians, there are four digits on the fore foot and five on thehind foot, but no claws on either. Some salamanders have fewer digits andthe amphiumas are eel-like in appearance with tiny, stubby legs. The sirensare aquatic salamanders with stumpy forelimbs and no hind limbs. Thecaecilians are limbless. They burrow in the manner of earthworms withzones of muscle contractions moving along the body. On the surface of theground or in water they move by undulating their body from side to side.[55]

In frogs, the hind legs are larger than the fore legs, especially so in thosespecies that principally move by jumping or swimming. In the walkers andrunners the hind limbs are not so large, and the burrowers mostly have shortlimbs and broad bodies. The feet have adaptations for the way of life, withwebbing between the toes for swimming, broad adhesive toe pads forclimbing, and keratinised tubercles on the hind feet for digging (frogsusually dig backwards into the soil). In most salamanders, the limbs are short and more or less the samelength and project at right angles from the body. Locomotion on land is by walking and the tail often swingsfrom side to side or is used as a prop, particularly when climbing. In their normal gait, only one leg isadvanced at a time in the manner adopted by their ancestors, the lobe-finned fish.[54] Some salamanders inthe genus Aneides and certain plethodontids climb trees and have long limbs, large toepads and prehensiletails.[45] In aquatic salamanders and in frog tadpoles, the tail has dorsal and ventral fins and is moved fromside to side as a means of propulsion. Adult frogs do not have tails and caecilians have only very shortones.[55]

Salamanders use their tails in defence and some are prepared to jettison them to save their lives in a processknown as autotomy. Certain species in the Plethodontidae have a weak zone at the base of the tail and usethis strategy readily. The tail often continues to twitch after separation which may distract the attacker andallow the salamander to escape. Both tails and limbs can be regenerated.[56] Adult frogs are unable to regrowlimbs but tadpoles can do so.[55]

Circulatory system

Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. Inthe juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps theblood through the gills where it is oxygenated, around the body and back to the heart in a single loop. In theadult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consistsof a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumpedthrough the pulmonary artery to the lungs. Continued contraction then pumps oxygenated blood around therest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.[57]

Nervous and sensory systems

The nervous system is basically the same as in other vertebrates, with a central brain, a spinal cord, andnerves throughout the body. The amphibian brain is less well developed than that of reptiles, birds andmammals but is similar in morphology and function to that of a fish. It consists of equal parts cerebrum,midbrain and cerebellum. Various parts of the cerebrum process sensory input, such as smell in the olfactorylobe and sight in the optic lobe, and it is additionally the centre of behaviour and learning. The cerebellum isthe centre of muscular coordination and the medulla oblongata controls some organ functions including


9 od 31 25.4.2015 22:11

Juvenile amphibian circulatory

systems are single loop systems

which resemble fish.

1 - Internal gills where the

blood is reoxygenated

2 - Point where the blood is

depleted of oxygen and returns

to the heart via veins

3 - Two chambered heart.

Red indicates oxygenated

blood, and blue represents

oxygen depleted blood.

Dissected frog:1 Right atrium,

2 Lungs, 3 Aorta, 4 Egg mass,

5 Colon, 6 Left atrium,

7 Ventricle, 8 Stomach, 9 Liver,

10 Gallbladder,

11 Small intestine, 12 Cloaca

heartbeat and respiration. The brain sends signals through the spinal cordand nerves to regulate activity in the rest of the body. The pineal body,known to regulate sleep patterns in humans, is thought to produce thehormones involved in hibernation and aestivation in amphibians.[58]

Tadpoles retain the lateral line system of their ancestral fishes, but this islost in terrestrial adult amphibians. Some caecilians possess electroreceptorsthat allow them to locate objects around them when submerged in water.The ears are well developed in frogs. There is no external ear, but the largecircular eardrum lies on the surface of the head just behind the eye. Thisvibrates and sound is transmitted through a single bone, the stapes, to theinner ear. Only high-frequency sounds like mating calls are heard in thisway, but low-frequency noises can be detected through anothermechanism.[54] There is a patch of specialized haircells, called papillaamphibiorum, in the inner ear capable of detecting deeper sounds. Anotherfeature, unique to frogs and salamanders, is the columella-operculumcomplex adjoining the auditory capsule which is involved in thetransmission of both airborne and seismic signals.[59] The ears ofsalamanders and caecilians are less highly developed than those of frogs asthey do not normally communicate with each other through the medium of sound.[60]

The eyes of tadpoles lack lids, but at metamorphosis, the cornea becomes more dome-shaped, the lensbecomes flatter, and eyelids and associated glands and ducts develop.[54] The adult eyes are an improvementon invertebrate eyes and were a first step in the development of more advanced vertebrate eyes. They allowcolour vision and depth of focus. In the retinas are green rods, which are receptive to a wide range ofwavelengths.[60]

Digestive and excretory systems

Many amphibians catch their prey by flicking out an elongated tongue witha sticky tip and drawing it back into the mouth before seizing the item withtheir jaws. Some use inertial feeding to help them swallow the prey,repeatedly thrusting their head forward sharply causing the food to movebackwards in their mouth by inertia. Most amphibians swallow their preywhole without much chewing so they possess voluminous stomachs. Theshort oesophagus is lined with cilia that help to move the food to thestomach and mucus produced by glands in the mouth and pharynx eases itspassage. The enzyme chitinase produced in the stomach helps digest thechitinous cuticle of arthropod prey.[61]

