In this article we will discuss about Amphibians:- 1. Origin of Amphibia 2. Factors that Caused Amphibian Evolution 3. Probable Ancestry 4. Structure of Amphibians 5. Digestive System of Amphibians 6. Respiratory System and Sound Production 7. Circulatory System 8. Nervous System 9. Urinogenital System 10. Reproduction and Development 11. 11. Reasons for Extinction.
- Origin of Amphibia
- Factors that Caused Amphibian Evolution
- Probable Ancestry
- Structure of Amphibians
- Digestive System of Amphibians
- Respiratory System and Sound Production in Amphibians
- Circulatory System of Amphibians
- Nervous System of Amphibians
- Urinogenital System of Amphibians
- Reproduction and Development of Amphibians
- Reasons for Extinction of Amphibians
1. Origin of Amphibia:
During the middle of Devonian time, the bony fishes had differentiated into the actinopterygians on one hand and into the dipnoans and crossopterygians on the other. The climax of evolution was reached when the descendants of some crossopterygians left water and invaded land. This event of transition from water to land ushered a new phase in vertebrate evolution, the beginning of the land vertebrates.
The amphibians started the beginning of tetrapod history. By invading a new environment on land, the amphibians opened broad avenues for further evolution over a wide range of structural and functional adaptations. Before going into the discussion on the ancestry of the amphibians, the steps the first amphibians have taken to meet the basic requirements for life on land are described below.
The problem of terrestrial locomotion with additional gravitational complications was solved by profound changes in all the parts of the tetrapod body.
(a) The amphibian head attained powerful musculature with corresponding changes of the articular processes of the skull and its adjacent endoskeleton.
(b) The lower jaw apparatus developed elaborate musculature for its operation and support.
(c) The vertebral column loses flexibility and gained strength and rigidity by ossification.
(d) The pectoral and pelvic girdles support the limbs and also protect the important visceral organs from injury which may result from new upward thrusts. Walking on land resulted upward pressure. This upward thrust caused the diminution of dermal skeleton of the girdles. Powerful scapula and triradiate pelvic girdle with elaborate ilium are some of the important modifications in the girdles. These modifications are supplemented by the development of endo-skeletal processes for firm attachment of associated muscles.
(e) The amphibians have developed two pairs of limbs. These are well-equipped with adequate muscles and strong girdles to lift the body away from the frictional contact of the ground. The carrying of the weight of body on the four limbs has caused great change in the vertebral column.
(f) The problems of breathing in air are solved by developing well-formed lungs for gaseous exchange in air. The moist skin in modern amphibians also acts as an accessory respiratory organ.
(g) All amphibians possess well-developed vascular system, a new scheme for the development of lungs, i.e., introduction of pulmonary circuit. This has caused tremendous change in the structure of the heart and the circulatory system as a whole.
(h) Development of a middle ear cavity with a bone to transmit the vibrations from tympanum to inner ear helps in the intensification of the sound waves of air.
(i) The skin becomes suited for terrestrial life to resist desiccation.
Search of ancestry:
The modern teleosts have reached the peak of evolutionary success amongst the primary water-living vertebrates. They have undergone extensive adaptive radiation. The air-breathing fishes, the dipnoans and the crossopterygians exhibit a close relationship with the amphibians and it is an apparent biological truth that a group of such lung-fishes gave rise to the new terrestrial population—the amphibians.
So the chance for dipnoans and crossopterygians to hold the significant position in amphibian ancestry needs consideration. How did the early amphibians meet the new requirements imposed upon them as a result of change from an aquatic to terrestrial life is to be solved first.
The early amphibians must have fulfilled the basic requirements for living on land by making the following modifications:
(a) Partial loss of armours although present in some earliest amphibia, e.g., Stegocephalians.
(b) Loss of unpaired fins.
(c) Development of terrestrial appendages by transforming the paired fins into limbs.
(d) Loss of internal gills and acquisition of lungs.
Dipnoi—foreshadowed amphibian organisation:
It was a general belief that the dipnoans stand in the direct line of tetrapod descend, because the dipnoans show many structural and functional resemblances with the amphibians. Although the pectoral and pelvic girdles in dipnoans cannot support the weight of the body on land, these girdles foreshadowed some amphibian features in several ways.
(a) The internal skeleton of the paired appendages is well-developed in dipnoans.
(b) Paired appendages articulate with the respective girdles by a single proximal bony piece, which can be compared with the humerus or femur.
(c) Outward extension of myomeric muscles into the paired fins is quite suggestive of the arrangement of musculature of the paired appendages of Amphibia.
(d) Ability to breathe air by lungs (modified swim-bladder).
(e) Pectoral girdle of Necturus resembles closely that of dipnoans.
