AmphibiansPosted on May 18, 2018 - Last modified: September 8, 2018
Amphibians (Amphibia) are any member of the group of Vertebrate animals characterized by its ability to live in both aquatic and terrestrial habitats.
Its name means "to live a double life" in classical Greek, it reflects this strategy of dual life, although some species are permanent inhabitants of the land, while other species have a completely aquatic mode of existence.
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The three orders of life of amphibians vary greatly in size and structure. The presence of a long tail and two pairs of limbs of approximately the same size distinguishes newts and salamanders (order Caudata) from other amphibians, although members of the eel-like family Sirenidae do not have hind limbs.
Newts and salamanders vary greatly in length; members of the Mexican genus Thorius measure 25-30 mm, while Andrias, a genus of giant aquatic salamanders endemic to China and Japan, reaches a length of more than 1,5 meters.
Frogs and toads (order Anura) are easily identified by their long hind legs and the absence of a tail. They only have five to nine presacral vertebrae. The West African goliath frog, which can reach 30 cm from snout to vent and can weigh up to 3,3 kg, is the largest anuran. Some of the smaller anurans include the South American brachycephalics, which have a muzzle-to-vent length of only 9,8 mm, and some microilids, which grow to 9 to 12 mm as adults. The long, slender, limbless Cacilians (order Gymnophiona) are animals that have adapted to fossil (burrowing) lifestyles by evolving a body segmented by annular grooves and a short, blunt tail. Cecilia can grow to over 1 meter (3 feet) long. The largest species, Caecilia thompsoni, reaches a length of 1,5 meters, while the smallest species, Idiocranium russeli, is only 90 to 114 mm.
Feeding of amphibians
Adult amphibians are almost entirely carnivores. They will eat practically everything they can catch and swallow. Legless amphibians can only open and close their jaws to catch prey. Also, many salamanders and frogs have long, sticky tongues that specialize in opening insects.
More than 6.500 species of living amphibians are known. They first appeared about 340 million years ago, and were one of the first groups to deviate from ancestral fish-tetrapod populations during the evolution of animals from strictly aquatic forms to terrestrial types. Today amphibians are represented by frogs and toads (order Anura), newts and salamanders (order Caudata), and caecilians (order Gymnophiona). These three orders of living amphibians are believed to derive from a single radiation from ancient amphibians, and although they are strikingly different in body shape, they are probably the closest relatives of each other. As a group, the three orders constitute the Lissamphibia subclass.
Neither lysaphibians nor any of the extinct amphibian groups were the ancestors of the group of tetrapods that gave rise to reptiles. Although some aspects of the biology and anatomy of the various groups may demonstrate characteristics that reptilian ancestors possess, amphibians are not the intermediate step in the evolution of reptiles from fish.
Modern amphibians are united by several unique traits. They typically have moist skin and are highly dependent on cutaneous (skin surface) respiration. They have a double canal auditory system, green rods in their retinas to discriminate shades, and pedicelled teeth (two-part). Some of these traits may also have existed in extinct groups.
The members of the three existing orders differ markedly in their structural appearance. Frogs and toads are tailless and squat, with long, powerful rear legs modified for jumping. Caecilians have no limbs, are like worms, and are highly adapted for an early riser existence. Salamanders and newts have tails and two pairs of limbs of approximately the same size; however, they are somewhat less specialized in body shape than the other two orders.
Life cycle: Reproduction, ovulation, larval stage and metamorphosis
Many amphibians have a biphasic life cycle that involves aquatic eggs and larvae that develop into terrestrial or semi-aquatic juveniles and adults.
They commonly lay large amounts of eggs in the water; clutches of tiger salamander (Ambystoma tigrinum) can exceed 5.000 eggs, and large bullfrogs (L. catesbeianus) can produce clutches of 45.000 eggs.
The size of the egg and the temperature of the water are important factors that influence the development time of the embryo. Eggs of many species of anurans laid in warm waters require only a day or two to develop, while eggs laid in cold mountain lakes or streams may not hatch for 30 to 40 days.
The development of salamander eggs often takes longer, and hatching occurs between 20 and 270 days after fertilization.
Many amphibians are required to breed in stagnant waters. The eggs are deposited in the water and the developing larvae are essentially free-living embryos; They must find their own food, escape predators, and perform other vital functions while continuing to develop.
As the larvae complete their embryonic development, they adopt an adult body plan that allows them to abandon aquatic habitats in favor of terrestrial ones. Although this metamorphosis from aquatic to terrestrial life occurs in members of all three groups of amphibians, there are many variants, and some taxa give birth to their live young. In fact, the approximately 6.200 living species of amphibians show more evolutionary experiments in reproductive mode than any other group of vertebrates.
Some taxa have aquatic eggs and larvae, while others embed their eggs in the skin on the back of the female; These eggs hatch as miniature tadpoles or frogs. In other groups, the young develop within the oviduct, and the embryos feed on the wall of the oviduct. In some species, the eggs develop inside the female's stomach.
Inside the egg, the embryo is enclosed in a series of semi-permeable gelatinous capsules and suspended in perivithelial fluid, a liquid that also surrounds the yolk. The hatching larvae dissolve these capsules with enzymes secreted by the glands at the tips of their snouts. The mass of the original yolk of the egg provides all the necessary nutrients for the development; however, various stages of development use different nutrients. In early development, fats are the main source of energy.
During gastrulation, an early stage of development in which the embryo consists of two layers of cells, there is an increasing dependence on carbohydrates. After gastrulation, there is a return to the use of fat. During the later stages of development, when morphological structures are formed, proteins are the primary source of energy.