Amphibians possess a pancreas, liver and gall bladder. The liver is usuallylarge with two lobes. Its size is determined by its function as a glycogen andfat storage unit, and may change with the seasons as these reserves are builtor used up. Adipose tissue is another important means of storing energy andthis occurs in the abdomen, under the skin and, in some salamanders, in thetail.[62]

There are two kidneys located dorsally, near the roof of the body cavity.Their job is to filter the blood of metabolic waste and transport the urine viaureters to the urinary bladder where it is stored before being passed outperiodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen asammonia in large quantities of dilute urine, while terrestrial species, with a greater need to conserve water,excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their


10 od 31 25.4.2015 22:11

The axolotl (Ambystoma

mexicanum) retains its larval

form with gills into adulthood

Male orange-thighed frog

(Litoria xanthomera) grasping

the female during amplexus

metabolic waste as uric acid.[63]

Respiratory system

The lungs in amphibians are primitive compared to those of amniotes,possessing few internal septa and large alveoli, and consequently having acomparatively slow diffusion rate for oxygen entering the blood. Ventilationis accomplished by buccal pumping. Most amphibians, however, are able toexchange gases with the water or air via their skin. To enable sufficientcutaneous respiration, the surface of their highly vascularized skin mustremain moist to allow the oxygen to diffuse at a sufficiently high rate.[61]

Because oxygen concentration in the water increases at both lowtemperatures and high flow rates, aquatic amphibians in these situations canrely primarily on cutaneous respiration, as in the Titicaca water frog and thehellbender salamander. 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.[61]

Reproduction

For the purpose of reproduction most amphibians require fresh wateralthough some lay their eggs on land and have developed various means ofkeeping them moist. A few (e.g. Fejervarya raja) can inhabit brackishwater, but there are no true marine amphibians. There are reports, however,of particular amphibian populations unexpectedly invading marine waters.Such was the case with the Black Sea invasion of the natural hybridPelophylax esculentus reported in 2010.[64]

Several hundred frog species in adaptive radiations (e.g.,Eleutherodactylus, the Pacific Platymantines, the Australo-Papuanmicrohylids, and many other tropical frogs), however, do not need anywater for breeding in the wild. They reproduce via direct development, anecological and evolutionary adaptation that has allowed them to becompletely independent from free-standing water. Almost all of these frogs live in wet tropical rainforestsand their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within theegg. Reproductive success of many amphibians is dependent not only on the quantity of rainfall, but theseasonal timing.[65]

In the tropics, many amphibians breed continuously or at any time of year. In temperate regions, breeding ismostly seasonal, usually in the spring, and is triggered by increasing day length, rising temperatures orrainfall. Experiments have shown the importance of temperature, but the trigger event, especially in aridregions, is often a storm. In anurans, males usually arrive at the breeding sites before females and the vocalchorus they produce may stimulate ovulation in females and the endocrine activity of males that are not yetreproductively active.[66]

In caecilians, fertilisation is internal, the male extruding an intromittent organ, the phallodeum, and insertingit into the female cloaca. The paired Müllerian glands inside the male cloaca secrete a fluid which resemblesthat produced by mammalian prostate glands and which may transport and nourish the sperm. Fertilisationprobably takes place in the oviduct.[67]

The majority of salamanders also engage in internal fertilisation. In most of these, the male deposits a


11 od 31 25.4.2015 22:11

spermatophore, a small packet of sperm on top of a gelatinous cone, on the substrate either on land or in thewater. The female takes up the sperm packet by grasping it with the lips of the cloaca and pushing it into thevent. The spermatozoa move to the spermatheca in the roof of the cloaca where they remain until ovulationwhich may be many months later. Courtship rituals and methods of transfer of the spermatophore varybetween species. In some, the spermatophore may be placed directly into the female cloaca while in others,the female may be guided to the spermatophore or restrained with an embrace called amplexus. Certainprimitive salamanders in the families Sirenidae, Hynobiidae and Cryptobranchidae practice externalfertilisation in a similar manner to frogs, with the female laying the eggs in water and the male releasingsperm onto the egg mass.[67]

With a few exceptions, frogs use external fertilisation. The male grasps the female tightly with his forelimbseither behind the arms or in front of the back legs, or in the case of Epipedobates tricolor, around the neck.They remain in amplexus with their cloacae positioned close together while the female lays the eggs and themale covers them with sperm. Roughened nuptial pads on the male's hands aid in retaining grip. Often themale collects and retains the egg mass, forming a sort of basket with the hind feet. An exception is thegranular poison frog (Oophaga granulifera) where the male and female place their cloacae in closeproximity while facing in opposite directions and then release eggs and sperm simultaneously. The tailed frog(Ascaphus truei) exhibits internal fertilisation. The "tail" is only possessed by the male and is an extension ofthe cloaca and used to inseminate the female. This frog lives in fast-flowing streams and internal fertilisationprevents the sperm from being washed away before fertilisation occurs.[68] The sperm may be retained instorage tubes attached to the oviduct until the following spring.[69]

Most frogs can be classified as either prolonged or explosive breeders. Typically, prolonged breederscongregate at a breeding site, the males usually arriving first, calling and setting up territories. Other satellitemales remain quietly nearby, waiting for their opportunity to take over a territory. The females arrivesporadically, mate selection takes place and eggs are laid. The females depart and territories may changehands. More females appear and in due course, the breeding season comes to an end. Explosive breeders onthe other hand are found where temporary pools appear in dry regions after rainfall. These frogs are typicallyfossorial species that emerge after heavy rains and congregate at a breeding site. They are attracted there bythe calling of the first male to find a suitable place, perhaps a pool that forms in the same place each rainyseason. The assembled frogs may call in unison and frenzied activity ensues, the males scrambling to matewith the usually smaller number of females.[68]