Although the dipnoans present some specializations towards a method of living out of water, the total evidences direct quite clearly to the fact that they are not on the direct line of emergence of amphibia from fishes. The dipnoans exhibit too many specialised features and such a specialised group cannot possibly hold the ancestry of another group of animals.
Striking similarities, especially in circulatory and respiratory systems, are possibly due to the physiological convergence for living in similar condition of life. The dipnoans, today, give an idea of the form that probably linked the fishes with the amphibians. The dipnoans are usually regarded as the collateral uncle of the amphibia but not the father of first tetrapod.
Recently considering the morphological and anatomical point of views, D. Rosen et al., (1981), and Duellman and Trueb (1986) opine that the nearest living relatives of recent amphibians are lung-fishes than the crossopterygians but this has been criticised by Jarvik (1980) and other scientists.
Crossopterygians — direct the channel of amphibian evolution:
To trace the direct line of amphibian origin from the fishes, the importance of the crossopterygians in holding the probable starting point needs consideration. The early crossopterygians, as exemplified by the Devonian genera, Osteolepis and Eusthenopteron furnish the strongest support. Because they possess many features which are certainly amphibian or lead towards amphibia.
The features are:
(a) Bony pattern of jaws and skull are comparable to that observed in early amphibians.
(b) Two large bones on the top of a skull can be homologized as the amphibian parietal bones.
The posterior skull table of Osteolepis and Eusthenopteron is more similar to that of early amphibians (e.g., Ichthyostega and Eryops) than that of dipnoans (Fig. 7.49).
(c) The jaws of Eusthenopteron possessed labyrinthodont teeth characterized by in-folding of a tooth wall around a central pulp cavity (Fig. 7.50).
(d) Pectoral girdle presents certain features which are prerequisites for amphibian fore- limbs.
(e) Skull of Eusthenopteron contains almost all the elements observed in the early amphibians.
(f) Pectoral fin of Eusthenopteron can be compared to the forelimbs of amphibia. The single proximal piece of bone can be homologized with the humerus and the next two pieces can be compared to radius and ulna.
(f) Various wrist and ankle bones, the bony elements of hand and foot have evolved from the distal bony complex of the crosspterygian fins.
Thus, in many respects, the crossopterygians show close similarities with the amphibian and it is expected that these fishes are the direct progenitors of the early amphibians.
Cladistic analysis in favour of osteolepiforms as the ancestor of early tetrapods does not support fully.
At present, a late Devonian crossopterygians, Panderichthyidae (e.g., Elpistostege and Panderichthys) seem to be in the line of direct ancestor of amphibians. The members of Panderichthyidae were crocodile-like fishes with fins instead of limbs.
Their hands, bodies and the skull roof were flattened and they had elongated snout. Their eyes were on the top of head and they had no dorsal and anal fins. Those features suggest a closer link with the first tetrapods.
The frontal bones were present in both Panderichthys and tetrapods but were absent in Osteolepiforms. The ribs of Panderichthys project ventrally from the vertebral column whereas in osteolepiforms the ribs of the vertebral column project dorsally.
The most dramatic and widely accepted event to note in amphibian evolution is the transformation of the crossopterygian paddles into amphibian limbs. The sequences indicating how amphibian limbs arose from a fish fin is a controversial issue. It is probable that the lobed fins of crossopterygians specially seen in Eusthenopteron have become transformed into the tetrapod limbs.
As the fish came on the land, the paired appendages perhaps first carried only the weight of the body and the muscles of the appendages became capable of only forward and backward movements.
With the evolution of the tetrapods on land, the limbs become elongated and shifted under the body to raise the body further away from the ground. While on land the muscles became modified and arranged around the shoulder and hip joints for balanced movement on land.
For the attachment of the muscles the girdle became expanded into plates consisting of different characteristic pieces. The shoulder girdle of an earliest amphibia, Eogyrius, inherited a shoulder girdle closely similar to the Osteolepis.
Fig. 7.51 gives an idea of the possible stages of transformation of the crossopterygian girdles and fins into the tetrapod girdles and limbs respectively. The actual and documentary transitional limb is still unknown. But the most ancient footprint of Thin-opus throws much light on the process of transformation.
2. Factors that Caused Amphibian Evolution:
What were the factors that led Crossopterygians to leave their primal aquatic home and to come on land? There are different views. A few of them are given below.
Barrell (1916) emphasised that the Devonian was a dry period whence many streams and ponds tended to dry up seasonally. Certain crossopterygians were capable of movement from drying pools to places where water was available. The periodic escapes from drying pools possibly caused the development of tetrapod limbs.
(ii) Desire for excessive water:
Romer (1958) rejected the view of droughtness in Devonian period. He suggested that amphibians and early reptiles were inhabitants of water until the Pennsylvanian period. He also suggested that it was the desire for more water that caused the first excursion of crossopterygians from one place to other.
