Amphibians

Amphibians are cold-blooded animals that possess backbones and display features that lie between those of fish and reptiles. They spend time both in water and on land. Their larvae (not yet fully developed offspring) mature in water and breathe through gills, like fish, while adults breathe air through lungs and skin. Amphibians are in the class Amphibia, which includes over 3,500 species. They are further divided into three orders: Anura (frogs and toads), Urodela (salamanders and newts), and Gymnophiona (caecilians, pronounced sih-SILL-yuhns, which are wormlike in appearance).

History

Amphibians evolved from fish about 400 million years ago, when the amount of dry land on Earth increased greatly. Certain fish adapted to these changing conditions by gradually developing limbs to crawl with and lungs to breathe with. Such organisms, capable of life both in water and on land, came to be called amphibians, a name that means "double life." Amphibians were the first vertebrates (animals with backbones) to live on land. However, they returned to the water to breed. The largest variety of amphibians occurred about 360 to 230 million years ago, when the environment was continually alternating between wet and dry conditions. Many of the species that developed during this period no longer exist. The groups of amphibians that survived to the present day can be traced back no further than 200 million years.

Characteristics

Amphibians are cold-blooded animals, meaning they do not have a constant body temperature but instead take on the temperature of their environment. They have moist, scaleless skin that absorbs water and oxygen, but that also makes them vulnerable to dehydration (loss of bodily fluids). Without moist conditions, their skin dries out and they die. Therefore, amphibians are most often found near ponds, marshlands, swamps, and other areas where freshwater is available. Some amphibians become inactive when conditions are unfavorable for survival. This period of inactivity is called estivation when it occurs during hot, dry weather and hibernation when it occurs in response to cold temperatures. Activity resumes when favorable conditions return.

The thin skin of amphibians contains many glands, among them poison glands that protect certain species against predators. The poison from the glands of the brightly colored poison-dart frog is particularly toxic and is used by South American Indians to coat the tips of their arrows. Some amphibians protect themselves from enemies by changing color to blend in with their surroundings.

Life cycle

The life cycle of most amphibians begins in water when the female lays eggs that are fertilized outside of her body. The eggs then hatch into larvae, or tadpoles, that breathe through external gills. The larvae grow flat tails and feed on vegetation. During a process called metamorphosis, physical changes occur and external gills give way to lungs. The tadpoles also change from plant-eating animals to meat eaters. Amphibians usually reach full adulthood at three to four years.

Words to Know

Estivation: State of inactivity during the hot, dry months of summer.

Gill: A bodily organ capable of obtaining oxygen from water.

Hibernation: State of rest or inactivity during the cold winter months.

Invertebrate: An animal lacking a spinal column.

Larva: An animal in its early form that does not resemble the parent and must go through metamorphosis, or change, to reach its adult stage.

Vertebrate: An animal having a spinal column.

Not all amphibians follow this pattern of reproduction. Some salamanders live out their entire lives on land, where they give birth to fully formed live young. Others lay their eggs in moist places on the forest floor, where they hatch as tiny versions of the adults. Some newts retain their external gills throughout their lives. The red-spotted newt of eastern North American spends its juvenile stage on land as the red eft, returning to water to develop and live as an adult.

Three major groupings

Anurans. Frogs and toads make up the order Anura, the largest group of living amphibians, comprising about 3,000 species. Anurans lack tails and have long hind legs that are well adapted for jumping and swimming. Most anurans live in areas where there is freshwater, although some are well adapted to drier habitats. Some common anurans of North America

include the bullfrog, spring peeper, American toad, and spadefoot toad. Frogs and toads differ in that toads have shorter legs and drier skin that appears warty in comparison to the smooth skin of frogs. Frogs range in size, the smallest measuring about 1 inch (2.5 centimeters) and the largest (the West African Goliath frog) measuring more than 1 foot (about 30 centimeters).

Frogs and toads live mainly on a diet of insects and other invertebrates. The largest frogs and toads also eat small mammals, birds, fish, and other amphibians.

Urodeles. The order Urodela contains about 250 species of newts and salamanders. Urodeles range in size from approximately 4 inches (about 10 centimeters) to the largest of all amphibians, the giant salamander of Japan, which grows to more than 5 feet (about 1.5 meters). Urodeles have long tails and small, underdeveloped legs. They are usually found in or near water and often reside in moist soil under rocks or logs. Adults usually spend most of their time on land and have a diet consisting of insects and worms.