Life cycle

Most amphibians go through metamorphosis, a process of significant morphological change after birth. Intypical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs,toads and salamanders all hatch from the egg as larvae with external gills. Metamorphosis in amphibians isregulated by thyroxine concentration in the blood, which stimulates metamorphosis, and prolactin, whichcounteracts thyroxine's effect. Specific events are dependent on threshold values for different tissues.[70]

Because most embryonic development is outside the parental body, it is subject to many adaptations due tospecific environmental circumstances. For this reason tadpoles can have horny ridges instead of teeth,whisker-like skin extensions or fins. They also make use of a sensory lateral line organ similar to that of fish.After metamorphosis, these organs become redundant and will be reabsorbed by controlled cell death, calledapoptosis. The variety of adaptations to specific environmental circumstances among amphibians is wide,with many discoveries still being made.[71]

Eggs

The egg of an amphibian is typically surrounded by a transparent gelatinous covering secreted by theoviducts and containing mucoproteins and mucopolysaccharides. This capsule is permeable to water andgases, and swells considerably as it absorbs water. The ovum is at first rigidly held, but in fertilised eggs the


12 od 31 25.4.2015 22:11

Frogspawn, a mass of eggs

surrounded by jelly

Amphibian egg:

1. Jelly capsule 2. Vitelline

membrane

3. Perivitelline fluid 4. Yolk

plug

5. Embryo

Early stages in the development

of the embryos of the common

frog (Rana temporaria)

innermost layer liquefies and allows the embryo to move freely. This alsohappens in salamander eggs, even when they are unfertilised. Eggs of somesalamanders and frogs contain unicellular green algae. These penetrate thejelly envelope after the eggs are laid and may increase the supply of oxygento the embryo through photosynthesis. They seem to both speed up thedevelopment of the larvae and reduce mortality.[72] Most eggs contain thepigment melanin which raises their temperature through the absorption oflight and also protects them against ultraviolet radiation. Caecilians, someplethodontid salamanders and certain frogs that lay eggs underground haveunpigmented eggs. In the wood frog (Rana sylvatica), the interior of theglobular egg cluster has been found to be up to 6 °C (43 °F) warmer than itssurroundings which is an advantage in its cool northern habitat.[73]

The eggs may be deposited singly or in small groups, or may take the formof spherical egg masses, rafts or long strings. In terrestrial caecilians, theeggs are laid in grape-like clusters in burrows near streams. The amphibioussalamander Ensatina attaches its similar clusters by stalks to underwaterstems and roots. The greenhouse frog (Eleutherodactylus planirostris) layseggs in small groups in the soil where they develop in about two weeksdirectly into juvenile frogs without an intervening larval stage.[74] Thetungara frog (Physalaemus pustulosus) builds a floating nest from foam toprotect its eggs. First a raft is built, then eggs are laid in the centre, andfinally a foam cap is overlaid. The foam has anti-microbial properties. Itcontains no detergents but is created by whipping up proteins and lectinssecreted by the female.[75][76]

Larvae

The eggs of amphibians are typically laid in water and hatch into free-livinglarvae that complete their development in water and later transform intoeither aquatic or terrestrial adults. In many species of frog and in mostlungless salamanders (Plethodontidae), direct development takes place, thelarvae growing within the eggs and emerging as miniature adults. Manycaecilians and some other amphibians lay their eggs on land, and the newlyhatched larvae wriggle or are transported to water bodies. Some caecilians,the Alpine salamander (Salamandra atra) and some of the Africanlive-bearing toads (Nectophrynoides spp.) are viviparous. Their larvae feedon glandular secretions and develop within the female's oviduct, often forlong periods. Other amphibians, but not caecilians, are ovoviviparous. Theeggs are retained in or on the parent's body, but the larvae subsist on the

yolks of their eggs and receive no nourishment from the adult. The larvae emerge at varying stages of theirgrowth, either before or after metamorphosis, according to their species.[77] The toad genus Nectophrynoidesexhibits all of these developmental patterns among its dozen or so members.[5]

Frogs

Frog larvae are known as tadpoles and typically have oval bodies and long, vertically flattened tails withfins. The free-living larvae are normally fully aquatic, but the tadpoles of some species such as (Nannophrysceylonensis) are semi-terrestrial and live among wet rocks.[78] Tadpoles have cartilaginous skeletons, gills forrespiration (external gills at first, internal gills later), lateral line systems and large tails that they use forswimming.[79] Newly hatched tadpoles soon develop gill pouches that cover the gills. The lungs developearly and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some


13 od 31 25.4.2015 22:11

Larva of the long-toed

salamander

(Ambystoma macrodactylum)

Larvae of the alpine newt

(Ichthyosaura alpestris)

species complete their development inside the egg and hatch directly into small frogs. These larvae do nothave gills but instead have specialised areas of skin through which respiration takes place. While tadpoles donot have true teeth, in most species, the jaws have long, parallel rows of small keratinized structures calledkeradonts surrounded by a horny beak.[80] Front legs are formed under the gill sac and hind legs becomevisible a few days later. Tadpoles are typically herbivorous, feeding mostly on algae, including diatomsfiltered from the water through the gills. They are also detritivores, stirring up the sediment at the pondbottom and ingesting edible fragments. They have a relatively long, spiral-shaped gut to enable them todigest this diet.[81] Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles andfish. Young of the Cuban tree frog (Osteopilus septentrionalis) can occasionally be cannibalistic, theyounger tadpoles attacking a larger, more developed tadpole when it is undergoing metamorphosis.[82]

