Editing Amphibian (section)

=== Larvae ===
[[File:Frog spawn time-lapse.gif|thumb|left|alt=Frog spawn development|Early stages in the development of the embryos of the [[common frog]] (''Rana temporaria'')]]
The eggs of amphibians are typically laid in water and hatch into free-living larvae that complete their development in water and later transform into either aquatic or terrestrial adults. In many species of frog and in most lungless salamanders (Plethodontidae), direct development takes place, the larvae growing within the eggs and emerging as miniature adults. Many caecilians and some other amphibians lay their eggs on land, and the newly hatched larvae wriggle or are transported to water bodies. Some caecilians, the [[alpine salamander]] (''Salamandra atra'') and some of the [[Nectophrynoides|African live-bearing toads]] (''Nectophrynoides spp.'') are [[Viviparity|viviparous]]. Their larvae feed on glandular secretions and develop within the female's oviduct, often for long periods. Other amphibians, but not caecilians, are [[Ovoviviparity|ovoviviparous]]. The eggs 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 their growth, either before or after metamorphosis, according to their species.{{sfn | Stebbins | Cohen | 1995 | pp=6–9 }} The toad genus ''Nectophrynoides'' exhibits all of these developmental patterns among its dozen or so members.<ref name="species"/> Amphibian larvae are known as [[tadpole]]s. They have thick, rounded bodies with powerful muscular tails.{{sfn | Dorit | Walker | Barnes | 1991 | p=849 }}

==== Frogs ====
Unlike in other amphibians, frog tadpoles do not resemble adults.<ref>{{cite book|author1=Vitt, Laurie J.|author2=Caldwell, Janalee P.|year=2013|title=Herpetology: An Introductory Biology of Amphibians and Reptiles|publisher=Academic Press|page=42|isbn=978-0123869197}}</ref> The free-living larvae are normally fully aquatic, but the tadpoles of some species (such as ''[[Nannophrys ceylonensis]]'') are semi-terrestrial and live among wet rocks.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?query_src=aw_lists_genera_&table=amphib&where-genus=Nannophrys&where-species=ceylonensis |title=''Nannophrys ceylonensis'' |author=Janzen, Peter |date=May 10, 2005 |publisher=AmphibiaWeb |access-date=July 20, 2012 |archive-date=June 7, 2022 |archive-url=https://web.archive.org/web/20220607143554/https://amphibiaweb.org/cgi-bin/amphib_query?query_src=aw_lists_genera_ |url-status=live }}</ref> Tadpoles have cartilaginous skeletons, gills for respiration (external gills at first, internal gills later), [[lateral line system]]s and large tails that they use for swimming.<ref>{{cite encyclopedia |url=https://www.britannica.com/eb/article-40603/Anura |title=Anura: From tadpole to adult |author1=Duellman, W. E. |author2=Zug, G. R. |encyclopedia=Encyclopædia Britannica |access-date=July 13, 2012 |archive-date=November 2, 2012 |archive-url=https://web.archive.org/web/20121102234820/https://www.britannica.com/EBchecked/topic/29023/Anura |url-status=live }}</ref> Newly hatched tadpoles soon develop gill pouches that cover the gills. These internal gills and operculum are not homologous with those of fish,<ref>[https://books.google.com/books?id=1kfzEAAAQBAJ&dq=tadpoles+external+operculum+homologous&pg=PA81 Comparative Anatomy and Developmental Biology of Vertebrates (Zoology Book)]</ref> and are only found in tadpoles as both salamanders and caecilians have external gills only.<ref>[https://books.google.com/books?id=uU2WDwAAQBAJ&dq=Urodela+Apoda+Anura+internal+external+gills&pg=PA20 Biological Systems in Vertebrates, Vol. 1 Functional Morphology of the Vertebrate Respiratory Systems]</ref> Combined with buccal pumping the internal gills has allowed tadpoles to adopt a [[filter feeder|filter feeding]] lifestyle, even if several species have since evolved other types of feeding strategies.<ref>[https://books.google.com/books?id=CzxVvKmrtIgC&dq=anuran+larvae+filter+feeding+buccal-pump+gills&pg=PA125 Biology of Amphibians]</ref> The lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place. While tadpoles do not have true teeth, in most species, the jaws have long, parallel rows of small keratinized structures called keradonts surrounded by a horny beak.{{sfn | Stebbins | Cohen | 1995 | pp=179–181 }} Front legs are formed under the gill sac and hind legs become visible a few days later.

