Amphibians are a group of cold-blooded tetrapods (four-limbed vertebrates), except those with an amniotic membrane (the amniotes). They are a paraphyletic group belonging to the class Amphibia. All living amphibians belong to the subclass Lissamphibia, which contains three orders: Anura (frogs and toads), Urodela (salamanders), and Gymnophiona (caecilians, the legless amphibians).
The term ‘amphibian’ is derived from the Greek word ‘amphíbios,’ meaning ‘both kinds of life,’ referring to their ability to inhabit terrestrial and aquatic environments.
About 8,000 species of amphibians are currently known, of which almost 90% are frogs.
They widely vary in size, with the smallest amphibian, the microhylid frog Paedophryne amauensis, measuring just 7.7 mm (0.30 inches) in length, while the largest living amphibian, the Chinese giant salamander, reaches up to 1.8 m (5 ft 11 inches).
Amphibians have thin, delicate, and moist skin that enables gaseous exchange with the surroundings (cutaneous respiration). The outer layer, the epidermis, is highly cornified and periodically shed and renewed through molting, a process regulated by the hormones of the pituitary and the thyroid glands.
While frogs have smooth skin, toads (their fellow Anurans) are characterized by a highly warty texture. Caecilians also have highly folded skin with mineralized dermal scales embedded in the inner dermal layer (which appear superficially similar to bony fish scales). However, no such dermal scales are observed in salamanders.
In the juvenile or tadpole stage, an amphibian’s heart is two-chambered, and pumps deoxygenated blood to the gills for purification. However, in adults, the heart is three-chambered, comprising a single ventricle and two atria. The lungs take over the function of gills, allowing deoxygenated blood to enter through the pulmonary artery. Here, the blood is oxygenated and then circulated throughout the body.
All amphibians use gills for respiration in their larval stages, and some, like the axolotl, retain them even in adulthood.
Although some adults breathe through their lungs, which have internal septa and large alveoli, most amphibians use their vascularized skin surface for respiration. For example, while breathing on land, where the oxygen concentration is high, some small species, like plethodontid salamanders, rely completely on their skin for gaseous exchange.
Amphibians possess a short, ciliated oeshophagus that passes partially broken food from the mouth to the stomach. The enzyme chitinase acts upon this mass and helps digest the chitin in the food, such as an insect’s exoskeleton.
They also possess accessory digestive glands, like a large, bilobed liver, pancreas, and gallbladder.
Both larval and adult forms of aquatic amphibians excrete their metabolic waste in the form of ammonia through highly diluted urine. In contrast, terrestrial species need to conserve water and thus expel waste as uric acid.
The kidneys filter the blood by removing metabolic wastes. The urine is transported to the urinary bladder through a pair of tubes, the ureters. In some species of salamanders and land-dwelling frogs, the bladder accounts for 20 to 50% of their body weight.
Although the term ‘Amphibian’ initially referred to all animals that could live on land or in water, including seals and otters, the class Amphibia traditionally includes all tetrapod vertebrates that are not amniotes. All modern amphibians are divided into three subclasses, of which two are extinct. The three extant orders of amphibians are listed under the only extant subclass, Lissamphibia.
However, many extinct amphibian groups, such as Embolomeri (Late Paleozoic large aquatic predators) and Seymouriamorpha (semiaquatic to terrestrial Permian forms), are not included in the above subclasses.
Although amphibians are cosmopolitan in distribution, they are absent in Antarctica, remote oceanic islands, and extremely dry, xeric deserts.
While most frogs and toads inhabit humid, tropical regions, salamanders are concentrated in the cool, montane forests of the Northern Hemisphere. Caecilians, abundant in Asia, Africa, and Central and South America, are mostly found in underground burrows and under plant debris.
They primarily feed on slow-moving prey populations, like beetles, caterpillars, earthworms, spiders, ants, and termites. The Brazilian tree frog is frugivorous, often adding fruits to its diet, while species in the genus Siren consume aquatic plants.
Frog larvae initially feed on the egg yolk but can also consume bacteria, algal crusts, detritus, and raspings from aquatic plants.
Many amphibians in the tropics breed continuously or at any particular time of the year, whereas in temperate regions, breeding usually occurs in spring.
Most frogs are either prolonged or explosive breeders. In prolonged breeding, the males usually arrive at a breeding site, aggregating other members to settle their territories. The females arrive at the site sporadically, laying the eggs after mating, and then leave for setting in another location.
Conversely, explosive breeders, like fossorial frog species, assemble in temporary pools in dry areas after rain. The males attract the females (usually small in number) through mating calls and courtship rituals and scramble to mate with them.
Except for a few species, like Limnonectes larvaepartus and Ascaphus truei, frogs undergo external fertilization. The males tightly grab the females with their forelimbs (amplexus) and orient their cloacae. The couples remain in this posture while the eggs are laid, and the male covers them with his sperm. The male then forms a basket-like structure to store the egg mass with his hind feet.
The males of some frog species, like Colostethus subpunctatus, protect the eggs after they have been laid in hidden sites, such as under a stone or log. After hatching, they glue the tadpoles to their backs using sticky mucous secretions and carry them to a temporary pool to drop them off. Similarly, male midwife toads wind egg strings around their thighs and carry the eggs for about eight weeks till they are ready to hatch.
Salamanders, too, undergo internal fertilization, but their reproductive process is different from that of caecilians. Before mating, the males compete with each other and engage in elaborate courtship displays to win the attention of the females. After mates have been chosen, the males deposit spermatophores (packets of sperm) on a suitable substrate, either on land or in water, while the females grab these packets using the lips of their cloaca and shove them into the cloacal opening. The sperm are stored in the spermatheca (sperm-storing organs), where they remain until ovulation.
Some salamanders, like male hellbenders, show parental care by seeking underwater nests and guiding the females to lay their eggs at the site. They then guard the site until the eggs hatch, fanning them to ensure a proper oxygen supply.
Cecaelians undergo internal fertilization, with the sperm fertilizing the eggs inside the female’s body. In most cases, the male extrudes an external copulatory organ called the ‘phallodeum’ and inserts it into the female’s cloaca, transferring the sperm into her reproductive tract. The fertilization usually occurs in the female’s oviduct.
Amphibians undergo metamorphosis, through which they transform from immature individuals to mature ones by going through intermediate developmental stages.
Usually laid in water, each egg contains an embryo suspended in perivitelline fluid and surrounded by gelatinous capsules. After the embryo derives nutrition from the yolk and develops into a larva, it is ready to hatch, and the capsules are dissolved by enzymes produced by specialized glands at the tip of the larva’s snout.
Amphibian larvae, often called tadpoles, emerge from the eggs with rounded bodies and strong, muscular tails. In most lungless salamanders and a few species of frogs, the larvae grow within the eggs and emerge directly as miniature adults (direct development). Some caecilians, like alpine salamanders and African live-bearing toads, are viviparous, and their larvae develop within the female’s oviduct, whereas Surinam toads are ovoviviparous.
Frogs
Salamanders
Caecilians
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