Successive stages in the development of common toad (Bufo bufo) tadpoles, finishing with metamorphosis

At metamorphosis, rapid changes in the body take place as the lifestyle of the frog changes completely. Thespiral‐shaped mouth with horny tooth ridges is reabsorbed together with the spiral gut. The animal developsa large jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, and a tongue is formed.There are associated changes in the neural networks such as development of stereoscopic vision and loss ofthe lateral line system. All this can happen in about a day. A few days later, the tail is reabsorbed, due to thehigher thyroxine concentration required for this to take place.[81]

Salamanders

At hatching, a typical salamander larva has eyes without lids, teeth in bothupper and lower jaws, three pairs of feathery external gills, a somewhatlaterally flattened body and a long tail with dorsal and ventral fins. Theforelimbs may be partially developed and the hind limbs are rudimentary inpond-living species but may be rather more developed in species thatreproduce in moving water. Pond-type larvae often have a pair ofbalancers, rod-like structures on either side of the head that may preventthe gills from becoming clogged up with sediment. Some members of thegenera Ambystoma and Dicamptodon have larvae that never fully developinto the adult form, but this varies with species and with populations. Thenorthwestern salamander (Ambystoma gracile) is one of these and,depending on environmental factors, either remains permanently in thelarval state, a condition known as neoteny, or transforms into an adult.[83]

Both of these are able to breed.[84] Neoteny occurs when the animal'sgrowth rate is very low and is usually linked to adverse conditions such aslow water temperatures that may change the response of the tissues to thehormone thyroxine.[85] Other factors that may inhibit metamorphosisinclude lack of food, lack of trace elements and competition fromconspecifics. The tiger salamander (Ambystoma tigrinum) also sometimesbehaves in this way and may grow particularly large in the process. Theadult tiger salamander is terrestrial, but the larva is aquatic and able tobreed while still in the larval state. When conditions are particularlyinhospitable on land, larval breeding may allow continuation of a population that would otherwise die out.There are fifteen species of obligate neotenic salamanders, including species of Necturus, Proteus andAmphiuma, and many examples of facultative ones that adopt this strategy under appropriate environmentalcircumstances.[86]


14 od 31 25.4.2015 22:11

The caecilian Ichthyophis

glutinosus with eggs and

developing embryo

The ringed caecilian

(Siphonops annulatus)

resembles an earthworm

Lungless salamanders in the family Plethodontidae are terrestrial and lay a small number of unpigmentedeggs in a cluster among damp leaf litter. Each egg has a large yolk sac and the larva feeds on this while itdevelops inside the egg, emerging fully formed as a juvenile salamander. The female salamander oftenbroods the eggs. In the genus Ensatinas, the female has been observed to coil around them and press herthroat area against them, effectively massaging them with a mucous secretion.[87]

In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae arealready carnivorous and continue to feed as predators when they are adults so few changes are needed totheir digestive systems. Their lungs are functional early, but the larvae don't make as much use of them as dotadpoles. Their gills are never covered by gill sacs and are reabsorbed just before the animals leave thewater. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of theskin, the development of eyelids, and certain changes in dentition and tongue structure. Salamanders are attheir most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails areencumbrances on land.[88] Adult salamanders often have an aquatic phase in spring and summer, and a landphase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to theland phase, thyroxine. External gills do not return in subsequent aquatic phases because these are completelyabsorbed upon leaving the water for the first time.[89]

Caecilians

Most terrestrial caecilians that lay eggs do so in burrows or moist places onland near bodies of water. The development of the young of Ichthyophisglutinosus, a species from Sri Lanka, has been much studied. The eel-likelarvae hatch out of the eggs and make their way to water. They have threepairs of external red feathery gills, a blunt head with two rudimentary eyes,a lateral line system and a short tail with fins. They swim by undulatingtheir body from side to side. They are mostly active at night, soon lose theirgills and make sorties onto land. Metamorphosis is gradual. By the age ofabout ten months they have developed a pointed head with sensorytentacles near the mouth and lost their eyes, lateral line systems and tails.

The skin thickens, embedded scales develop and the body divides into segments. By this time, the caecilianhas constructed a burrow and is living on land.[90]

In the majority of species of caecilians, the young are produced by vivipary.Typhlonectes compressicauda, a species from South America, is typical ofthese. Up to nine larvae can develop in the oviduct at any one time. Theyare elongated and have paired sac-like gills, small eyes and specialisedscraping teeth. At first, they feed on the yolks of the eggs, but as this sourceof nourishment declines they begin to rasp at the ciliated epithelial cells thatline the oviduct. This stimulates the secretion of fluids rich in lipids andmucoproteins on which they feed along with scrapings from the oviductwall. They may increase their length sixfold and be two-fifths as long astheir mother before being born. By this time they have undergonemetamorphosis, lost their eyes and gills, developed a thicker skin andmouth tentacles, and reabsorbed their teeth. A permanent set of teeth grow through soon after birth.[91][92]

The ringed caecilian (Siphonops annulatus) has developed a unique adaptation for the purposes ofreproduction. The progeny feed on a skin layer that is specially developed by the adult in a phenomenonknown as maternal dermatophagy. The brood feed as a batch for about seven minutes at intervals ofapproximately three days which gives the skin an opportunity to regenerate. Meanwhile they have beenobserved to ingest fluid exuded from the maternal cloaca.[93]