Berrill (1955) is inclined, to think that the enemies in water forced crossopterygians to leave for land. Other factors were the abundance of food in land, lure for atmospheric oxygen and recurrence of unfavorable environment.
It seems that the real cause is neither safety nor food nor the desire to breathe atmospheric air, but an adaptation which has been imposed repeatedly upon the crossopterygians by recurrence of hostile environment.
Most probably, during late Devonian period, due to excessive periodic drought, crossopterygians were forced to search for new fresh-water streams and lakes, where they can live and thus escape the risk of survival. By this way they had to cross dry land to find suitable water. From such a start the amphibians evolved in the geological age and became adapted to the new terrestrial environment.
3. Probable Ancestry:
During the Devonian time, some of the crossopterygians came to land from aquatic home. This group who was successful to come to land from water was probably the rhizodonts being represented by Osteolepis and Eusthenopteron. It was a very significant step to come into a completely new environment.
On coming to land these advanced air-breathing fishes became transformed into primitive amphibians. Fig. 7.52 shows the evolution of amphibian vertebral structures from the crosspterygian fishes. The most primitive amphibians, known the Ichthyostegid whose components of vertebrae have the similarity to the crossopterygians.
Due to the similarity of the components of vertebrae, between Ichthyostega and crossopterygians, it is supposed that amphibians have evolved from crossopterygians and from this basic type a radiation of several types of vertebrae among other amphibians and also in other groups of vertebrates may have evolved.
Figure 7.53 shows the phylogenetic tree of the amphibians. Ichthyostega exhibited a transitional phase by possessing an admixture of piscine and amphibian characters. Ichthyostega, although retained many distinct piscine characters, has paved the path of amphibian evolution.
4. Structure of Amphibians:
The living amphibians exhibit diverse structural adaptation. The caecilians have a degenerated structural organisation and they furnish all the basic modifications for fossorial life. They are the most primitive forms amongst the living amphibians.
The modern urodeles include a large number of families and genera. Although built on a common fundamental plan, each family is characterised by having peculiar anatomical structures.
The urodeles include many giant forms. The largest amphibia is the Megalobatrachus of Japan and China which may even reach a length of about 1.60 m. Most of the urodeles are aquatic. The terrestrial forms have limbs and are plantigrade.
Anatomically, the urodeles occupy an intermediate position between the caecilians and the anurans. The anurans constitute the highly specialised forms and show wide range of adaptive radiation. Hyla shows an adaptation for arboreal life and possesses adhesive discs at the tip of the digits.
The plate-like adhesive discs are not suctorial in action, but have a moist, corrugated antiskid surface which helps in adhesion to the tree. During climbing, a sticky secretion is expelled from the adhesive discs by the action of collagenous fibres which operate the glands.
The adjustment of the adhesive discs is facilitated by the development of intercalary cartilage between the terminal and penultimate joints. Another tree-frog, Chiromantis, has opposable digits.
In most of the tree-frogs, the webs between the toes are absent or reduced, excepting in Rhacophorus of East Africa where the elongated digits are webbed. It has been recorded that Rhacophorus malabaricus can glide from a height of more than 9 m and Hyla venulosa can glide from a height of 42 m quite effectively. In Hyla venulosa, the digits are not webbed.
The skin of amphibians consists of epidermis and dermis. The epidermis consists of several layers and is renewed by ecdysis. This process of renewal is controlled by the pituitary and thyroid glands. Localised thickening in epidermis is observed in the larvae, specially in the formation of the horny larval jaws and teeth. The warts of toad are also the instances of such thickenings.
The skin of modern amphibians is naked and remains moist due to the secretion of integumentary glands. The moist skin is necessary for respiration and also possibly for temperature regulation. There are two types of skin glands in amphibians. These are: mucous glands and poison glands. The mucus secreted by the mucous glands keeps the skin moist.
The poison glands are well- developed in toad and salamanders. The parotoid glands of toad are the best examples of the poison glands. Most of the warts on the dorsal surface of the toad open to the exterior by a minute opening which leads into poison gland.
The gland produces active toxins. The secretion is venomous and causes nausea, respiratory and cardiac dysfunctions. The toxins are isolated as the bufogin and bufotalin. The poison of Dendrobates acts on the nervous system. The secretion of the dorsal glands of a warty Newt (Triturus cristatus) is venomous. The poison glands are defensive organs.
The skin of the larval amphibians is ciliated. The colour of the skin of amphibia may vary from dull to brilliant. The urodela usually shows brilliant colouration which has a protective value. The green colouration of tree-frogs is a protective device, because it harmonizes with the surrounding green fliage.
The spotted salamander and some frogs exhibit warning colouration. In some tree- frogs, the brightness of the body colouration may vary with the change of intensity of light.