Iodine and T4 (over stimulate the spectacular [[apoptosis]] [programmed cell death] of the cells of the larval gills, tail and fins) also stimulate the [[evolution of nervous systems]] transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting.<ref>{{Cite journal|author=Venturi, Sebastiano|title=Evolutionary Significance of Iodine|journal=Current Chemical Biology|volume=5|pages=155–162|year=2011|issn=1872-3136|doi=10.2174/187231311796765012|issue=3|doi-broken-date=March 26, 2025 |url=https://www.researchgate.net/publication/272210714|access-date=November 10, 2016|archive-date=December 9, 2019|archive-url=https://web.archive.org/web/20191209171647/https://www.researchgate.net/publication/272210714_Evolutionary_Significance_of_Iodine|url-status=live}}</ref><ref>{{Cite journal|author=Venturi, Sebastiano|title=Iodine, PUFAs and Iodolipids in Health and Disease: An Evolutionary Perspective|journal=Human Evolution|volume= 29 |issue= 1–3|pages=185–205|year=2014|issn=0393-9375}}</ref>

In fact, tadpoles developing in ponds and streams are typically [[herbivore|herbivorous]]. Pond tadpoles tend to have deep bodies, large caudal fins and small mouths; they swim in the quiet waters feeding on growing or loose fragments of vegetation. Stream dwellers mostly have larger mouths, shallow bodies and caudal fins; they attach themselves to plants and stones and feed on the surface films of [[algae]] and bacteria.<ref name="EBAnura"/> They also feed on [[diatom]]s, filtered from the water through the [[gill]]s, and stir up the sediment at bottom of the pond, ingesting edible fragments. They have a relatively long, spiral-shaped gut to enable them to digest this diet.<ref name="EBAnura">{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/29023/Anura |title=Anura |author1=Duellman, William E. |author2=Zug, George R. |year=2012 |encyclopedia=Encyclopædia Britannica |access-date=March 26, 2012 |archive-date=November 2, 2012 |archive-url=https://web.archive.org/web/20121102234820/https://www.britannica.com/EBchecked/topic/29023/Anura |url-status=live }}</ref> Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles and fish. Young of the [[Cuban tree frog]] (''Osteopilus septentrionalis'') can occasionally be [[Cannibalism (zoology)|cannibalistic]], the younger tadpoles attacking a larger, more developed tadpole when it is undergoing metamorphosis.<ref>{{cite journal |author=Crump, Martha L. |year=1986 |title=Cannibalism by younger tadpoles: another hazard of metamorphosis |journal=Copeia |volume=1986 |pages=1007–1009 |jstor=1445301 |doi=10.2307/1445301 |issue=4 }}</ref>

{{wide image|Bufobufotadpoles.jpg|800px|alt=Metamorphosis|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. The spiral-shaped mouth with horny tooth ridges is reabsorbed together with the spiral gut. The animal develops a 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 of the lateral line system. All this can happen in about a day. A few days later, the tail is reabsorbed, due to the higher thyroxine concentration required for this to take place.<ref name="EBAnura" />