Parental care


15 od 31 25.4.2015 22:11

Male common rocket frog

(Colostethus panamensis)

carrying tadpoles on his back

Male common midwife toad

(Alytes obstetricans) carrying

eggs

The care of offspring among amphibians has been little studied but, ingeneral, the larger the number of eggs in a batch, the less likely it is that anydegree of parental care takes place. Nevertheless, it is estimated that in upto 20% of amphibian species, one or both adults play some role in the careof the young.[94] Those species that breed in smaller water bodies or otherspecialised habitats tend to have complex patterns of behaviour in the careof their young.[95]

Many woodland salamanders lay clutches of eggs under dead logs or stoneson land. The black mountain salamander (Desmognathus welteri) does this,the mother brooding the eggs and guarding them from predation as theembryos feed on the yolks of their eggs. When fully developed, they breaktheir way out of the egg capsules and disperse as juvenile salamanders.[96] The male hellbender, a primitivesalamander, excavates an underwater nest and encourages females to lay there. The male then guards thesite for the two or three months before the eggs hatch, using body undulations to fan the eggs and increasetheir supply of oxygen.[44]

The male Colostethus subpunctatus, a tiny frog, protects the egg clusterwhich is hidden under a stone or log. When the eggs hatch, the maletransports the tadpoles on his back, stuck there by a mucous secretion, to atemporary pool where he dips himself into the water and the tadpoles dropoff.[97] The male midwife toad (Alytes obstetricans) winds egg strings roundhis thighs and carries the eggs around for up to eight weeks. He keeps themmoist and when they are ready to hatch, he visits a pond or ditch andreleases the tadpoles.[98] The female gastric-brooding frog (Rheobatrachusspp.) reared larvae in her stomach after swallowing either the eggs orhatchlings; however, this stage was never observed before the speciesbecame extinct. The tadpoles secrete a hormone that inhibits digestion inthe mother whilst they develop by consuming their very large yolk

supply.[99] The pouched frog (Assa darlingtoni) lays eggs on the ground. When they hatch, the male carriesthe tadpoles around in brood pouches on his hind legs.[100] The aquatic Surinam toad (Pipa pipa) raises itsyoung in pores on its back where they remain until metamorphosis.[101] The granular poison frog (Oophagagranulifera) is typical of a number of tree frogs in the poison dart frog family Dendrobatidae. Its eggs arelaid on the forest floor and when they hatch, the tadpoles are carried one by one on the back of an adult to asuitable water-filled crevice such as the axil of a leaf or the rosette of a bromeliad. The female visits thenursery sites regularly and deposits unfertilised eggs in the water and these are consumed by thetadpoles.[102]

Feeding and diet

With a few exceptions, adult amphibians are predators, feeding on virtually anything that moves that theycan swallow. The diet mostly consists of small prey that do not move too fast such as beetles, caterpillars,earthworms and spiders. The sirens (Siren spp.) often ingest aquatic plant material with the invertebrates onwhich they feed[103] and a Brazilian tree frog Xenohyla truncata includes a large quantity of fruit in itsdiet.[104] The Mexican burrowing toad (Rhinophrynus dorsalis) has a specially adapted tongue for pickingup ants and termites. It projects it with the tip foremost whereas other frogs flick out the rear part first, theirtongues being hinged at the front.[105]

Food is mostly selected by sight, even in conditions of dim light. Movement of the prey triggers a feedingresponse. Frogs have been caught on fish hooks baited with red flannel and green frogs (Rana clamitans)have been found with stomachs full of elm seeds that they had seen floating past.[106] Toads, salamandersand caecilians also use smell to detect prey. This response is mostly secondary because salamanders have


16 od 31 25.4.2015 22:11

Northwestern salamander

(Ambystoma gracile) eating a

worm

Edible frog (Pelophylax

esculentus) exhibiting

cannibalism

Male treefrog (Dendropsophus

microcephalus) inflating his air

sac as he calls

been observed to remain stationary near odoriferous prey but only feed if itmoves. Cave-dwelling amphibians normally hunt by smell. Somesalamanders seem to have learned to recognize immobile prey when it hasno smell, even in complete darkness.[107]

Amphibians usually swallow food whole but may chew it lightly first tosubdue it.[33] They typically have small hinged pedicellate teeth, a featureunique to amphibians. The base and crown of these are composed ofdentine separated by an uncalcified layer and they are replaced at intervals.Salamanders, caecilians and some frogs have one or two rows of teeth inboth jaws, but some frogs (Rana spp.) lack teeth in the lower jaw, and toads(Bufo spp.) have no teeth. In many amphibians there are also vomerineteeth attached to a facial bone in the roof of the mouth.[108]

The tiger salamander (Ambystoma tigrinum) is typical of the frogs andsalamanders that hide under cover ready to ambush unwary invertebrates.Others amphibians, such as the Bufo spp. toads, actively search for prey,while the Argentine horned frog (Ceratophrys ornata) lures inquisitive preycloser by raising its hind feet over its back and vibrating its yellow toes.[109]

Among leaf litter frogs in Panama, frogs that actively hunt prey havenarrow mouths and are slim, often brightly coloured and toxic, whileambushers have wide mouths and are broad and well-camouflaged.[110]

Caecilians do not flick their tongues, but catch their prey by grabbing itwith their slightly backward-pointing teeth. The struggles of the prey and

further jaw movements work it inwards and the caecilian usually retreats into its burrow. The subdued preyis gulped down whole.[111]

