Amphibians are members of the class Amphibia, subphylum Vertebrata, phylum Chordata. The class Amphibia includes frogs, toads, salamanders, newts and caecilians. Amphibians are characterized by a glandular skin without external scales, by gills during development (and in adulthood in some), and by eggs that may have jelly coats but develop without formation of extraembryonic membranes such as the amnion. Most amphibians also have four limbs. Limbs and lungs are adaptations for life on land; the limbs evolved from the ancestral fishes' lobed fins. The scales and amniote egg evolved by reptiles are further adaptations for life on land and distinguish reptiles from amphibians.
The class Amphibia comprises three living orders and several extinct ones. The living members of the class include those forms which have been mentioned above. Amphibians are thought to have arisen from lobe-finned crossopterygian fishes. Considerable conjecture exists, however, as to whether amphibians actually arose from several lineages. Some Swedish experts tend to favor separate lineages for each of the modern orders; certain British and U.S. scientists support the idea of two lineages, one giving rise to frogs, the other to salamanders and caecilians. The characteristics of vertebrae, teeth, and skull bones form the basis for these hypotheses. Additional developmental and variational data are necessary to support these ideas.
Two major subclasses of extinct amphibians are found in the fossil record. They are the Labyrinthodontia and the Lepospondyli. The amphibians of the Labyrinthodontia, which lived during the late Devonian through Triassic periods (345 to 190 million years ago), include the most primitive amphibians represented by the genus Ichthyostega. They were fresh-water carnivorous animals, with tail fins, small scales, and a fishlike vertebral column. Their skulls had many bones, as did those of their presumed crossopterygian ancestor. The Labyrinthodontia, according to the U.S. paleontologist Alfred S. Romer, include three extinct orders: the Ichthyostegalia, the Temnospondyli, and the Anthracosauria. The Anthracosauria are thought to be the ancestors of reptiles and hence of modern birds and mammals. The Temnospondyli are thought by some scientists to be the ancestors of the modern frogs.
The amphibians of the subclass Lepospondyli, which lived during the Mississippian through lower Permian periods (340 to 270 million years ago), include the extinct orders Nectridea, Aistopoda, and Microsauria. Members of the latter two orders were elongate. Some had limbs, some had reduced limbs, and some had no limbs. Many scientists suggest that the ancestors of modern salamanders and caecilians are among the lepospondyls.
The modern, extant orders of the Amphibia are placed in the subclass Lissamphibia. The superorder Salientia includes extinct froglike forms, and the extant order Anura consists of the frogs and toads. The superorder Uordela and order Caudata include the salamanders. The superorder Gymnophiona includes a newly discovered fossil with small limbs and the living limbless caecilians, order Apoda. These three groups are allied in the Lissamphibia by characteristics of tooth, skin, and fat body structure. Vertebral and skull structure, as well as other characteristics, differ markedly among the three groups. This suggests to many scientists separate origins for each of the three modern orders.
HABITAT AND DISTRIBUTION
The distribution of amphibians is worldwide, except in Antarctica and Greenland. They are found on landmasses and in fresh water. Frogs are the most widespread of the three groups, occurring on all major landmasses inhabited by amphibians. They live in a great variety of habitats, ranging from deserts to rain forests, permanent ponds to high mountain meadows. With some exceptions, an aquatic situation is required for breeding and tadpole development.
Salamanders are primarily a north temperate group, occurring in North America, Europe, and Asia. Some species are found in Africa north of the Sahara, and one group of some 200 species has had an extensive adaptive radiation in the New World tropics in Central and South America to northern Argentina. Salamanders occupy a variety of terrestrial and aquatic habitats; those found in dry environments, such as the Mexican plateau, usually live in ponds. When on land, they are often found in moist leaf litter and under rocks and logs. In northern areas several species are found in ponds, streams, or rivers. Many species require water for breeding and development, but many other species breed on land and forgo the larval stage.
The caecilians are tropical in distribution. They occur in the New World from southern Mexico to northern Argentina, in tropical Africa, and in Southeast Asia. These limbless elongate, virtually tailless animals are usually burrowers. They are found in stream banks, under debris, and along root channels from sea level to 3,000 m (9,840 ft). One group in South America is aquatic and lives in streams, especially in the drainage basin of the Amazon River. The larvae of certain species in Southeast Asia and South America, and probably Africa, also inhabit streams.
The order Anura or Salientia, the frogs and toads, includes 22 to 24 families, according to current classifications. Familiar species are the leopard and the grass frog (genus Rana), which is used in many biology laboratory exercises; the brown, warty-skinned, drier-adapted toads (genus Bufo); and the aquatic, tongueless African clawed frog (genus Xenopus). This frog was once used in pregnancy tests and is now used in research on genetics and development. Several species are found in the eastern United States. Breeding choruses after the spring rains may include the following: two species of toads; leopard frogs, green frogs, pig frogs, and gopher frogs (Rana); woods tree frogs, green tree frogs, squirrel tree frogs, barking tree frogs, and little grass frogs (Hyla); cricket frogs (Acris); and eastern narrow-mouthed toads (Microhyla). Among the more bizarre of the world's frogs are the following: the giant frog of Africa (Conraua or Giganturana goliath), which may reach 1 m (39 in) in body length; Sminthillus, the smallest frog, at 12 mm (0.5 in) in body length, which lays one egg a year; Centrolene of Central America, whose skin is translucent and whose bones are green; and Pseudis paradoxicus, whose tadpoles are in fact larger than the adult frogs into which they metamorphose.
