Amphibians

Class Amphibia:  

    

Amphibian, common name for any animal of the vertebrate class lying between fishes and reptiles on the evolutionary scale. Emerging from the oceans almost 400 million years ago, amphibians were the first vertebrates (animals with a backbone) to venture onto land. The class, with about 4400 existing species, includes three living orders: the tailed amphibians, consisting of the salamanders (including newts) and sirens; the tailless amphibians, comprised of frogs and toads; and the caecilians, which are wormlike amphibians that are limbless and have vestigial eyes.

With their slender bodies and long tails, some amphibians, such as the salamanders, may be mistaken for lizards and other reptiles. Unlike reptiles, however, amphibians have no epidermal scales, and most must stay close to water to survive.

The name amphibian refers to the double life that most amphibians lead (amphi- means both; bio- means life). Amphibian larvae are fishlike aquatic animals that breathe through gills, whereas adult amphibians are terrestrial carnivores that breathe through lungs and skin. Of course, there are exceptions: Some amphibians are completely terrestrial; others are completely aquatic. But the majority of amphibians live in water for the first part of their life and on land as adults.

The aquatic larva is one reason that most amphibians must live in moist areas. However, it is not the only reason. Another reason amphibians are strongly tied to water is that their eggs do not have a shell. Thus they tend to dry out unless they are laid in water or in very moist places. Yet another reason is that amphibian skin does not bear scales, fur, or any other protective covering that would help prevent drying out. (However, the skin does contain mucous glands whose secretions help keep the skin moist. ) In addition, the skin of almost all adult amphibians is used in respiration and thus must remain moist. If the skin dries out, most amphibians will suffocate.

Amphibians can be defined as vertebrates that are aquatic as larvae and terrestrial as adults, breathe with lungs as adults, have a moist skin that contains many glands, and lack scales and claws. This functional definition is not perfect. As you will soon discover, there are exceptions to almost every "rule" about amphibians.

Appeared early in the Silurian Period. By the time amphibians appeared during the Devonian, there were well-established forests of mosses and ferns. Arthropods too had gone ashore, and many species of insects had already evolved. This meant that any vertebrates whose legs and lungs allowed them to spend time on land had lots of food and no competitors. The first land dwellers had at their disposal an environment full of empty niches!

The heyday of the amphibians was short-lived, however. Climate changes ultimately caused many of the low, swampy amphibian habitats to disappear. Most of the amphibian groups became extinct by the end of the Permian Period (about 245 million years ago), leaving behind four groups of land vertebrates-reptiles, which evolved from amphibians early in the Carboniferous Period, and three orders of small amphibians.

Form and Function in Amphibians

Living amphibians have evolved many adaptations that help them overcome the problems of living both in water and on land. As we examine the essential life functions in amphibians, we will focus our attention on the structures found in frogs. Keep in mind that although the majority of amphibians perform their life functions in ways that are similar to that of a typical frog, there are a number of species that function in unusual ways.

FEEDING Tadpoles, the larvae of frogs and toads, are typically filter feeders, which devour tiny floating plants and bits of organic matter, or herbivores, which use teeth on their lower jaw to graze on attached algae. Some tadpoles eat so much so quickly that up to half of their body mass is in their guts! Tadpoles are mostly herbivorous, so their long, coiled intestines are extremely important in helping them break down their hard-to-digest plant food. Tadpoles, of course, have to grow quickly, for those that lag behind may starve or die if their puddle dries out.

Adult amphibians are almost entirely carnivorous. Salamanders and legless amphibians can only snap their jaws open and shut to catch prey, but many frogs have a long sticky tongue specialized to capture insects.

From the mouth, food slides down the esophagus into the stomach. The stomach connects with the small intestine, where digestive enzymes are manufactured and dissolved food is absorbed. Tubes connect the intestine with organs that also produce digestive enzymes, such as the liver, pancreas, and gall bladder. The small intestine leads to the large intestine, or colon. At the end of the large intestine is a muscular cavity called the cloaca (kloh-^Y-kah), which stores wastes until they are expelled.