When they are newly hatched, frog larvae feed on the yolk of the egg. When this is exhausted some moveon to feed on bacteria, algal crusts, detritus and raspings from submerged plants. Water is drawn in throughtheir mouths, which are usually at the bottom of their heads, and passes through branchial food trapsbetween their mouths and their gills where fine particles are trapped in mucus and filtered out. Others havespecialised mouthparts consisting of a horny beak edged by several rows of labial teeth. They scrape and bitefood of many kinds as well as stirring up the bottom sediment, filtering out larger particles with the papillaearound their mouths. Some, such as the spadefoot toads, have strong biting jaws and are carnivorous or evencannibalistic.[112]

Vocalization

The calls made by caecilians and salamanders are limited to occasional softsqueaks, grunts or hisses and have not been much studied. A clicking soundsometimes produced by caecilians may be a means of orientation, as inbats, or a form of communication. Most salamanders are consideredvoiceless, but the California giant salamander (Dicamptodon ensatus) hasvocal cords and can produce a rattling or barking sound. Some species ofsalamander emit a quiet squeak or yelp if attacked.[113]

Frogs are much more vocal, especially during the breeding season whenthey use their voices to attract mates. The presence of a particular speciesin an area may be more easily discerned by its characteristic call than by afleeting glimpse of the animal itself. In most species, the sound is producedby expelling air from the lungs over the vocal cords into an air sac or sacs in the throat or at the corner of themouth. This may distend like a balloon and acts as a resonator, helping to transfer the sound to the


17 od 31 25.4.2015 22:11

American toad, (Anaxyrus

americanus) singing

The red back salamander

(Plethodon cinereus) defends a

territory against intruders.

atmosphere, or the water at times when the animal is submerged.[113] Themain vocalisation is the male's loud advertisement call which seeks to bothencourage a female to approach and discourage other males from intrudingon its territory. This call is modified to a quieter courtship call on theapproach of a female or to a more aggressive version if a male intruderdraws near. Calling carries the risk of attracting predators and involves theexpenditure of much energy.[114] Other calls include those given by afemale in response to the advertisement call and a release call given by amale or female during unwanted attempts at amplexus. When a frog isattacked, a distress or fright call is emitted, often resembling a scream.[115]

The usually nocturnal Cuban tree frog (Osteopilus septentrionalis)produces a rain call when there is rainfall during daylight hours.[116]

Territorial behaviour

Little is known of the territorial behaviour of caecilians, but some frogs and salamanders defend homeranges. These are usually feeding, breeding or sheltering sites. Males normally exhibit such behaviour thoughin some species, females and even juveniles are also involved. Although in many frog species, females arelarger than males, this is not the case in most species where males are actively involved in territorial defence.Some of these have specific adaptations such as enlarged teeth for biting or spines on the chest, arms orthumbs.[117]

In salamanders, defence of a territory involves adopting an aggressiveposture and if necessary attacking the intruder. This may involve snapping,chasing and sometimes biting, occasionally causing the loss of a tail. Thebehaviour of red back salamanders (Plethodon cinereus) has been muchstudied. 91% of marked individuals that were later recaptured were withina metre (yard) of their original daytime retreat under a log or rock.[118] Asimilar proportion, when moved experimentally a distance of 30 metres(98 ft), found their way back to their home base.[118] The salamanders leftodour marks around their territories which averaged 0.16 to 0.33 squaremetres (1.7 to 3.6 sq ft) in size and were sometimes inhabited by a male andfemale pair.[119] These deterred the intrusion of others and delineated the

boundaries between neighbouring areas. Much of their behaviour seemed stereotyped and did not involveany actual contact between individuals. An aggressive posture involved raising the body off the ground andglaring at the opponent who often turned away submissively. If the intruder persisted, a biting lunge wasusually launched at either the tail region or the naso-labial grooves. Damage to either of these areas canreduce the fitness of the rival, either because of the need to regenerate tissue or because it impairs its abilityto detect food.[118]

In frogs, male territorial behaviour is often observed at breeding locations; calling is both an announcementof ownership of part of this resource and an advertisement call to potential mates. In general, a deeper voicerepresents a heavier and more powerful individual, and this may be sufficient to prevent intrusion by smallermales. Much energy is used in the vocalization and it takes a toll on the territory holder who may bedisplaced by a fitter rival if he tires. There is a tendency for males to tolerate the holders of neighbouringterritories while vigorously attacking unknown intruders. Holders of territories have a "home advantage" andusually come off better in an encounter between two similar-sized frogs. If threats are insufficient, chest tochest tussles may take place. Fighting methods include pushing and shoving, deflating the opponent's vocalsac, seizing him by the head, jumping on his back, biting, chasing, splashing, and ducking him under thewater.[120]


18 od 31 25.4.2015 22:11

Cane toad (Bufo marinus) with

poison glands behind the eyes

The fire salamander

(Salamandra salamandra), a

toxic species, wears warning

colours.

