Amphibia is one of the five major classes of vertebrates. There are three orders in the amphibia group, two of which are widely familiar, frogs (Anura) and salamanders (Caudata), and one of which is less well-known, the tropical caecilians (Gymnophiona). The name "amphibian" refers to the use of both aquatic and terrestrial habitats, and the life history patterns of species in the group.
Amphibian Characteristics and Life History
The earliest tetrapods (four-legged, terrestrial vertebrates) were amphibians, and living amphibian species retain some of the primitive characteristics of the first terrestrial vertebrates, which invaded land habitats during the geologic period known as the Devonian, which was approximately 408 million years ago. For example, unlike other terrestrial vertebrates, amphibians lack scales and claws, and are instead characterized by a moist, glandular skin composed of living cells. The skin is involved in respiration to some degree in most amphibians. Certain lungless salamanders rely largely on the skin for gas exchange. In many species, skin glands secrete noxious, or sometimes highly poisonous, substances that serve as deterrents to predators. This is sometimes associated with a warning coloration that advertises toxicity, and some species have even evolved specific defensive postures to show off effectively their warning coloration. One example of this is the "unken reflex," in which the animal throws back its forelimbs to display the bright warning colors on its belly. The possession of skin with living cells limits many amphibian species to fairly moist habitats, although certain species have drier skins and are able to tolerate drier habitats. Amphibian eggs also require moist environments. These lack the water-conserving eggshells found in other terrestrial vertebrate groups and are instead covered with a gelatinous capsule.
Many amphibian species make use of both aquatic and terrestrial habitats, either simultaneously or sequentially during different life stages. A typical amphibian life cycle involves semiterrestrial adults that breed and lay eggs in water. Eggs then develop into aquatic larvae. After a period of growth, the larvae undergo metamorphosis, or transformation, and become semiterrestrial adults. Metamorphosis is triggered by changes in concentrations of circulating hormones . Numerous anatomical changes occur during metamorphosis. These include the loss of gills; the development of lungs; the development of eyelids; the loss of aquatic sensory systems such as the lateral line, which is responsible for underwater "hearing"; tongue development; maturation of the kidneys and gonads ; and changes in the skin, which becomes thicker and develops dermal glands. Of the three amphibian clades , or orders, metamorphosis is most extreme among frogs, where there is a striking difference between the larva (the tadpole) and the adult. Particularly striking is the rapid development of fore-and hindlimbs and the loss of the tail. In addition, there are anatomical changes associated with the transition from an herbivorous tadpole to a carnivorous adult. These include reshapings of the jaws and mouthparts, as well as a shortening of the digestive tract. The rapid and extreme nature of metamorphosis in frogs results in unusually high mortality levels during this period. Locomotion is difficult both in the water and on land because of the simultaneous presence of limbs and a tail.
Although aquatic larvae, metamorphosis, and semiterrestrial adults make up a typical amphibian life history, it is important to remember that there are numerous exceptions to this pattern. These include egg laying in drier habitats, including trees, rather than in aquatic habitats; bypassing the larval stage via direct development of eggs into miniature versions of adults; and even live-bearing in select species. In fact, the diversity of reproductive modes is greater among amphibians than other vertebrate groups.
All amphibians are carnivorous, with the exception of larval frogs (tad-poles), which are primarily herbivorous. Most amphibians are generalists, and will eat anything they can capture and ingest. Frogs and salamanders capture prey with their tongues, which are highly developed. Certain salamander species have specialized projectile tongues that they fire with impressive accuracy to capture prey.
Major Amphibian Groups
The three orders of amphibians are salamanders, frogs, and caecilians.