Frog SkeletonRESPIRATION Adult amphibians typically breathe using lungs, mouth cavities, and skin. In many amphibians, such as frogs and toads, the lungs are reasonably well developed: Their

INTERNAL TRANSPORT The circulatory system in adult amphibians is closely linked to the development of lungs. In adult amphibians and other air-breathing vertebrates, the circulatory system forms what is known as a double loop. The first loop carries oxygen-poor blood from the heart to the lungs and takes oxygen-rich blood from the lungs back to the heart. The second loop transports oxygen-rich blood from the heart to the rest of the body and oxygen-poor blood from the body back to the heart.

The amphibian heart has three separate chambers: left atrium, right atrium, and ventricle. Blood returning from most of the body collects in a large vein called the vena cava. The vena cava and other veins draining the head and skin empty into the sinus venosus. The sinus venosus, in turn, empties into the right atrium. Blood returning from the lungs in the pulmonary vein enters into the left atrium.

Treefrog's padded-toe front footWhen the atria contract, they empty their blood into a single ventricle. The ventricle then pumps blood into a single large vessel called the bulbus cordus. The bulbus cordus quickly divides into a series of aortic arches that lead to the major body arteries.

Tadpoles have two-chambered hearts and single-loop circulatory systems, much like bony fishes. In a single-loop system, blood travels from the heart to the gills to the rest of the body and back to the heart. When tadpoles mature into adults, the circulatory system changes into a double-loop system.

EXCRETION Amphibians use kidneys to eliminate wastes from their bloodstream. The kidneys are dark-colored oval structures that lie against the dorsal part of the body wall. Structures in the kidneys filter nitrogenous wastes from the blood. The excretory product of the kidneys--urine--travels through tubes called ureters into the cloaca. From there it can be passed directly to the outside or it may be stored in a small urinary bladder.

RESPONSE Amphibians have well-developed nervous and sensory systems. Their eyes are large and often bulge outward from the sides of the head. Amphibians can move their eyes around in their sockets quite well. The surface of the eye is protected from damage under water and kept moist on land by a transparent nictitating (NIHK-tih-tayt-ihng) membrane. This membrane is located inside the regular eyelid, which can also be closed over the eye. Frogs have keen vision for spotting moving insects, but they probably do not see color as well as fishes do.

Webbed hind foot, Rana catesbienna, American bullfrogAlthough amphibian ears have no external sound collectors, they are often very sensitive. Many frogs and toads use croaks, peeps, and a variety of other calls to find a mate, so hearing is vital to their survival and reproduction. Many amphibian larvae and adults also have a lateral line system like that of fishes that detects water movements.

Amphibians respond to adverse conditions in their environment in a number of ways. Because amphibians do not have an internal mechanism for regulating their body temperature, they deal with seasons that are too hot or too cold by hiding in a sheltered spot, such as an underground burrow, and entering a dormant state.

Predators might seem to be a major problem for clawless, soft-skinned amphibians. However, amphibians have many ways of protecting themselves. Some amphibians hide; others run away quickly; still others are well-camouflaged. Many produce distasteful or toxic chemicals that are secreted in their mucous coatings. The more toxic amphibians usually have warning coloration--bright patterns that tell potential predators that the brilliantly colored animal is poisonous or otherwise dangerous. In addition, some poisonous amphibians respond to a threat by waving their tail or freezing in a pose that shows off their warning colors.

Toad's four-toed front footREPRODUCTION When frogs reproduce, the male climbs onto the female's back and squeezes. In response to this stimulus, the female releases as many as 200 eggs, which the male then fertilizes. The embryos are surrounded with a sticky, transparent jelly that attaches the egg mass to underwater plants and nourishes the developing embryos. The eggs typically hatch into tadpoles after one to three weeks.

Not all amphibians have external fertilization and are oviparous like frogs. Many have internal fertilization and are either oviparous, ovoviviparous, or viviparous. Some salamanders have an unusual form of internal fertilization in which the male never needs to come into direct contact with the female! Instead, the male deposits a packet of sperm on the ground and through an elaborate courtship dance persuades the female to pick up the sperm packet with her cloaca.