Experts recognize 10 families in the order Caudata, the salamanders and newts. Among the best known genera are the Eastern North American red eft (Notophthalmus); the European newts (Triturus); and the Spanish Pleurodeles, used in developmental studies. Many species are found in the Appalachian Mountains of the southeastern United States, where salamander communities have been analyzed. The largest salamander is the Japanese giant salamander (Andrias), which extends up to 135 cm (53 in) in length; the smallest is the tropical genus Thorius, at 16 mm (0.6 in).
The order Gymnophiona or Apoda includes the little-known caeciliansÑthe limbless, mostly tailless, elongate, tropical amphibians that burrow or swim. These are considered to look wormlike because of their body shape and the presence of many rings, which resemble the segments of worms. The smallest caecilians (Idiocranium) are mature at 90 mm (3.6 in); the largest (Caecilia) are more than 1,300 mm (52 in).
STRUCTURE AND FUNCTION
The three orders of amphibians are allied by structure of their teeth and their skins particularly. Their teeth have two partsÑan upper crown and a pedicel (base) that is attached to the jaws. The skin is smooth and contains numerous mucous and toxic glands. The outer layer of the skin is keratinized but does not form the epidermal scales seen in reptile skin. In fact, amphibians lack scales except for some species of caecilians that have fishlike scales embedded in the skin. The organ systems of amphibians modify the typical vertebrate plan. They have a relatively straight digestive system with a short intestine, and they have a three-chambered heart; they respire by lungs and sometimes skin as adults and by gills as larvae. Their digestive, excretory, and reproductive systems empty into a common terminal chamber, the cloaca. Their nervous systems are complex and have attributes to facilitate both aquatic and terrestrial life. Larvae and some aquatic adults have multidirectional sensory systems. All amphibians hear. Frogs and salamanders (except those which live in permanently dark locations) have good vision. Vision is nearly lost in caecilians, whose eyes are covered by skin and sometimes bone. They perceive by chemical cues, using their unique tentacles to conduct stimuli to olfactory centers.
Voice production is largely an attribute of frogs. Salamanders and caecilians produce noises, coughs, and grunts, but apparently not for communication. Frogs have complex sound production and perception systems, with species-specific warning, defensive, and breeding communications.
Amphibians are poikilotherms, cold-blooded organisms; their body temperature is slightly higher than that of the environment. They are active at optimal temperatures and withdraw from extreme heat or cold.
Mode of locomotion varies greatly among amphibians. Limbless caecilians burrow, using their heads like shovels and gaining thrust through contraction of muscles to force the body against the soil. The caecilians that swim, and most swimming salamanders, are like eels, using body muscle contraction to propel themselves through the water. Salamanders use their limbs very little in swimming. On land, the limbs give considerable propulsion to the body, and several salamander gaits have been described. The limbs are sprawled out from the body, and the middle of the trunk usually rests on the groundÑin contrast to the faster lizards, whose legs are under the body and raise it above the ground. Frogs make use of their long hind-limbs to effect several kinds of locomotionÑjumping, hopping, swimming, burrowing, and climbing.
LIFE CYCLE AND BEHAVIOR
Breeding in most amphibians is seasonal, usually correlated with temperature and moisture optima. Frogs court by using sounds or actions as attractants; salamanders depend primarily on visual and tactile cues. It is not known how male and female caecilians find each other. The modes of fertilization vary. Most frogs practice external fertilization, the females laying eggs in water and the males spraying sperm over them. Primitive salamanders have external fertilization, but most have internal fertilization, with the female taking up the male's spermatophore. All caecilians have internal fertilization, and the male deposits sperm in the cloaca of the female by using the end of his cloaca as an intromittent organ.
Other trends in amphibian reproductive biology include the following: (1) Many frogs lay large numbers of eggs. Few salamanders and fewer caecilians do. (2) Most frog species have tadpoles, a free-living larval stage; a few have direct development or retain developing young in the body of the female. (3) Many salamander species have larvae; several have direct development. Only a few retain the young. (4) Few caecilians have larvae; some have direct development. Several have a mode of viviparity in which the female nourishes her developing young by oviductal secretions, and they are born after metamorphosis. (5) A number of species of salamanders and caecilians brood their egg clutches. Few frogs do, but some carry eggs on their backs or legs, or in vocal sacs until hatching or metamorphosis. (6) The species that have direct development, brooding, or maternal retention have reduced litter sizes compared to those which lay eggs and abandon them.The larvae of the three orders are morphologically very different, although all are free-living, foraging animals.
Protective behaviors of amphibians include hiding or staying still in the presence of danger and having coloration matching the environment so that the animal is not obvious. Frogs have warning calls that alert other members of the population. Some salamanders and frogs, when disturbed, arch their backs, stiffen, and rock on their bellies. Other salamanders and frogs have flash colors that warn predators away. Several species of each kind of amphibian have toxic skins; predators learn to avoid them. Some nontoxic species mimic the coloration of toxic species so that they too are not consumed. Some species of salamanders have tails that break off when they are attacked. As the predator pursues the twitching tail, the salamander quietly escapes.
The importance of amphibians is evident throughout the world. They are major components of their ecosystem, both predator and prey. They are food for several human cultures. Interest in them ranges from their aesthetic appeal to their use as a source of arrow poison. They are of considerable importance to science, furnishing material for study of cell function, genetics, and development. As laboratory specimens they are used in the study of anatomy, behavior, ecology, and evolution. It is becoming recognized that many amphibians are barometers of the degradation of the environment, as populations are dying because of such factors as environmental contaminants and their effect, and changes in land use. These factors can modify the effects of diseases, predators, and even evolution, especially through human actions.
Amphibians are no longer present in some areas where they were recently abundant, including relatively pristine, undisturbed habitats. This is occurring in many areas of the world. At the same time, amphibian populations are stable and growing in other regions. Research is under way to determine why some populations are declining and others are not.