Amphibian
Amphibians are ectothermic, anamniotic, four-limbed vertebrates that make up the class Amphibia. A wildlife hospital receives a frog or toad for treatment, and the people who brought it made a common mistake. They placed the animal in a water-filled aquarium with no way out. The result can be drowning, because adult amphibians generally cannot breathe underwater. That single misunderstanding points to something strange about these creatures. The smallest known vertebrate on Earth is one of them, a frog from New Guinea measuring just 7.7 mm. The largest amphibian that ever lived stretched up to 9 metres. They sit between water and land, belonging fully to neither. What separates them from the reptiles they resemble? Why do their numbers now fall sharply across the globe? And how did a fish hauling itself onto a Devonian shore become the ancestor of every land vertebrate alive today?
The word amphibian comes from the Ancient Greek amphibios, meaning both kinds of life, joining amphi, of both kinds, with bios, life. Early naturalists used the term loosely for any animal that could live on land or in water, including seals and otters. The modern class draws a sharper line. Reptiles, birds and mammals are amniotes, surrounded in the egg by membranes that resist drying out. Amphibians lack those membranes, so they require water bodies to reproduce. This is the trait that divides them from the lizards they superficially resemble. All living amphibians belong to the subclass Lissamphibia, divided into three orders. Anura covers the frogs and toads. Urodela, also called Caudata, covers the salamanders and newts. Gymnophiona, also called Apoda, covers the caecilians. The study of amphibians is called batrachology, while the study of both reptiles and amphibians together is called herpetology. Counting species is contentious, and two systems compete. One is maintained by the website AmphibiaWeb at the University of California, Berkeley. The other comes from herpetologist Darrel Frost and the American Museum of Natural History, published as Amphibian Species of the World. Following Frost, the total number of known living amphibian species as of the 31st of March 2019 was exactly 8,000. Nearly 90% of them are frogs.
The first major amphibian groups appeared in the Devonian period, around 370 million years ago. They descended from lobe-finned fish resembling the modern coelacanth and lungfish. These fish had multi-jointed, leg-like fins with digits, letting them crawl along the sea bottom. Some had developed primitive lungs to breathe air when stagnant Devonian swamps ran low on oxygen. Ichthyostega was one of the first primitive amphibians, with nostrils, more efficient lungs, four sturdy limbs, a neck, and a tail with fins. Its skull closely resembled that of the lobe-finned fish Eusthenopteron. At the end of the Devonian, 360 million years ago, the land held early plants but no vertebrates. Animals like Ichthyostega may have hauled themselves out using their forelimbs, dragging their hindquarters like an elephant seal. The fish hyomandibula bone, sitting behind the gills, shrank and became the stapes of the amphibian ear, an adaptation needed for hearing in air. In the early Carboniferous, between 360 and 323 million years ago, warm wet swamps spread with mosses, ferns and horsetails. Air-breathing arthropods invaded the land and fed the carnivorous amphibians adapting there. With no other tetrapods present, amphibians sat at the top of the food chain, some filling roles now held by crocodiles. They still returned to water to lay shell-less eggs. The amniotic egg, which prevents the embryo drying out, would later let reptiles reproduce on land and dominate.
Lissamphibia, the only surviving amphibian lineage, first appeared during the Early Triassic, around 250 million years ago. Its origins have long been disputed. The most popular hypothesis traces modern amphibians to temnospondyls, the most diverse group of prehistoric amphibians, during the Permian. Another hypothesis points instead to lepospondyls. A 2005 molecular phylogeny based on rDNA analysis suggested salamanders and caecilians are more closely related to each other than to frogs. The three groups appear to have diverged in the Paleozoic or early Mesozoic, before the supercontinent Pangaea broke apart. The fossil record carries large gaps, which makes single discoveries valuable. In 2008, the dissorophoid temnospondyl Gerobatrachus was found in Early Permian rocks in Texas, showing many features of modern frogs. The oldest true frog with hopping adaptations is Prosalirus bitis, from the Early Jurassic Kayenta Formation of Arizona. The oldest known caecilians, Funcusvermis gilmorei and Eocaecilia micropodia, also come from Arizona. The earliest salamander, Beiyanerpeton jianpingensis, comes from the Late Jurassic of northeastern China. A fourth lissamphibian group, the Albanerpetontidae, survived until about 2 million years ago before going extinct.
Amphibian skin is permeable to water and works as a secondary respiratory interface. Gas exchange through the skin lets adults respire without rising to the surface and even hibernate at the bottom of ponds. To enable this cutaneous respiration, the highly vascularised skin must stay moist so oxygen can diffuse fast enough. Some small terrestrial salamanders and frogs lack lungs entirely and rely solely on their skin. The plethodontid salamanders are the most famous example, having neither lungs nor gills. Mucous glands, found mainly on the head, back and tail, keep the skin damp. Most amphibians also carry granular glands that secrete distasteful or poisonous substances, some lethal to humans. The main poison glands, the parotoids, produce the neurotoxin bufotoxin. They sit behind the ears of toads, along the backs of frogs, behind the eyes of salamanders, and on the upper surface of caecilians. Colour comes from three layers of pigment cells called chromatophores. The deepest are melanophores, then guanophores producing a blue-green colour, then the yellow lipophores nearest the surface. Hormones from the pituitary gland trigger colour change. Because the nervous system does not directly control the pigment cells, the change happens more slowly than in fish. A vividly coloured skin usually warns predators that the animal is toxic. The outer skin is shed periodically in one piece, unlike the flakes of mammals and birds, and amphibians often eat the sloughed skin.