==== Salamanders ====
[[File:Ambystoma macrodactylum 26597.JPG|thumb|alt=Larva of the long-toed salamander|Larva of the [[long-toed salamander]] <br/>(''Ambystoma macrodactylum'')]]
[[File:Ambystoma macrodactylum sigillatum, Plumas County, CA.jpg|thumb|alt=Adult of the long-toed salamander|Larva of the [[long-toed salamander]] <br/>(''Ambystoma macrodactylum'')]]
[[File:Mesotriton alpestris apuanus04.jpg|thumb|alt=Larvae of the alpine newt|Larvae of the [[alpine newt]] <br/>(''Ichthyosaura alpestris'')]]
At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills, and a long tail with [[Anatomical terms of location#Dorsal and ventral|dorsal]] and [[Anatomical terms of location#Dorsal and ventral|ventral]] fins. The forelimbs may be partially developed and the hind limbs are rudimentary in pond-living species but may be rather more developed in species that reproduce in moving water. Pond-type larvae often have a pair of balancers, rod-like structures on either side of the head that may prevent the gills from becoming clogged up with sediment.<ref name=EB-Caudata/><ref>{{cite journal |author1=Valentine, Barry D. |author2=Dennis, David M. | year=1964 | title=A comparison of the gill-arch system and fins of three genera of larval salamanders, ''Rhyacotriton'', ''Gyrinophilus'', and ''Ambystoma'' | journal=Copeia | volume=1964 | issue=1 | pages=196–201 | jstor=1440850 | doi=10.2307/1440850 }}</ref> Both of these are able to breed.<ref>{{cite web |url=http://www.amphibiaweb.org/cgi-bin/amphib_query?where-genus=Ambystoma&where-species=gracile |title=''Ambystoma gracile'' |author=Shaffer, H. Bradley |year=2005 |publisher=AmphibiaWeb |access-date=November 21, 2012 |archive-date=December 26, 2012 |archive-url=https://web.archive.org/web/20121226135357/http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Ambystoma&where-species=gracile |url-status=live }}</ref> Some have larvae that never fully develop into the adult form, a condition known as [[neoteny]].<ref name=biology1994/> Neoteny occurs when the animal's growth rate is very low and is usually linked to adverse conditions such as low water temperatures that may change the response of the tissues to the hormone thyroxine.<ref>{{cite web |url=http://pages.uoregon.edu/titus/herp_old/neoteny.htm |title=Metamorphosis vs. neoteny (paedomorphosis) in salamanders (Caudata) |author=Kiyonaga, Robin R |access-date=November 21, 2012 |archive-date=August 20, 2012 |archive-url=https://web.archive.org/web/20120820014148/http://pages.uoregon.edu/titus/herp_old/neoteny.htm |url-status=live }}</ref> as well as lack of food. There are fifteen species of [[wikt:obligate|obligate]] neotenic salamanders, including species of ''[[Necturus]]'', ''[[Olm|Proteus]]'' and ''Amphiuma'', and many examples of [[wikt:facultative|facultative]] ones, such as the [[northwestern salamander]] (''Ambystoma gracile'') and the [[tiger salamander]] (''A. tigrinum'') that adopt this strategy under appropriate environmental circumstances.<ref name=biology1994>{{cite book |title=Biology of Amphibians |last=Duellman |first=William E. |author2=Trueb, Linda |author-link2=Linda Trueb|year=1994 |publisher=JHU Press |isbn=978-0-8018-4780-6 |pages=191–192 |url=https://books.google.com/books?id=CzxVvKmrtIgC&q=salamander+obligate+neoteny&pg=PA191 |access-date=October 15, 2020 |archive-date=January 26, 2021 |archive-url=https://web.archive.org/web/20210126161600/https://books.google.com/books?id=CzxVvKmrtIgC&q=salamander+obligate+neoteny&pg=PA191 |url-status=live }}</ref>

Lungless salamanders in the family Plethodontidae are terrestrial and lay a small number of unpigmented eggs in a cluster among damp leaf litter. Each egg has a large yolk sac and the larva feeds on this while it develops inside the egg, emerging fully formed as a juvenile salamander. The female salamander often broods the eggs. In the genus ''[[Ensatinas]]'', the female has been observed to coil around them and press her throat area against them, effectively massaging them with a mucous secretion.{{sfn | Stebbins | Cohen | 1995 | p=196 }}

In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems. Their lungs are functional early, but the larvae do not make as much use of them as do tadpoles. Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of the skin, the development of eyelids, and certain changes in dentition and tongue structure. Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are encumbrances on land.<ref>{{cite journal |author1=Shaffer, H. Bradley |author2=Austin, C. C. |author3=Huey, R. B. |year=1991 |title=The consequences of metamorphosis on salamander (''Ambystoma'') locomotor performance |journal=Physiological Zoology |volume=64 |issue=1 |pages=212–231 |jstor=30158520 |doi=10.1086/physzool.64.1.30158520 |s2cid=87191067 |url=https://pdfs.semanticscholar.org/35a9/f72917535be00155ac93824e53b1876583da.pdf |archive-url=https://web.archive.org/web/20200210143955/https://pdfs.semanticscholar.org/35a9/f72917535be00155ac93824e53b1876583da.pdf |url-status=dead |archive-date=2020-02-10 }}</ref> Adult salamanders 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, thyroxine. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.<ref name=EB-Caudata>{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/100353/Caudata |title=Caudata |author=Wake, David B. |year=2012 |encyclopedia=Encyclopædia Britannica |access-date=March 26, 2012 |archive-date=May 16, 2013 |archive-url=https://web.archive.org/web/20130516165633/https://www.britannica.com/EBchecked/topic/100353/Caudata |url-status=live }}</ref>