The colour change is caused by the physiological adjustment of the deep-seated melanophores, guanophores and overlying lipophores. The skin of Gymnophiona is thick and contains groups of granular dermal scales enclosed in sacs and large multicellular poison glands (Fig. 7.44).
The exoskeleton was present in fossil amphibians. But in modern forms it is restricted to majority of the caecilians and some anurans. In caecilians, clusters of small dermal scales lie in the skin in most of the cases (Fig. 7.44) In a few toads, bony plates remain embedded in the skin of the back and these are dermal in origin.
In Brachycephalus of Brazil, the dermal plates on the back become fused with the neural spines. Small and horny claws are present in the larval stage of an Asiatic urodele, Onychodactylus and in an African toad, Xenopus. In Xenopus, claws are present at the tips of first three digits of his hind limb. Claws have been recorded in some fossil amphibians too.
The claws present in these amphibians, although fore-shadowed the emergence of claws in higher classes of vertebrate, are not true claws. In Pelobates, the highly cornified areas on the feet can be compared with the epidermal scales.
The skull in the living amphibians varies greatly. There is a general tendency towards reduction in the thickness and number of dermal elements in the skull. The inter-pterygoid vacuities and the orbits are greatly enlarged. The rami of the lower jaw are short and the skull becomes much flattened. The skull of the anurans is highly specialised among the amphibians.
The inter-parietal foramen (present in fossil amphibians) is totally absent in modern amphibians. The skull of toads is devoid of teeth, but in Amphignathodon true teeth are present on the lower jaw. In frogs, teeth are present on the lower jaw. The skull of urodeles has certain peculiar features of its own. It is less specialised than that of anurans and differs in many important respects.
The chondrocranium is lower in organisation with many degenerative or paedomorphic features. The parietals and frontals are separate and in some forms both lacrimals and pre-frontals are persistent. The skull of urodeles differs from that of anurans by having a large prevomer.
In caecilians, the skull is peculiar. It has large investing bones and a small but complete orbit is present there. The skull is a rigid structure. The compactness is correlated with the burrowing habit. The lower as well as the upper jaws bear teeth. Like that of urodeles, a tooth-bearing coronoid is present in the mandible.
The vertebral column in amphibia is largely bony and the vertebrae are articulated together. The flexibility of the vertebral column is lost to give more strength. This modification is due to the lifting of the body on the limbs. In urodeles which spend much of the time in water, the vertebrae lack ossification and notochord persists to help in swimming.
The transition from aquatic to terrestrial life causes the shortening of the vertebral column. In anurans, the vertebral column is composed of nine vertebrae and an un-segmented urostyle behind. The vertebral column is composed of nine vertebrae and an un-segmented urostyle behind.
The vertebral column is differentiated into:
(a) The cervical region represented by the atlas,
(b) The thoracolumbar region with variable number of vertebrae,
(c) A sacral region containing a large vertebra and
(d) A caudal region comprised of the tail vertebrae. The urostyle represents the caudal region in anurans. The transverse processes and zygapophyses are well-developed for the attachment of muscles.
The number of vertebrae is variable. It may extend up to 250 in urodeles and caecilians. In anurans, the vertebrae are procoelous except the ninth vertebra of Rana which is peculiar. The anterior surface of centrum is convex while the posterior surface bears a double convexity. But in urodeles, two types of vertebrae are encountered.
The primitive urodele as exemplified by Ambystoma has amphicoelous vertebrae and the higher urodeles possess opisthocoelous vertebrae. Some primitive frogs exemplified by Ascaphus and Liopelma possess free ribs. The skeletal features of the girdles and limbs have changed considerably in amphibians but the basic plan of the limbs and girdles remains same throughout the group.
In caecilians, these are secondarily lost due to fossorial adaptation. In fishes, the girdles and paired fins are small and are largely cartilaginous, but in amphibians the girdles have become greatly enlarged and modified due to their weight-bearing function. The appendicular skeleton in urodeles is greatly simplified, but in anurans these are highly developed and are quite suitable for terrestrial mode of life.
5. Digestive System of Amphibians:
Adult amphibians feed mostly on the arthropods, but the larval forms are usually omnivorous. They may be cannibals. As a result of similar food habits, the digestive system shows little variation. In most amphibia excepting toads, the teeth are present. The teeth are borne on the premaxillae, maxillae and vomer.
The teeth are very small and pointed and are used only to catch the prey. Biting teeth are present in adult Ceratophrys ornata. Amphignathodon, a South American tree-frog, possesses teeth on the lower as well as on the upper jaws. The salivary glands are absent but some oral glands are present which produce mucus.
In terrestrial amphibians, cilia are present in the oral cavity which keep the oral fluid in movement. In case of many urodeles, tongue is immovably fixed. It may be movable as seen in most anurans but it is free behind and fixed anteriorly.