There are approximately 400 species of salamanders worldwide. Salamanders retain a fairly primitive vertebrate body plan , with slender bodies, four limbs, and tails. Some species have undergone limb reduction, and a few species lack hindlimbs, possessing only two front limbs. Although a few species of salamanders are characterized by external fertilization of eggs, the majority use internal fertilization via spermatophore. After an often extensive courtship, in which mating pairs swim or walk together, the male releases a spermatophore that consists of a gelatinous pedestal capped with a ball of sperm. The female walks over the spermatophore and picks it up with the lips of her cloaca, the common chamber into which digestive, urinary, and reproductive tracts discharge. Sperm are then stored internally in the spemathecae, where they are used to fertilize eggs that are laid later. A large number of salamander species are actually direct developing, with eggs hatching directly into miniature adults. In this way, the aquatic larval stage is bypassed. Other species are perrenibranchiate, and never metamorphose . These species retain the larval morphology their entire lives. Among amphibians, only salamanders include perrenibranchiate species. The axolotl, a well-studied organism, is perrenibranchiate. There are also a small number of live-bearing salamanders, with development taking place in the oviduct of the female.
There are over 4000 species of frogs, making it the most diverse of the amphibian orders. Frogs have made striking modifications in the ancestral amphibian body plan, many of these associated with their saltatory (jumping) mode of locomotion. These include the loss of the tail, a shortened vertebral column, and the fusion of many bones of the forelimbs and hindlimbs. Frogs are ecologically diverse, and have adapted to a wide variety of habitats. Some species are highly aquatic, with adults never making use of terrestrial habitats. Highly aquatic species are characterized by streamlined bodies and extensively webbed hindfeet. They swim by kicking their hindlimbs simultaneously, which essentially represents the use of saltatory motions in the water. Several separate lineages of frogs have adapted to arboreal habitats. These species are characterized by adaptations such as expanded toe pads and opposable thumbs. They also tend to locomote by walking hand over hand rather than jumping. A few arboreal species are even able to parachute from tree to tree. These "flying frogs" have extensive webbing between their fingers and toes, which they extend as they parachute. Finally, some frogs are fossorial and live underground. These species often possess digging specializations such as muscular forearms or hard, keratinized spade-shaped digging tools on the hindfeet. Certain fossorial frogs are even able to survive in desert environments. Individuals remain torpid in burrows much of the time, but come out once a year during heavy rains to reproduce.
Frogs are unique among amphibians in that courtship in most species involves vocal calling by males. Calls are species specific, and considerable research has been done on sexual selection in certain species. Mating typically involves amplexus, in which the male clasps the female either at the shoulders or at the hindlegs. Fertilization is most commonly external, although certain species use internal fertilization.
The third group of amphibians is the caecilians, which are found only in tropical regions. There are approximately 200 species. Caecilians are wormlike amphibians that lack limbs. They have rings on the outside of the body that cause them to appear segmented. Many species lack a tail, and the cloaca is at the end of the body. They vary in size, with the largest species attaining lengths of up to three feet. Caecilians are fossorial or aquatic, and eyes are smaller than in the other two groups. In certain species the eyes are buried beneath the skin or even beneath skull bones. Caecilians also possess a unique sense organ called the tentacle, which is found between the nostril and the eye. It functions as a chemical sensor (detector). Caecilians feed primarily on earthworms and other invertebrates. Unlike frogs and salamanders, caecilians do not use tongue projection to capture prey. Rather, they catch a worm with their jaws, spin it lengthwise in order to remove and ingest a piece, and then try to catch the worm again.
Also in contrast to frogs and salamanders, more than half of caecilian species are viviparous , or live bearing. Fertilization is internal, via a male organ called the phalloideum. Free-swimming larvae hatch within the maternal oviduct, where they live and feed until birth. Caecilian larvae are characterized by special larval teeth that they use to scrape the walls of the oviduct. This stimulates maternal glands to secrete a fatty milk on which the larvae feed. The adult teeth develop prior to birth. Larvae in viviparous species also possess large gills that they flatten against the wall of the oviduct to obtain oxygen.
Recent Amphibian Declines
In the last few decades of the twentieth century, scientists began observing a rapid and alarming global decline in numerous species of amphibians. This is particularly disturbing in that certain previously abundant species have apparently gone extinct in relatively undisturbed pristine habitats. Many causes have been suggested as possibly being involved in the declines, including increased levels of ultraviolet radiation due to destruction of the ozone layer through human activities; declining pH levels from acid rain ; parasites; disease; and habitat loss . It is quite possible that a combination of these factors is responsible.
see also Phylogenetic Relationships of Major Groups.
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