Some species of urodeles are aquatic (live in water), including those of the genus Siren. These North American amphibians are shaped like eels, have small forelegs and no hind legs or pelvis. They breathe through external gills as well as lungs and burrow in mud at the bottom of marshes.

Gymnophions. Caecilians of the order Gymnophiona are blind, legless amphibians shaped like worms. They burrow in moist soil in tropical habitats of Africa and South America, feeding on soil invertebrates such as worms. There are at least 160 species of caecilians, ranging in size from 4 inches (about 10 centimeters) to 4.5 feet (about 1 meter) in length, but most are rarely seen despite their size.

Recent decline

In the last half of the twentieth century, scientists noted the alarming decline in the numbers of amphibians and amphibians species around the world. They theorized the decline was due to a number of factors: pollution of freshwater ecosystems, the destruction of amphibian habitats by ever-spreading human populations, and, possibly, increased ultraviolet radiation due to ozone depletion. Amphibians are known as indicator species, or species whose health is an indicator or sign of the health of the ecosystem they inhabit. As their numbers decrease, so do the number of healthy ecosystems around the world, which in turn results in the loss of many other animal and plant species.

amphibians Amphibians are tetrapods that still retain a connection to the water because of their need to lay their eggs in moist conditions. Although modern amphibians, frogs and toads, salamanders, and caecilians (legless amphibians) are familiar organisms today, their Palaeozoic ancestors looked quite different and there are difficulties in establishing relationships between the two groups. The earliest amphibians are known from East Greenland and from Russia. They are exemplified by Ichthyostega from the Late Devonian of East Greenland, an animal about 0.5–1 m long that may have breathed air and moved about on land, but retained a fish-like tail and the sensory canal system and gills of its osteolepiform (bony fish) ancestors. These features indicate that it spent most of its time in the water. Sharp conical teeth in the jaws indicate that this animal was a predator, while a number of skeletal adaptations attest to its ability to support its body on land. These include enlarged ribs to support the viscera, and the replacement of the notochord by vertebrae that consisted of a neural arch and a ventral half-ring connected by bony nodules.

A major division of the amphibians occurred in the Early Carboniferous with the development of two groups, the labyrinthodonts and the lepospondyls (Fig. 1). Labyrinthodonts are named for the complex folding of their tooth enamel. Most of them were stocky, short-legged animals up to 2 m in length, with large skulls and jaws lined with sharp conical teeth. They had notches in the back of the skull to accommodate the tympanic membrane (ear-drum). This was joined to the inner ear by a small bone called the stapes, which had been a jaw support (the hyomandibular) in the osteolepiforms. The large body size and short legs suggest that these animals lived mostly in the water, where they adopted a crocodilian life-style. Typical labyrinthodonts of this type are called temnospondyls. This group also includes some forms that developed aquatic characteristics, such as laterally flattened tails and reduced limbs, which indicate a return to a fully aquatic way of life. Temnospondyls were important carnivores during the Carboniferous and Permian and persisted into the Mesozoic. The last examples are known from the Cretaceous of Australia, where they were contemporaries of the dinosaurs and the mammals. The other group of labyrinthodonts was the anthracosaurs, terrestrial amphibians that gave rise to the reptiles in the Early Carboniferous. In anthracosaurs such as Seymouria from the Permian of Texas, the skull and dentition are still essentially amphibian while the post-cranial skeleton shows a large number of features characteristic of early reptiles. Whether animals such as this were really amphibians or reptiles ultimately depends on whether they laid amniote (hard-shelled) eggs or not; and the answer to this question is not yet known.

The second major group of Palaeozoic amphibians, the lepospondyls, were mostly rather small animals less than 0.3 m long, in which the teeth lacked the labyrinthodont structure and the skull lacked a notch for the tympanic membrane. They are named for the spool-shaped centrum or bony disc surrounding the notochord that contrasts with the labyrinthodont vertebrae formed of several pieces. This group developed elongated bodies and reduced limbs as an adaptation to a completely aquatic existence. One group, the diplocaulids, developed skulls that were greatly extended laterally, giving them a boomerang shape. Hydrodynamic studies have shown that this shape provided lift and allowed the animal to rise rapidly from its resting-place on the bottom to attack fish swimming above it.