Parental care in amphibians varies even more than their methods of fertilizing eggs. As in other animals, parental care is a way of ensuring that more young will survive. In many amphibians, it may also be an adaptation to a lack of suitable bodies of water in which aquatic larvae can develop. Some frogs incubate their young in their mouth, vocal sac, or stomach. Male midwife toads wrap sticky strings of fertilized eggs around their hind legs and carry them about until the eggs are ready to hatch. The Surinam toad and marsupial frog have special structures on their back in which the young develop. And in certain tree frogs, tadpoles cling to their parent's back with a suckerlike mouth and are carried between pools of rainwater that collect among the leaves of certain plants.

PHYSICAL CHARACTERISTICS  

The skin of many vertebrates is protected by fur, feathers, or epidermal scales, but amphibians have no such skin covering. Amphibian skin is generally smooth and moist; in some caecilians, small scales are embedded in the skin folds of the body. The inner portion of the skin of amphibians has many blood vessels, which aid respiration, and numerous mucous glands that keep the Moist slimy frog skin (left), and somewhat drier, wartier toad skin (right.) skin moist. Some amphibians have granular glands that secrete a fluid that is irritating and poisonous. Color cells in the skin can, by contraction or expansion, cause skin color changes, as seen in some tree frogs. The outer part of the skin is continually being renewed, and it is sometimes shed in large patches that may be eaten by the amphibian. Besides growing new skin, some salamanders can regenerate whole limbs. Male and female amphibians often differ in size and color, and male frogs and toads may have vocal sacs, swollen thumbs, and skin frills.

INTERNAL STRUCTURE

Because the skeletal, muscular, digestive, nervous, and other systems of the typical amphibian are similar to those of higher animals, biology students study the frog to learn about these systems. The amphibian brain, however, is notable in that the cerebellum is a mere connecting band. The adult heart consists of a muscular ventricle and two auricles, but during the gill-breathing larval stage the circulation resembles that of fishes. The teeth and tongue vary in form and are sometimes absent altogether.

BODY TEMPERATURE  

Like reptiles, amphibians are described as cold-blooded; body temperature varies with the temperature of the environment and is generally much lower than that of birds and mammals. Because they rely on external sources of warmth, amphibians in cool regions hibernate through the winter.

BEHAVIOR  

In mating and feeding, amphibians are usually quite active at night. Most spend at least part of their lives in moist surroundings, and their fragile, jellylike eggs are usually laid in water. The eggs of most species hatch into larvae; the young of frogs and toads are called tadpoles. These gill-breathing larvae commonly metamorphose—that is, their bodies change and they grow into air-breathing adults. Some amphibians, however, become sexually mature in the larval stage and never metamorphose. Adult amphibians are carnivorous, eating mainly insects, slugs, and worms; the larvae are mainly herbivorous. Most amphibians live in warm, moist regions, but a few live in the Temperate Zones, with some frogs ranging far north.

EVOLUTION  DIAGRAM-Amphibian Evolution

Amphibians appear to have evolved from lobe-finned fishes during the early Devonian Period. One hypothesis suggests that during droughts these creatures probably crawled out of drying pools and in time became less dependent on water. Amphibians flourished during the Carboniferous Period, but many had died out by the time modern amphibians appeared in the Mesozoic Era.

A wide variety of amphibians still exist, but with fewer species than any other class of land vertebrates. Although they have thrived since prehistory, amphibians are now threatened by a decline in global populations, noted since about 1980. In addition to the problem of declining populations, reports of amphibians with malformations, such as extra limbs and missing or malformed organs, are increasing. Frogs, toads, and salamanders in many parts of North America display such characteristics. Scientists have been unable to determine the cause for the malformations or population decline, although possible factors may include habitat destruction and fragmentation, parasites, pesticides, pollution, climate change, acid rain, depletion of the ozone layer, or a combination of these factors.

Scientific classification: Amphibians make up the class Amphibia. The tailed amphibians (salamanders and sirens) make up the order Caudata, the tailless amphibians (frogs and toads) the order Anura (or Salientia), and the caecilians the order Cymnophiona.