Perhaps the most poisonous

animal in the world, the golden

poison frog Phyllobates

terribilis is endemic to

Colombia.[128]

Defence mechanisms

Amphibians have soft bodies with thin skins, and lack claws, defensivearmour, or spines. Nevertheless they have evolved various defencemechanisms to keep themselves alive. The first line of defence insalamanders and frogs is the mucous secretion that they produce. Thiskeeps their skin moist and makes them slippery and difficult to grip. Thesecretion is often sticky and distasteful or toxic.[121] Snakes have beenobserved yawning and gaping when trying to swallow African clawed frogs(Xenopus laevis), which gives the frogs an opportunity to escape.[121][122]

Caecilians have been little studied in this respect, but the Cayenne caecilian(Typhlonectes compressicauda) produces toxic mucus that has killedpredatory fish in a feeding experiment in Brazil.[123] In some salamanders, the skin is poisonous. The rough-skinned newt (Taricha granulosa) from North America and other members of its genus contain theneurotoxin tetrodotoxin (TTX), the most toxic non-protein substance known and almost identical to thatproduced by pufferfish. Handling the newts does not cause harm, but ingestion of even the most minuteamounts of the skin is deadly. In feeding trials, fish, frogs, reptiles, birds and mammals were all found to besusceptible.[124][125] The only predators with some tolerance to the poison are certain populations ofcommon garter snake (Thamnophis sirtalis). In locations where both snake and salamander co-exist, thesnakes have developed immunity through genetic changes and they feed on the amphibians withimpunity.[126] Coevolution occurs with the newt increasing its toxic capabilities at the same rate as the snakefurther develops its immunity.[125] Some frogs and toads are toxic, the main poison glands being at the side ofthe neck and under the warts on the back. These regions are presented to the attacking animal and theirsecretions may be foul-tasting or cause various physical or neurological symptoms. Altogether, over 200toxins have been isolated from the limited number of amphibian species that have been investigated.[127]

Poisonous species often use bright colouring to warn potential predators oftheir toxicity. These warning colours tend to be red or yellow combinedwith black, with the fire salamander (Salamandra salamandra) being anexample. Once a predator has sampled one of these, it is likely to rememberthe colouration next time it encounters a similar animal. In some species,such as the fire-bellied toad (Bombina spp.), the warning colouration is onthe belly and these animals adopt a defensive pose when attacked,exhibiting their bright colours to the predator. The frog Allobates zaparo isnot poisonous, but mimics the appearance of other toxic species in itslocality, a strategy that may deceive predators.[129]

Many amphibians are nocturnal and hide during the day, thereby avoidingdiurnal predators that hunt by sight. Other amphibians use camouflage toavoid being detected. They have various colourings such as mottledbrowns, greys and olives to blend into the background. Some salamandersadopt defensive poses when faced by a potential predator such as the NorthAmerican northern short-tailed shrew (Blarina brevicauda). Their bodieswrithe and they raise and lash their tails which makes it difficult for thepredator to avoid contact with their poison-producing granular glands.[130]

A few salamanders will autotomise their tails when attacked, sacrificing thispart of their anatomy to enable them to escape. The tail may have aconstriction at its base to allow it to be easily detached. The tail isregenerated later, but the energy cost to the animal of replacing it issignificant.[131]

Some frogs and toads inflate themselves to make themselves look large and


19 od 31 25.4.2015 22:11

The extinct golden toad (Bufo

periglenes), last seen in 1989

The Hula painted frog

(Discoglossus nigriventer) was

believed to be extinct but was

rediscovered in 2011.

fierce, and some spadefoot toads (Pelobates spp) scream and leap towards the attacker.[33] Giantsalamanders of the genus Andrias, as well as Ceratophrine and Pyxicephalus frogs possess sharp teeth andare capable of drawing blood with a defensive bite. The blackbelly salamander (Desmognathusquadramaculatus) can bite an attacking common garter snake (Thamnophis sirtalis) two or three times itssize on the head and often manages to escape.[132]

Conservation

Dramatic declines in amphibian populations, including population crashesand mass localized extinction, have been noted since the late 1980s fromlocations all over the world, and amphibian declines are thus perceived tobe one of the most critical threats to global biodiversity.[133] In 2004, theInternational Union for Conservation of Nature (IUCN) reported statingthat currently birds,[134] mammals, and amphibians extinction rates were atmaximum 48 times greater than natural extinction rates—possibly 1,024times higher. In 2006 there were believed to be 4,035 species of amphibiansthat depended on water at some stage during their life cycle. Of these,1,356 (33.6%) were considered to be threatened and this figure is likely tobe an underestimate because it excludes 1,427 species for which there was insufficient data to assess theirstatus.[135] A number of causes are believed to be involved, including habitat destruction and modification,over-exploitation, pollution, introduced species, climate change, endocrine-disrupting pollutants, destructionof the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs ofamphibians), and diseases like chytridiomycosis. However, many of the causes of amphibian declines are stillpoorly understood, and are a topic of ongoing discussion.[136]

With their complex reproductive needs and permeable skins, amphibiansare often considered to be ecological indicators.[137] In many terrestrialecosystems, they constitute one of the largest parts of the vertebratebiomass. Any decline in amphibian numbers will have an impact on thepatterns of predation. The loss of carnivorous species near the top of thefood chain will upset the delicate ecosystem balance and may causedramatic increases in opportunistic species. In the Middle East, a growingappetite for eating frog legs and the consequent gathering of them for foodwas linked to an increase in mosquitoes.[138] Predators that feed onamphibians are affected by their decline. The western terrestrial gartersnake (Thamnophis elegans) in California is largely aquatic and dependsheavily on two species of frog that are diminishing in numbers, theYosemite toad (Bufo canorus) and the mountain yellow-legged frog (Rana

muscosa), putting the snake's future at risk. If the snake were to become scarce, this would affect birds ofprey and other predators that feed on it.[139] Meanwhile, in the ponds and lakes, fewer frogs means fewertadpoles. These normally play an important role in controlling the growth of algae and also forage on detritusthat accumulates as sediment on the bottom. A reduction in the number of tadpoles may lead to anovergrowth of algae, resulting in depletion of oxygen in the water when the algae later die and decompose.Aquatic invertebrates and fish might then die and there would be unpredictable ecologicalconsequences.[140]