The modern amphibians are separated by a large gap in time and morphology from the major Palaeozoic groups. The earliest frogs and salamanders are found in the Jurassic and look very much like their modern descendents. A possible ancestral frog from the Triassic of Madagascar, Triadobatrachus, shows specializations in the ear region similar to those found in some temnospondyl labyrinthodonts, which suggests that they are likely ancestors. On the other hand, it has been argued that salamanders and caecilians were derived from the microsaurs, a lepospondyl group. If this is correct, then modern amphibians are polyphyletic (derived from more than one ancestor), despite the fact that there appear to be a number of characters linking them, including the development of respiration through the skin, cylindrical vertebral centra, and the presence of pedicellate teeth in which a zone of weakness between base and crown allows them to break easily.

David K. Elliott

Bibliography

Carroll, R. L. (1988) Vertebrate paleontology and evolution. W. H. Freeman, New York.
Colbert, E. H. and and Morales, M. (1991) Evolution of the vertebrates. Wiley-Liss, Inc., New York.

Amphibian

The class Amphibia consists worldwide of nearly 4,700 species, contained in three major orders: Caudata (salamanders), Gymnophiona (caecilians), and Anura (frogs and toads). Salamanders are composed of about 415 species worldwide, and are typically characterized by their long tails and four limbs of nearly equal size. They first appeared in the fossil record over 190 million years ago in the late Triassic.

The caecilians consist of about 165 species. They have a mostly pantropical distribution, and are characterized by their elongated, annulated (ringed) bodies and lack of legs, resembling worms. These amphibians first appeared in the fossil record nearly 190 million years ago in the early Jurassic.

By far the most successful of the three orders with about 4,100 species worldwide, frogs and toads are characterized by lack of a true tail and by generally having comparatively enlarged hind limbs. The order Anura first surfaced in the fossil record about 230 million years ago in the early Triassic.

Amphibians have relatively moist, scaleless skin and rely heavily on cutaneous respiration and/or the presence of a buccopharyngeal pump (a muscular pump in the throat) to force air into their mouth and lungs, features not found in other classes of terrestrial vertebrates. In addition, most amphibians produce eggs that develop and hatch outside their bodies laying gelatinous, unshelled eggs in water or moist places. Many undergo a larval aquatic existence before metamorphosis into adults (unlike other classes of terrestrial vertebrates). In a few species, the female retains the eggs in her body where they are nourished directly by her before she gives birth to her young, or they develop by absorbing their own yolk (a phenomenon also known to occur in at least one species of sea snake, class Reptilia).

Some populations of amphibians have disappeared or begun to decline, and this has raised concern among biologists worldwide. It is unknown if this phenomenon is uniformly widespread across all continents, or is occurring only in selected areas.

see also Crocodilians; Reptile; Tuatara; Turtle

Joseph T. Collins

Bibliography

Halliday, Tim R., and Kraig Adler. The Encyclopedia of Reptiles and Amphibians. Facts on File, Inc., 1986.

Pough, F. Harvey, R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.

Stebbins, Robert C., and Nathan W. Cohen. A Natural History of Amphibians. Princeton, NJ: Princeton University Press, 1995.

amphibian in zoology, cold-blooded vertebrate animal of the class Amphibia. There are three living orders of amphibians: the frogs and toads (order Anura, or Salientia), the salamanders and newts (order Urodela, or Caudata), and the caecilians , or limbless amphibians (order Apoda, or Gymnophiona), a little known tropical group. Amphibians, the most primitive of the terrestrial vertebrates, are intermediate in evolutionary position between the fish and the reptiles . Typically they undergo a metamorphosis from an aquatic, water-breathing, limbless larva (called a tadpole) to a terrestrial or partly terrestrial, air-breathing, four-legged adult. The eggs are usually deposited in water or in a protected place where their moisture will be conserved; they have neither shells nor the sets of membranes that surround the eggs of reptiles and other higher vertebrates. Some amphibians lay their eggs in dry places, and the young undergo the larval stage within the egg, emerging as small adults; in these the eggs have evolved various protective structures. Adult amphibians differ from reptiles in having moist skins, without scales or with small, hidden scales. All living amphibians are specialized for their way of life, none representing the main amphibian stock from which the reptiles evolved. The salamanders and newts are superficially the most similar to ancestral amphibians, having long tails and front and hind legs of approximately equal size. Frogs and toads are highly modified for jumping, with large, muscular hind legs and no tails, while the caecilians have lost all external traces of limbs.