Orders of Amphibia:

 
Order Apoda. This order contains amphibians without legs. Examples of this order are the caecilians. Most of these organisms live in the moist soil, while some tropical species live in ponds and streams.
 
Order Urodela. These are tailed amphibians. Included in this group are the salamanders and newts. These animals contain a tail and neck. Some of these organisms never develop lungs and must rely on external gills.
 
Order Anura. Frogs and toads represent these tailless organisms. Frogs use powerful hind legs for movement. Their long sticky tongue is attached to the tip of the lower jaw. They exhibit a variety of colorful skin, which is sometimes poisonous. Frogs live in many areas of the world. The male is the only one of the species that makes sounds.

Salamanders Salamander Feeding Movies --- Tiger Salamander --- Eastern Newt ---

These amphibians keep their tails even as adults. Both adults and larvae are carnivores. Although many fossil salamanders were more than 3 meters long, most modern salamanders are about 15 centimeters long. Most hatch as fully aquatic larvae with gills. As adults they live in moist woods, where they tunnel under rocks and rotting logs.

Some salamanders, such as the mud puppy and the axolotl (AK-soh-laht'l), never lose their gills and live in water all their life. Some newts, like the crimson-spotted newt, switch back and forth between water and land. Starting as aquatic larvae, they emerge and live entirely on land in a form called the red eft. After a year or two, the red eft changes its colors to green with red spots and returns to the water to breed.

Structure

Salamanders are characterized by the possession of a notochord, paired pharyngeal clefts (gill slits), and a dorsal, hollow nerve cord. These structures are present either throughout life or at some stage of the animal's development. The notochord is a flexible rod like support that consists of densely packed cells and lies just beneath the nerve cord. The pharyngeal gill slits are openings through the lateral, or side, walls of the pharynx. Other features include an elongated, bilaterally symmetrical body composed of an anterior enlargement of the nerve cord (brain) and an internal body cavity (COELOM) found between the digestive tract and the body wall. Late in embryonic life, a segmented vertebral column develops as a replacement to the notochord. The spine is divided into 5 parts. The cervical, dorsal, sacral, sacro-caudal and caudal regions respectively. The cervical region is composed of a single vertebrae joined to the skull at 4 points, unlike the 2 points in other amphibians. Ribs can be found along the dorsal and in some cases the sacro-caudal regions. The front legs are attatched to the thoracic girdle and the back legs to the pelvic girdle. Salamanders possess a well-developed head, with brain, brain case, and paired sense organs.

 

Systems

Salamanders have three-chambered hearts (two atria and one ventricle), but the flow pattern in such hearts permits effective function of the heart as two separate heart pumps. Left-to-right shunting of blood in the heart serves a useful function in adapting to excessive heat. They have closed circulatory systems. Some are arranged with the respiratory organs and the general body tissues combined in a single circuit. In other, more advanced species, the blood makes a double transit through the heart; one system carries the blood through the respiratory organs (gills or lungs), and the other takes the blood to the other body tissues.

The kidneys of the salamander lie in the forepart of the body, open into the body cavity, and filter materials from both the bloodstream and fluids in the cavity, or coelom while other vertebrates filter only the bloodstream. The urine-forming unit of the kidney is called the nephron, which is a coiled tubule full of complicated twists and turns. The tubules of the various nephrons empty urine into collecting tubules , which in turn empty into the central cavity of the kidney, or kidney pelvis. A large duct finally empties the urine into the cloaca, which is a common chamber for eliminating urine and feces and for copulation. Salamanders excrete urea, which must be expelled in a watery urine.

Sperm storage glands, spermatathecae, occur in the cloacae of all female salamanders. The spermathecae are simple tubuloalveolar glands except in plethodontidae in which spermathecae are compound alveolar glands. There are many hypothesises concerning the evolution of cloacal sperm storage in salamanders but none have yet to be proven.