A global strategy to stem the crisis was released in 2005 in the form of the Amphibian Conservation ActionPlan. Developed by over eighty leading experts in the field, this call to action details what would be requiredto curtail amphibian declines and extinctions over the following five years and how much this would cost.The Amphibian Specialist Group of the World Conservation Union (IUCN) is spearheading efforts toimplement a comprehensive global strategy for amphibian conservation.[141] Amphibian Ark is anorganization that was formed to implement the ex-situ conservation recommendations of this plan, and they


20 od 31 25.4.2015 22:11

have been working with zoos and aquaria around the world, encouraging them to create assurance coloniesof threatened amphibians.[141] One such project is the Panama Amphibian Rescue and Conservation Projectthat built on existing conservation efforts in Panama to create a country-wide response to the threat ofchytridiomycosis.[142]

See also

List of threatened reptiles and amphibians of the United States

List of amphibians

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Cited texts

Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology. Saunders College Publishing.

ISBN 978-0-03-030504-7.

Stebbins, Robert C.; Cohen, Nathan W. (1995). A Natural History of Amphibians

(http://books.google.com/?id=0v47ou53yVsC&pg=PR11&dq=Caudata+Urodela#v=onepage&

q=Caudata%20Urodela&f=false). Princeton University Press. ISBN 978-0-691-03281-8.

Further reading

Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. W. H. Freeman.

ISBN 978-0-7167-1822-2.

Carroll, Robert L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. Johns Hopkins

University Press. ISBN 978-0-8018-9140-3.

Duellman, William E.; Linda Trueb (1994). Biology of Amphibians. Johns Hopkins University Press.

ISBN 978-0-8018-4780-6.

Frost, Darrel R.; Grant, Taran; Faivovich, Julián; Bain, Raoul H.; Haas, Alexander; Haddad, Célio

F.B.; De Sá, Rafael O.; Channing, Alan; Wilkinson, Mark; Donnellan, Stephen C.; Raxworthy,

Christopher J.; Campbell, Jonathan A.; Blotto, Boris L.; Moler, Paul; Drewes, Robert C.; Nussbaum,

Ronald A.; Lynch, John D.; Green, David M.; Wheeler, Ward C. (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 (https://dx.doi.org

/10.1206%2F0003-0090%282006%29297%5B0001%3ATATOL%5D2.0.CO%3B2). hdl:2246/5781

(http://hdl.handle.net/2246%2F5781).

Pounds, J. Alan; Bustamante, Martín R.; Coloma, Luis A.; Consuegra, Jamie A.; Fogden, Michael P.

L.; Foster, Pru N.; La Marca, Enrique; Masters, Karen L.; Merino-Viteri, Andrés; Puschendorf,

Robert; Ron, Santiago R.; Sánchez-Azofeifa, G. Arturo; Still, Christopher J.; Young, Bruce E. (2006).

"Widespread amphibian extinctions from epidemic disease driven by global warming". Nature 439

(7073): 161–167. Bibcode:2006Natur.439..161A (http://adsabs.harvard.edu

/abs/2006Natur.439..161A). doi:10.1038/nature04246 (https://dx.doi.org/10.1038%2Fnature04246).


Stuart, Simon N.; Chanson, Janice S.; Cox, Neil A.; Young, Bruce E.; Rodrigues, Ana S. L.; Fischman,

Debra L.; Waller, Robert W. (2004). "Status and trends of amphibian declines and extinctions

worldwide". Science 306 (5702): 1783–1786. Bibcode:2004Sci...306.1783S (http://adsabs.harvard.edu

/abs/2004Sci...306.1783S). doi:10.1126/science.1103538 (https://dx.doi.org


30 od 31 25.4.2015 22:11

Wikimedia Commons hasmedia related to Amphibia.

Wikispecies has informationrelated to: Amphibia

The WikibookDichotomous Key has apage on the topic of:Amphibia

/10.1126%2Fscience.1103538). PMID 15486254 (https://www.ncbi.nlm.nih.gov/pubmed/15486254).

Stuart, S. N.; Hoffmann, M.; Chanson, J. S.; Cox, N. A.; Berridge, R. J.; Ramani, P.; Young, B. E., ed.

(2008). Threatened Amphibians of the World (http://www.amphibians.org/publications/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.

External links

Amphibians (http://www.animalspot.net/category/amphibians)

- AnimalSpot.net

ArchéoZooThèque : Amphibians skeletons drawings

(http://photos.archeozoo.org/index/category

/54-amphibiens_langen_amphibians_lang_langes_anfibios_lang_?lang=en_UK) : available in vector,

image and PDF formats

Amphibian Specialist Group (http://www.amphibians.org/)

Amphibian Ark (http://www.amphibianark.org/)

AmphibiaWeb (http://www.amphibiaweb.org/)

Global Amphibian Assessment (http://www.globalamphibians.org/)

Amphibian vocalisations on Archival Sound Recordings (http://sounds.bl.uk

/Browse.aspx?category=Environment&collection=Amphibians)

Retrieved from "http://en.wikipedia.org/w/index.php?title=Amphibian&oldid=659186954"

Categories: AmphibiansAnimal classes

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