amphibians, vehicles for amphibious warfare, produced mostly by the USA. The most successful were the Alligator, an amphibious tractor officially designated a Land Vehicle, Tracked (LVT); and a six-wheeled, boat-shaped truck called from its factory serial numbers the DUKW (D = model year, U = amphibian, K = all-wheel drive, W = dual rear axles). The LVT developed from vehicles used for rescue purposes in the Florida swamp, was used in the Pacific war to lift personnel across coral, mud, or swamp, while armoured, gun-carrying LVTs gave covering fire. The British, who called them ‘buffaloes’, used them during the Scheldt Estuary battle. DUKWs, of which more than 20,000 were built, were used first in the Sicilian campaign. The British, who had their own, unsuccessful, amphibious truck called the ‘terrapin’, acquired 2,000 of them through Lend-Lease. The Germans manufactured an amphibious Volkswagen car and an amphibious tractor, but the former was used only for reconnoitring Soviet lakes and rivers during the German–Soviet war, and few of the latter were constructed.

Japan, the UK, the US, and the USSR also converted tanks for amphibious use. British and American ones with propellers and detachable flotation screens were known as DD (duplex drive) tanks, and were widely employed during, and after, the Normandy landings in June 1944 (see OVERLORD).

am·phib·i·an / amˈfibēən/ • n. Zool. a cold-blooded vertebrate animal of a class (Amphibia) that comprises the frogs, toads, newts, and salamanders, distinguished by having an aquatic gill-breathing larval stage followed (typically) by a terrestrial lung-breathing adult stage. ∎  a seaplane, tank, or other vehicle that can operate on land and on water. • adj. Zool. of or relating to this class of animals.

amphibian Class of egg-laying vertebrates, whose larval stages (tadpoles) are usually spent in water but whose adult life is normally spent on land. Amphibians have smooth, moist skin and are cold-blooded. Larvae breathe through gills; adults usually have lungs. All adults are carnivorous but larvae are frequently herbivorous. There are three living orders: Urodela (newts and salamanders); Anura (frogs and toads) and Apoda or caecilians.

amphibian creature that lives both on land and in water. XVII. f. modL. amphibium (sc. animal) — Gr. amphíbion (sc. zôion), sb. use of n. of adj. amphíbios (see prec., BIO-).
So amphibious XVII.

amphibian n. a small craft capable of moving on land or water driven by propellers and wheels or air cushion.

amphibian in aviation: see seaplane .

amphibians See AMPHIBIA.

amphibians See AMPHIBIA.

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quadragenarian, quinquagenarian, quodlibetarian, Rastafarian, riparian, rosarian, Rotarian, sabbatarian, Sagittarian, sanitarian, Sauveterrian, sectarian, seminarian, septuagenarian, sexagenarian, topiarian, totalitarian, Trinitarian, ubiquitarian, Unitarian, utilitarian, valetudinarian, vegetarian, veterinarian, vulgarian •Adrian, Hadrian •Assyrian, Illyrian, Syrian, Tyrian •morion • Austrian •Dorian, Ecuadorean, historian, Hyperborean, Nestorian, oratorian, praetorian (US pretorian), salutatorian, Salvadorean, Singaporean, stentorian, Taurean, valedictorian, Victorian •Ugrian • Zarathustrian •Cumbrian, Northumbrian, Umbrian •Algerian, Cancerian, Chaucerian, Cimmerian, criterion, Hesperian, Hitlerian, Hyperion, Iberian, Liberian, Nigerian, Presbyterian, Shakespearean, Siberian, Spenserian, Sumerian, valerian, Wagnerian, Zairean •Arthurian, Ben-Gurion, centurion, durian, holothurian, Khachaturian, Ligurian, Missourian, Silurian, tellurian •Circassian, Parnassian •halcyon • Capsian • 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