The tongue is used to capture the prey. The adhesive power of the tongue is enhanced, particularly in the frogs, by the secretion from the lingual and inters- nasal glands. The tongue is altogether absent in Xenopus and Pipa.
The oesophagus is a simple tube and is not sharply distinguishable from the stomach. The stomach is simple with folded mucous layer. Simple tubular gastric glands open in the folds. These glands are composed only of one type of cells. The glands produce pepsin and hydrochloric acid.
The intestine is short in adult amphibians and is marked off from the stomach by having a well-developed pyloric sphincter. But the intestine in omnivorous larval forms is much coiled like the spring of a clock. Caecum is absent in amphibian alimentary canal. But in some anurans, especially in Hyla arborea the large intestine has a conspicuous anterior caecum.
The liver and the pancreas have typical histological picture and produce bile and pancreatic juices respectively. The liver is basically a single massive gland with right and left lobes. The gall-bladder lies just right of the midline of the notch between the lobes.
The liver is attached to the duodenum and stomach by gastro-hepatic ligament. The pancreas is a thin and elongated structure along the duodenum on the side away from stomach. The amphibians can live for a considerable period of time without taking any food. Proteus, Typhlomolge are the typical examples of cave-swelling animals (troglodytes). Axolotl larva may remain alive for about 650 days in starvation.
6. Respiratory System and Sound Production in Amphibians:
Adult amphibians are lung-breathers. The skin acts as an accessory respiratory organ both in water and on land. The skin is highly vascular and specially so in the buccopharyngeal cavity. The larval amphibians respire in water by the gills. Such gills are retained in many adult urodeles. Few urodeles retain external gills as the respiratory organs in adults.
Both external and internal gills are present in anuran larvae. Ascaphus, living in the mountain stream of U.S.A., has reduced lungs which help the animal to live in water. In per-ennibranchiate urodeles, the lungs are simple saccular organs and the hydrostatic function is predominant. In Salamandra atra and Desmognathus, the lungs are absent.
In Astylosternus, an African frog, the lungs are vestigial. In caecilians, the tracheal lung may be present but the left one is always rudimentary. In aquatic urodeles, the lungs act secondarily as hydrostatic organ. In all these above cases, respiration is exclusively pharyngeal and/or cutaneous.
In almost all amphibians cutaneous respiration is a remarkable supplementary respiratory adaptation. In Cryptobranchus, there are vascular folds in the epidermis into which blood capillaries penetrate. The amphibians are virtually the pioneers where true voice is produced by the vocal organ.
The production of sound is a protective response for fear and the males call the females during breeding season. The noise is produced by the vibration of the vocal cords in the laryngotracheal chamber. The vocal sacs in the males of some anurans, developed as the buccal outgrowths, serve as resonator.
7. Circulatory System of Amphibians:
The heart of amphibia consists of a sinus venosus, two auricles, an undivided ventricle and a conus arteriosus. The conus arteriosus is made up of two regions: pylangium and synangium. The portion of the conus next to ventricle is called pylangium while the distal part is designated as synangium. The pylangium is more muscular than synangium.
The distal end of truncus arteriosus becomes expanded as bulbus arteriosus in some urodeles. The left auricle is absent in the plethodontid urodeles where the lungs and the pulmonary veins are missing. The auricles are completely separated by a complete inter-auricular septum. It is perforated in Salamandra or may be fenestrated with intervening spaces in lung less urodeles.
The venous blood returns to the right auricle while the left auricle receives oxygenated blood. The spiral valve is present in anuran heart but it may be reduced in most urodeles or may be absent as in Necturus, Cryptobranchus and the caecilians. In all the amphibians where the conus arteriosus is present there are two sets of valves which prevent the back flow of blood.
The trabeculae carni (strands of muscle making up the muscular walls of heart) are observed in amphibians. These trabeculae are best developed in the walls of auricles. In the urodeles where gills are retained in adults, the pattern of circulatory system is essentially fish-like. The venous system of the urodeles represents a transitional stage between the fish and the anurans.
The RBC in amphibia are nucleated and oval. One of the notable features to record is the presence of largest RBC amongst the vertebrates. The RBC of Proteus measures about 58 pm in diameter. There are three types of leucocytes in amphibians.
These are lymphocytes, monocytes and polymorphs. The red blood cells are produced mainly in kidneys and are destroyed in the liver and spleen. The bone- marrow also serves as an important centre for formation of red blood cells in adults particularly in males during breeding season. The spleen is a source of lymphocytes.
8. Nervous System of Amphibians:
The brain of amphibia is basically built on the same fundamental plan in all forms. The prosencephalon is represented by two large evaginated cerebral hemispheres. The pineal organ is a simple sac in most cases, but in a few amphibians this forms a retina-like structure. The optic lobes are well-developed. As the amphibians are sluggish animals, the cerebellum is simple.