Anura takes its name from the Greek for without and tail. Frogs and toads have long folding hind limbs, shorter forelimbs, webbed clawless toes, no tails, large eyes and moist glandular skin. Smooth-skinned members are called frogs and warty ones toads, though the split is not formal and exceptions abound. The family Bufonidae are the true toads. About 96% of the more than 5,000 frog species belong to the suborder Neobatrachia. Caudata, from the Latin for tail, consists of the salamanders, elongated low-slung animals that resemble lizards only superficially. They are no more related to lizards than to mammals. Most salamanders stay under 15 cm long, hiding by day under stones or logs and emerging at night to forage for worms and insects. The largest family, Plethodontidae, the lungless salamanders, includes 60% of all salamander species. The name newt applies to members of the subfamily Pleurodelinae within the family Salamandridae. The Chinese giant salamander, Andrias davidianus, reaches 1.8 m, while Thorius pennatulus from Mexico seldom exceeds 20 mm. Gymnophiona, from the Greek for naked and serpent, comprises the caecilians, long limbless animals with a worm-like form. Adults range from 8 to 75 centimetres, except Thomson's caecilian, Caecilia thompsoni, which can reach 150 cm. Caecilians have rudimentary eyes covered in skin and a pair of short tentacles near the eye with tactile and olfactory functions. Most live underground in burrows in damp soil, and they have a mostly Gondwanan distribution across tropical Africa, Asia, and Central and South America.
Metamorphosis is regulated by thyroxine concentration in the blood, which stimulates the change, and prolactin, which counteracts it. Specific events depend on threshold values for different tissues. Frog tadpoles, unusually, do not resemble adults. They have cartilaginous skeletons, gills, lateral line systems and large swimming tails. While they lack true teeth, most have long parallel rows of small keratinized structures called keradonts surrounded by a horny beak. At metamorphosis, the spiral mouth and spiral gut are reabsorbed, the animal develops a large jaw, the gills vanish, and eyes, legs and a tongue appear. Stereoscopic vision develops and the lateral line system is lost. All this can happen in about a day, with the tail reabsorbed a few days later under higher thyroxine concentrations. Salamander metamorphosis is less dramatic, since the larvae are already carnivorous and keep feeding as predators when adult. Some salamanders never fully develop into the adult form, a condition called neoteny, usually linked to low water temperatures or lack of food. There are fifteen species of obligate neotenic salamanders, including species of Necturus, Proteus and Amphiuma. Caecilian metamorphosis is gradual. In Ichthyophis glutinosus from Sri Lanka, eel-like larvae hatch with three pairs of external red feathery gills and make their way to water. By about ten months they have developed a pointed head with sensory tentacles and lost their eyes, lateral line systems and tails. Unisexual female mole salamanders of the genus Ambystoma, common in the Great Lakes region, are the oldest known unisexual vertebrate lineage, having emerged about 5 million years ago.
It is estimated that in up to 20% of amphibian species one or both adults play some role in caring for the young. The general rule is that the larger the batch of eggs, the less likely any parental care takes place. The male midwife toad, Alytes obstetricans, winds egg strings around his thighs and carries them for up to eight weeks. He keeps them moist, then visits a pond or ditch to release the tadpoles. The male hellbender, a primitive salamander, excavates an underwater nest, encourages females to lay there, and guards the site for two or three months, fanning the eggs with body undulations to raise their oxygen. The aquatic Surinam toad, Pipa pipa, raises its young in pores on its back, where they stay until metamorphosis. The female gastric-brooding frog, Rheobatrachus, reared larvae in her stomach after swallowing eggs or hatchlings, the tadpoles secreting a hormone that inhibited her digestion. That stage was never observed before the species became extinct. The ringed caecilian, Siphonops annulatus, evolved maternal dermatophagy, in which the young feed on a specially developed layer of the adult's skin. They feed as a batch for about seven minutes at intervals of roughly three days, giving the skin time to regenerate. With permeable skins and complex reproductive needs, amphibians serve as ecological indicators of habitat conditions, and recent decades have brought a dramatic decline in their populations across many species worldwide.
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Common questions
What are amphibians and what makes up the class Amphibia?
Amphibians are ectothermic, anamniotic, four-limbed vertebrates that make up the class Amphibia. All living amphibians belong to the subclass Lissamphibia, which has three orders: Anura (frogs and toads), Urodela or Caudata (salamanders), and Gymnophiona or Apoda (caecilians).
How many amphibian species are there?
Following the classification by Darrel Frost and the American Museum of Natural History, the total number of known living amphibian species as of the 31st of March 2019 was exactly 8,000. Nearly 90% of them are frogs.
Can amphibians breathe underwater?
Adult amphibians generally cannot breathe underwater. Wildlife hospitals report that people often place frogs and toads in water-filled aquaria with no way to climb out, and as a result the animals may drown. Amphibians do use their permeable skin as a secondary respiratory interface.
What is the smallest and largest amphibian?
The smallest amphibian and smallest known vertebrate is Paedophryne amauensis, a frog from New Guinea measuring just 7.7 mm. The largest living amphibian is the 1.8 m Chinese giant salamander, Andrias davidianus, while the largest amphibian that ever existed, the extinct Prionosuchus, reached up to 9 m.
When and how did amphibians evolve?
The first major amphibian groups appeared in the Devonian period, around 370 million years ago, descending from lobe-finned fish similar to modern coelacanths and lungfish. These fish had leg-like fins with digits and primitive lungs, and Ichthyostega was one of the first primitive amphibians with nostrils and more efficient lungs.
Why are amphibians considered ecological indicators?
Amphibians are ecological indicators because of their permeable skins and complex reproductive needs, which make them sensitive to habitat conditions. In recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
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