Regeneration

The term regeneration has been used to describe a variety of biological repair processes ranging from the continuous replacement of dying cells to the regrowth of an amputated tail or limb. The ability to replace lost cells through mitosis (cell division) of those remaining cells is primary to all living systems. Salamanders are amongst the best regenerators of appendages. Most Amphibians can regenerate a tail, but only frog tadpoles and salamanders can produce new limbs. At metamorphosis most frogs lose this capability.

An important feature of appendage regeneration in amphibians is the fact that the normal limb tissues seem to revert (dedifferentiate) to an embryonic type of cell in the region of amputation. These cells accumulate and produce a bump called the blastema which in turn will recreate the lost structures, even regenerating new bone tissue from previously non bone tissue. This shows that cells are not irreversibly committed to a particular life-style. The wound epidermis or surface, if left to itself and open to the environment, seems to promotes mitosis and somehow causes the distal accumulation of blastema cells. Human children below the age of about 11 have been observed to regenerate lost fingertips if the wounds are not prematurely covered. Scientists may eventually learn how to make any cell become another type and thus may be able to bring about the replacement of lost limbs in humans.

An other point of interest is the rarity of tumors in appendages capable of regeneration. In fact, injection of carcinogens into salamander limbs produces extra limbs rather than tumors. Some scientists think that regeneration-controlling mechanisms are powerful enough to prevent carcinogen-activated cells from becoming tumorous and can revert them instead into life producing cell structures.

Salamander skin secretions

Most salamanders produce a mucous like skin secretion which tends to aid in the prevention of body moisture loss. If a salamanders skin drys the respiratory processes fail. The secretions also helps to maintain correct body fluid levels and lubrication while in the water. But some secretions have evolved into a variety of anti predator mechanisms. The skin secretions of these amphibians can be toxic, repulsive or both. Many of the skin secretions are very sticky and serve an antipredator function by adhereing the attacker to the surrondings or itself, thus distracting the predator and allowing the prey to escape. Many species of the Ambystomatadae family have these adhesive secretions produced by the granular glands. Studies showed that adhesive qualities varied according to species and size and that some species adhesive strengths were found to be equal to or stronger than rubber cement, with extremely rapid bond rates. These secretions were able to adhere a variety of predators for up 40 minutes or to distract them totally from the business at hand. Salamanders may use the adhesive secretions in connection with other defenses such as posturing, autotomizing the tail, aposematic or pseudoaposematic coloration and producing toxic or noxious secretions. Predators learn quickly which salamanders to avoid.

Suggestions indicate that the granular glands contain secretions that serve not only as an antipredator device but store nutrients as well. The size of the granular glands of Ambystomatidae decrease during times of food deprevation and the product found in these tail glands is composed of protein. Indicating that it could be using this product as a nutritional source in times of need. Ambystomatidae can also have hedonic glands, secreting pheromones, which play an important part during courtship and mating.

Molting

Salamander molting may occur from every few days to every few weeks except in very cold weather, when it ceases completely. They can molt their skins gradually in small patches or quickly shed large body sections , depending on the species. Salamanders usually eat their shed skins. The mechanisms involved have a natural internal rhythm of occurrence, influenced by light and temperature. In the salamanders the anterior lobe of the pituitary gland secretes a thyrotrophic hormone, which stimulates the thyroid gland to release its hormone or hormones, triggering the molting process.

Frogs and Toads Frog Exploratorium --- American Toad --- Bullfrog --- Northern Leopard Frog --- MPEG movie of frog rotating --- Frog System Diagrams: Skeletal -- Organs -- Digestive -- Nervous

Frogs and Toads: What's the difference? Technically speaking, toads are frogs. Both are members of the order Anura, a group that experts usually refer to simply as frogs. Within this large group, the name "toad" is given to those with dry, warty skin and short hind legs for walking instead jumping. Meanwhile, those with smooth, moist skin and long, strong, webbed hind legs for swimming and jumping are called frogs. In general, frogs live in moist climates and lay their eggs in clusters, while toads live in drier climates and lay their eggs in long chains. But be warned: drawing the line between frogs and toads can be tricky. For example, there are frogs with warty skin, and toads with slimy skin. Many species fit equally well into both categories

Most of us are familiar with the frogs and toads that live all over the United States. Their mating calls fill the night air in many parts of the country. Of these animals, frogs are more closely tied to water. Even adult frogs spend much of their time in or near ponds and streams. Adult toads, on the other hand, often live in moist woods. Some toads have even managed to invade relatively dry places by using the water permeability of their skin to good advantage. These animals burrow deep into moist soil and press their skin against the walls of their burrows. Their skin then functions just like the root hairs of plants. Because of osmotic pressure, water moves out of the soil into the toad.