Sense Organs in Amphibians:
The sense organs are well-developed. Many aquatic adult amphibians and the larvae possess simple lateral line organs in the form of clusters of cells in an open pit. The skin contains tactile sense organs and chemoreceptors.
The olfactory organ Works both in water and on land. Organ of Jacobson is present in most amphibians. It is a special sensory sac developed as a diverticulum from the olfactory chamber. It serves to test the scent of the food taken inside the mouth. It is absent in Proteus and Necturus.
The eyes of amphibia exhibit certain modifications due to transition from water to land. In water, the eyes were adapted for shortsighted vision but on land long-sighted vision becomes necessary. The eyes are extremely degenerated in caecilians and also in cave- dwelling urodeles. The eye ball is more or less spherical with a rounded cornea.
The lens is flattened in terrestrial amphibians, but in aquatic forms it is rounded. The eyelids are usually present in terrestrial forms excepting some primitive members. In tree-frogs, the eyelids may be transparent. The lacrimal glands are present in all the terrestrial amphibians. In aquatic amphibians, the lacrimal glands are absent but the lacrimal ducts are still retained in many cases.
In some caecilians, a single lacrimal gland occupies a position in the sightless eye-socket to lubricate the sensory tentacle. In all amphibians, the skin is also sensitive to light. This is highly developed in cave-dwelling urodeles.
The membranous labyrinth is composed of an utriculus with three semicircular canals, a sacculus with an outgrowth, called lagena. The middle ear consists of a funnel like cavity which communicates with the pharyngeal cavity by Eustachian tube. In this cavity, a rod (columella) is present which transmits the sound waves to the internal ear.
The columella fits into the fenestra ovalis by a broad foot (otostapes). The fenestra ovalis is partly occupied by a plate (operculum). So the transmitting rod is divided into an inner part, named as otostapes and operculum, a medium part called mediostapes and an outer part, designated as extra-columella.
In urodeles, caecilians and some anurans, the tympanic cavity and extra-columella may be absent. Cryptobranchus lacks the operculum. This is also absent in Xenopus and Pipa. In a terrestrial anuran, Bombinator, the tympanum and columella are greatly reduced.
9. Urinogenital System of Amphibians:
The kidneys in amphibia are of opisthonephric type and retain the characteristics of fishes. The shape of the kidneys corresponds to the shape of the body. In urodeles and in a primitive frog, Ascaphus the kidneys are elongated.
Each kidney is divided into an anterior narrow non-renal part and a broad posterior renal part. In caecilians, the kidneys are extremely elongated and occupy the whole length of the body cavity. In case of anurans, the kidneys become condensed and divided into lobes. In amphibians, the genital organs develop from the genital ridges. Each such ridge is situated on the ventromedian aspect of the developing mesonephros.
The genital ridge is divided into three sectors:
(a) Anterior part (progonalis),
(b) Middle part (gonalis) and
(c) Posterior (epigonalis).
The gonads proper develop from the gonalis. In anurans, the fat bodies develop from the progonalis while in the urodeles and caecilians, the fat bodies develop from the entire genital ridge. The testes in different amphibia assume various shapes.
The testes are smoothly rounded mass in anurans but in urodeles these may be elongated and lobed structures. In Desmognathus, each elongated testis is separated into a series of testicules which are budded off towards the anterior end, one for every year.
In caecilians, each testis is an elongated body and looks like a string of beads. The internal construction of testes is simple and consists of short seminiferous tubules. Each seminiferous tubule has a wide lumen and ends blindly.
The efferent ducts vary in number and extend up to the marginal canal of the kidney. Each marginal canal is developed from outgrowth of the capsule of primary mesonephric tubule of anterior portion of mesonephric kidney.
The mesonephric tubules usually extend to epididymal duct. The kidney tubules serving as the carriers of sperms may retain their glomeruli in caecilians and a Salamander, Spelerpes, but in most amphibians the glomeruli are lost.
The testes discharge through the kidneys by the vasa efferentia. So the Wolffian or mesonephric duct serves as a urinogenital duct in males and as a ureter in females. In toads, a special Bidder’s organ is present. This organ is better developed in males.
The function of this organ is not known. It is assumed to be an endocrine organ, because it undergoes a cycle of size change. This organ is capable of developing into an ovary after castration in either sex. The Bidder’s organs develop from the gonalis sector just anterior to the gonad proper.
In females, the ovary is an irregular mass. The eggs are discharged into the body cavity. The Mullerian duct becomes swollen and convoluted to become the gonoduct. It extends through the entire length of the body cavity. The eggs, after being discharged into the body cavity, enter the ostium (opening of gonoduct) and traverse the duct.