In the neuronal stage, an embryonic stage in which nervous tissue develops, cilia appear in the embryo, and the graceful movement of these hairy structures rotates the embryo within the perivithelial fluid. The larvae of directly developing and life-bearing caciliae, salamanders and some anurans have external gills that press against the inner wall of the egg capsule, allowing an exchange of gases (oxygen and carbon dioxide) with the outside air or with maternal tissues. During development, ammonia is the main form of nitrogen waste, and it is diluted by a constant diffusion of water in the perivithelial fluid.
Limb development in aquatic salamander embryos begins in the head region and proceeds in a wave down the body, with digits appearing sequentially on both sets of limbs. Salamanders that lay their eggs in streams produce embryos that develop both sets of limbs before hatching, but salamanders that lay their eggs in standing water have embryos that develop only the forelimbs before hatching.
Shortly after the appearance of the front legs, most pond-dwelling salamanders develop an ectodermal projection known as a balancer on each side of the head. These rod-shaped structures arise from the mandibular arch, contain nerves and capillaries, and produce a sticky discharge. They prevent the newly hatched larvae from sinking into the sediment and help the salamander maintain balance before its forelimbs develop. After the appearance of the forelimbs, the balancers degenerate.
During the embryonic and early larval stages in anurans, paired adhesive organs arise from the hyoid arch, located at the base of the tongue. The sticky mucus they secrete can form a thread-like bond between a newly hatched tadpole and the egg capsule or vegetation. Consequently, the still developing tadpole can remain in a stable position until it is able to swim and feed on its own, after which the adhesive organs degenerate.
The amphibian larva represents a morphologically distinct stage between the embryo and the adult. The larva is a free-living embryo. It must find food, avoid predators, and participate in all other aspects of free existence while completing its embryonic development and growth. Salamander and cacilian larvae are carnivorous, and have a morphology more similar to that of their respective adult forms than anuran larvae. Shortly after hatching from their egg capsules, larval salamanders, which have four fully developed limbs, begin feeding on small aquatic invertebrates. Salamander larvae are smaller versions of adults, although they differ from their adult counterparts by the presence of external gills, a tail fin, distinctive larval dentition, a rudimentary tongue, and the absence of eyelids. Larval cacilia, also smaller adult models, have external gills, a lateral line system (a group of epidermal sensory organs located on the head and along the sides of the body), and thin skin.
In anurans, tadpoles are like fish when they hatch. They have short, generally ovoid bodies and long laterally compressed tails that are composed of a central axis of musculature with dorsal and ventral fins. The mouth is terminally located (recessed), ringed with an oral disc that is often bordered by papillae and has many rows of keratin denticles. Tadpoles often have horny beaks. Its gills are internal and covered by an operculum.
Water aspirated through the mouth passes through the gills and is expelled through one or more expiracular openings on the side of an opercular chamber. Anuran larvae are micro-phagic and therefore feed heavily on bacteria and algae that cover aquatic plants and debris.
Salamander larvae typically reach full size within two to four months, although larvae can remain for two to three years before metamorphosis occurs. Some large aquatic species, such as the fern (Cryptobranchus alleganiensis) and the mud pup (Necturus maculosus), never fully metamorphose and retain the larval characteristics of adults. The development of tadpoles varies in length between species. Some species of anurans that live in xeric (dry) habitats, where ephemeral ponds can exist for only a few weeks, develop and metamorphose within two to three weeks; however, most species require at least two months. Species that live in cold mountain streams or lakes often require much more time. For example, development of the tailed frog (Ascaphus truei) takes three years to complete.
Metamorphosis involves an abrupt and profound change in the physiology and biochemistry of an animal, with consequent structural and behavioral modifications. These changes mark the transformation from embryo to juvenile and the completion of development. Ultimately, hormones control all larval growth and metamorphosis events, and in many cases, development is accompanied by a change from a fully aquatic to a semi-aquatic or fully terrestrial life.
Although salamanders undergo many structural modifications, these changes are not dramatic. The skin thickens as the dermal glands develop and the caudal fin is reabsorbed. The gills are reabsorbed and closed as the lungs develop and the gill circulation is altered. The eyelids, tongue, and a jawbone are formed, and teeth develop in the maxillary and parasphenoid bones. The changes that occur in caecilians (closure of the gill opening, degeneration of the tail fin, and the development of a tentacle and skin glands) are also minor.
Skeletal changes are much more dramatic in anurans because tadpoles make an abrupt and radical transition to their adult form. The extremities complete their development, and the forelimbs break through the opercular wall, early in metamorphosis. The tail shrinks as the body reabsorbs it, dermal glands develop, and the skin thickens.
As the lungs and pulmonary ventilation develop, the gills and their associated blood circulation disappear. The oral parts of adults replace their degenerated larval counterparts, and hyalingeal structures develop.
All anurans except the pipids (family Pipidae) develop a tongue. In the newly differentiated digestive tract, the intestine is shortened. The eyes are enlarged and structurally altered; eyelids appear. These extreme changes in the metamorphosis of the anuran clearly demarcate the change from an aquatic to a terrestrial way of life.
Other less obvious, although radical, modifications of the larval skull and hyobranchial apparatus (that is, the part of the skeleton that serves as the base for the tongue on the floor of the mouth) occur to make room for the newly developed sense organs. These modifications also facilitate the transition from larval feeding and respiration modes to adult modes.
During metamorphosis, the urogenital system of all amphibians is also modified. A mesonephric or opistonephric kidney (which uses nephrons located in the middle or at the end of the nephric ridge in the developing embryo) replaces the rudimentary and degenerated proonephric kidney. This transition is linked to the transition from the production of a large volume of diluted ammonia to a small amount of concentrated urea. Gonads and associated ducts also appear and begin to mature.