==== Caecilians ====
[[File:CaecilianNHM.png|thumb|left|alt=''Ichthyophis glutinosus''|The caecilian ''Ichthyophis glutinosus'' with eggs and developing embryo]]
Most terrestrial caecilians that lay eggs do so in burrows or moist places on land near bodies of water. The development of the young of ''[[Ichthyophis glutinosus]]'', a species from Sri Lanka, has been much studied. The eel-like larvae hatch out of the eggs and make their way to water. They have three pairs 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 undulating their body from side to side. They are mostly active at night, soon lose their gills and make sorties onto land. Metamorphosis is gradual. By the age of about ten months they have developed a pointed head with sensory tentacles 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 caecilian has constructed a burrow and is living on land.<ref>{{cite journal |author1=Breckenridge, W. R. |author2=Nathanael, S. |author3=Pereira, L. |year=1987 |title=Some aspects of the biology and development of ''Ichthyophis glutinosus'' |journal=Journal of Zoology |volume=211 |issue=3 |pages=437–449 |doi=10.1111/jzo.1987.211.3.437 }}</ref>

[[File:Siphonops annulatus.jpg|thumb|alt=Ringed caecilian|The [[Siphonops annulatus|ringed caecilian]] (''Siphonops annulatus'') resembles an earthworm]]
In the majority of species of caecilians, the young are produced by viviparity. ''[[Typhlonectes compressicauda]]'', a species from South America, is typical of these. Up to nine larvae can develop in the oviduct at any one time. They are elongated and have paired sac-like gills, small eyes and specialised scraping teeth. At first, they feed on the yolks of the eggs, but as this source of nourishment declines they begin to rasp at the ciliated epithelial cells that line the oviduct. This stimulates the secretion of fluids rich in [[lipid]]s and mucoproteins on which they feed along with scrapings from the oviduct wall. They may increase their length sixfold and be two-fifths as long as their mother before being born. By this time they have undergone metamorphosis, lost their eyes and gills, developed a thicker skin and mouth tentacles, and reabsorbed their teeth. A permanent set of teeth grow through soon after birth.<ref>{{cite journal |author=Wake, Marvalee H. |year=1977 |title=Fetal maintenance and its evolutionary significance in the Amphibia: Gymnophiona |journal=Journal of Herpetology |volume=11 |issue=4 |pages=379–386 |jstor=1562719 |doi=10.2307/1562719 |url=https://www.researchgate.net/publication/265101247 |access-date=November 10, 2016 |archive-date=October 4, 2018 |archive-url=https://web.archive.org/web/20181004081631/https://www.researchgate.net/publication/265101247 |url-status=live }}</ref><ref>{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/29797/Gymnophiona |title=Gymnophiona |author=Duellman, William E. |year=2012 |encyclopedia=Encyclopædia Britannica |access-date=March 26, 2012 |archive-date=January 14, 2012 |archive-url=https://web.archive.org/web/20120114043346/https://www.britannica.com/EBchecked/topic/29797/Gymnophiona |url-status=live }}</ref>

Gills are only necessarily during embryonic development, and in species that give birth the offspring is born after gill degeneration. In egg laying caecilians the gills are either reabsorbed before hatching, or, in species that hatch with gill remnants still present, short lived and only leaves behind a gill slit. For species with scales under their skin, the scales does not form before during metamorphosis.<ref>[https://books.google.com/books?id=nCfSBwAAQBAJ&dq=caecilians+gill+degeneration+migration+water&pg=PA309 Metamorphosis: A Problem in Developmental Biology]</ref>

The ringed caecilian (''[[Siphonops annulatus]]'') has developed a unique adaptation for the purposes of reproduction. The progeny feed on a skin layer that is specially developed by the adult in a phenomenon known as maternal dermatophagy. The brood feed as a batch for about seven minutes at intervals of approximately three days which gives the skin an opportunity to regenerate. Meanwhile, they have been observed to ingest fluid exuded from the maternal cloaca.<ref>{{cite journal |author1=Wilkinson, Mark |author2=Kupfer, Alexander |author3=Marques-Porto, Rafael |author4=Jeffkins, Hilary |author5=Antoniazzi, Marta M. |author6=Jared, Carlos |year=2008 |title= One hundred million years of skin feeding? Extended parental care in a Neotropical caecilian (Amphibia: Gymnophiona) |journal=[[Biology Letters]] |doi=10.1098/rsbl.2008.0217 |volume=4 |pages=358–361 |issue=4 |pmc=2610157 |pmid=18547909}}</ref>