In oviparous forms, the eggs get their jelly coating within the tube. In viviparous forms (Salamandra, Spelerpes fuscus, Typlonectes compressicauda, Dermophis thomensis), the eggs develop in the tube. The tube is divided into different parts in different amphibians.
(b) Infundibulum (wide lumen, thin wall and no glands),
(c) Tube (beset with glands in oviparous forms or without glands but mucous cells in viviparous forms),
(d) Uterus (wide lumen and much folded epithelium and
(e) Vagina (short section between uterus and cloaca).
The different parts of female gonoduct become modified in accordance with the modes of reproduction. In anurans, the uteri exhibit great modification. Bhaduri (1953) has classified the uterus of anurans into three broad categories (Fig. 7.45):
(i) Uterus separatus,
(ii) Uterus septatus and
(iii) Uterus communis.
In Rana and Xenopus two uteri remain separate along their course and open into the cloaca by independent openings. This condition is called the uterus separatus. In Bufo and Rhino-derma a septum runs anteroposterior between two uteri, but posteriorly forms a common uterus by fusion at the terminal ends.
This condition is called uterus septatus. The two uteri have a common opening into the cloaca. The degree of fusion varies greatly and finally in Dendrobates, two uteri become confluent into an unpaired common uterus. The median partition wall between the uteri is absent.
This type of uterine condition is called the uterus communis. The uterus separatus is comparable with the duplex type, the uterus septatus with the uterus bipartite and uterus bicornis types and the uterus communis with the uterus simplex of the mammals.
Hermaphrodism, though occasional, is observed in adult amphibians. Hermaphrodism occurs in adults as the consequence of failure of sex-directing mechanism to convert indifferent gonad into the specific sex. Because of dis-balance the anterior portion of the gonad remains as female while the posterior part becomes male. These two parts are usually separated by non-gonadal tissue bridge.
10. Reproduction and Development of Amphibians:
In majority of the amphibians, external fertilization is the rule. They are oviparous. But several instances of ovoviviparous condition are encountered. In Spelerpes fuscus, Typhlonectes compressicauda, Dermophis thomensis and Salamandra atra, the eggs are retained inside the oviduct where intra-uterine development occurs.
In most urodeles, the spermatozoa are transferred to the body of the female in the form of spermatophores. In almost all amphibians, the ontogenic development is indirect, i.e., accompanied by well-marked metamorphosis.
Most of the amphibians undergo complete metamorphosis but some urodeles retain the larval features and become neotenic. The most remarkable instance of neotenous form is the Axolotl which breeds in larval state.
11. Reasons for Extinction of Amphians:
Amphibians are undoubtedly a neglected group as humans have a general tendency to dislike these creatures. They certainly deserve their share of attention as they perform a vital role in ecological balance and form an important link in the food chain. They also called “bio-indicator of pollution”.
The amphibian population throughout the world is decreasing alarmingly day by day. ‘The exact reasons for the declines are not known, though some local reasons are considered by herpetologists. We do not know the status of the Indian species as very little work has been carried out.
Habitat loss and alteration are the main threats to the amphibians. Breeding grounds are being altered at a fast rate, owing to the filling up of the aquatic habitats for the construction of modern complexes in the suburb areas of the cities and towns. Aquatic habitats are also being destroyed mainly by siltation and sewage contamination.
The forests are being rapidly converted to agricultural and cattle grazing purposes. Extensive use of insecticides and herbicides for agricultural purposes may be the reasons for amphibian declines.
The acid precipitation, and increased ultraviolet radiation are being linked to global declines. The reasons for amphibian vanishing are different in different countries. A brief discussion of the reasons of the declination in different countries is given here.
In Great Britain, the disappearance of some frog species with thermal pollution results from the hot effluents of nuclear power plant cooling systems. More frequently it is the chemical contamination that contributes to amphibian declines and disappearances. Progressive acidification of ponds has been responsible for the disappearance of numerous colonies of Bufo calamita in Great Britain.
General absence of amphibians of some regions of France and Belgium are extreme pollution by waste products, pesticides in agricultural zones, and heavy metals, etc. In Denmark Bombina bombina, Hyla arborea, Pelobates fuscus, Bufo viridis are declining fast than the previous years.
Holm Anderson (1995) described that a team of professional biologists resurveyed 1300 localities from 1977-1986. This survey revealed that 19% of the breeding ponds had disappeared but a further 40% had been altered to some extent. However, amphibians disappeared to a much greater extent than did the ponds; about 50% of populations disappeared from 1945 to 1980.
In Hungary, Miklos Puky (1995) has studied urban amphibian populations around Budapest since 1988, and has come to conclusion that marked declines in several species are due to both human impact and drought.