Many toads and frogs produce potent toxins. For example, from glands behind its head, the marine toad can squirt toxins up to 1 meter, blinding or otherwise injuring predators. Some tropical tree frogs make a poison so powerful that it can kill humans and other large animals. Native tribes in the tropics often poison their arrow tips by rubbing them on these frogs. For this reason, these brightly colored amphibians are called poison arrow frogs. One species of poison arrow frog produces a toxin so powerful that 0. 00001 gram can kill an adult human.

Virtual Frog Builder Site

Digital Frog Site

Virtual Frog Dissection Site

Frog Embryology
DIAGRAM-External Anatomy of Frog --- DIAGRAM-External Anatomy of Frog  --- DIAGRAM-Frog Mouth and Feeding   --- DIAGRAM-Frog Breathing   --- DIAGRAM-Frog Skeleton   --- DIAGRAM-Frog Life Cycle

General view of the entire frog with the body cavity opened. --- Bullfrog colon and small intestine --- Bullfrog kidney, dorsal aorta, renal arteries and small intestine. --- Bullfrog stomach, pancreas and duodenum --- Ventral view of visceral cavity of the bullfrog. Note the: gall bladder, lung and small intestine --- Ventral view of visceral cavity of the bullfrog. Note: stomach, small intestine, pancreas, etc. --- Ventral view of the visceral cavity of the bullfrog. Note the: small and large intestines and fat body.   --- Ventral view of the bullfrog heart showing: the ventricle, left atrium, conus arteriosus, systemic arches and various arteries --- Ventral musculature in the anterior region of the frog. The integument has been removed --- The distal portion of the frog hind limb. Note: the musculature and bones (metatarsae and phalanges). --- Musculature of the distal region of the frog hind leg --- Musculature of the proximal region of the frog hind leg. --- Breathing in the frog - lung expansion etc.   --- Musculature of the frog forelimb --- Ventral view of the thoracic cavity of the frog. Note: liver, heart, gall bladder, stomach, small intestine and eggs.   --- Dorsal (upper) and ventral (lower) views of the general body musculature of the frog, integument removed. ---

 
Tadpole and Adult Comparison
Characteristics Adult Frog Tadpole
Type of limbs legs fins
Heart chambers 3 2
Type of food Carnivorous Vegetarian
Respiratory structure Lungs and Skin Gills

How Amphibians Fit into the World

Adult amphibians throughout the world today prey on animals that have been most abundant for millions of years: insects. As tadpoles, amphibians also devour large quantities of algae, powerfully affecting the energy balance of many bodies of water.

Humans do not often interact with amphibians, although frog legs are considered a delicacy by some. In tropical rain forests, native hunters tip their arrows with the toxic secretions of the poison arrow frogs. This enables them to kill large animals such as jaguars and deer with small weapons. In laboratories, researchers are studying the action of poison arrow frog toxins for clues to the way in which the nervous system works. Amphibians have also been the subject of studies on regeneration. It is still a mystery why salamanders can regenerate lost limbs, but closely related frogs cannot. By solving that mystery, researchers hope to develop new ways of treating humans who have lost limbs due to accidents or birth defects.

SECTION REVIEW

1. In what ways are amphibians adapted to life on land? What characteristics restrict most of them to water?

2. How does a frog carry out its essential life functions?

3. How are frogs and toads similar to salamanders? How are they different?

4. Major changes take place in a tadpole's digestive and circulatory systems during metamorphosis. Why do you think these changes are necessary?

5. Some frogs never go through a free-swimming tadpole stage. Instead, they hatch as tiny frogs. What kind of conditions might have brought about this adaptation?