The annual meetings of Societas Europaea Herpetologica that held in Bonn, Germany in 1995 included a symposium on declining amphibian populations. T. Hayes of Berkeley, U.S.A. reported work on the role of oestrogen mimics as possible endocrine disruptors.
In Russia, Vladmir Ischenko of Ekatermburg (1995) suggested on the basis of skeleto- chronological studies of a number of species that short lived species may be more vulnerable to local extinction than long lived species.
The third Latin American Congress of Herpetology was held at the University of Campinas, Sao Paolo, Brazil, in December in 1993 and the researchers analysed the decline of Latin American amphibian species. They reported that Atelopus, Melanophryniscus, Dendrobates, Hylodes, Telmatobius, Batrachophrynus and Centrolene are extremely dependent on water bodies.
One of the reasons for local declines is overexploitation. Human consumption of Telmatobius arequipensis, T. marmoratus and Batrachophrynus macrostomum in Peru and Caudiverbera caudiverbera in Chile is depleting fast as compared with former large populations. Other reason is extraction and exportation as reported for Chile. In 1985, 236 anurans were reported, while in 1992, there was an exportation of 1,00,000.
Another reason given for declines is the introduction of non-native fauna. Xenopus laevis, Rana catesbeiana and Triturus sp. in many places over the Brazil, Peru and Chile and rainbow trout (Oncorhynchus mykiss) along the Andes are seen as non-native faunas.
Less than 40 years ago, thousands of Amargosa toad (Bufo nelsoni), inhabited the Oasis valley in Southern Nevada, U.S.A. In 1994, this species consists of fewer than 100 individuals. Some of the factors believed to adversely affect the toad and its habitat include grazing, off road vehicle use, grading for flood control and modification by heavy equipment for the development of commercial enterprises.
The introduction and existence of non-native predators such as cat-fish and crayfish pollution, and diversion of spring water have also directly affected toad populations. Three species of U.S.A. are in most danger and need of listing.
They are the Amargosa toad, the Western boreal toad (Bufo boreas boreas (Southern Rocky mountains populations), and the great basin population of the spotted frog (Rana pretiosa). In addition the Wyoming toad (Bufo hemiophrys baxteri) could become extinct in the next few years.
Boreal toads (Bufo boreas boreas) experienced a massive die off in the Rocky Mountains of Colorado in the late 1970’s and early 1980’s. Red leg syndrome, caused by a variety of bacteria or fungi, has been identified as the proximal cause of death. Chytrid fungi are killing amphibians in the wild.
Recently the deaths of endangered boreal toads in the southern Rocky Mountains have been linked to a chytrid fungus, as also being responsible for amphibian die offs in Central America and Australia. Chytrid fungus in amphibians was first identified in 1998 by Green and other researchers from the U.S., Great Britain and Australia.
The Post Metamorphic Death Syndrome (PDS) is considered for the mortality of all or post metamorphic individuals in a short period of time. This disease agent may be the primary cause of certain amphibian declines in Northern West America. The proximal causes of death are usually widespread pathogens such as Aero monas (Red leg disease pathogen).
In Canada, factors related to human overpopulation, environmental contamination and habitat destruction have clearly been shown to be detrimental to amphibians, though the severity of the effect varies from species to species. In the North Eastern Greenland region, UV-B is linked to the amphibian declines.
It is considered a global phenomenon. The increased amount of ultraviolet radiation has reached the earth’s surface, as a result of destruction of ozone layer in stratosphere by the chemical pollutants, such as CFC and greenhouse effect.
The effects were confined at the poles at first, gradually are spreading into lower latitudes in both hemispheres. Ultraviolet light, in between 290-320 nanometer UV-B band, kills amphibian eggs and embryos.
In India, no sufficient work has done on the decline of amphibians. Maximum work has emphasised on survey.
K. Vasudevan (1998) reported the reasons of declines in Kalakad Mundanthurai Tiger Reserve, Southern India that the possible threat to amphibian species may be removal of top soil in large quantity by brick industries, and use of chemical fertilisers and pesticides, and human interference in the reserve forest which ultimately results in habitat loss.
From Bangladesh 13 species have been reported of which the population of Microhyla ornata, Microhyla rubra, Rana cyanophlictis, Rana hexadactyla and Rana tigrina are declining fast for habitat destruction and use of insecticides.
In Vietnam 112 amphibian species have been recorded of which many species are valuable economically and scientifically, have become rare.
Some are in danger of extinction or serious decrease such as Ichthyophis glutinosus, Paramesotriton deloustali, Bombina maxima, Rana chaepensis, Rana fansipani, Rana cancrivora, Rana kokchange, Rana tomanoffi, Rhacophorus appendiculatus and Rhacophorus nigropalmatus. The main reasons of declines are habitat destruction, overhunting and inappropriate exploitation.
Since the late 1970s, at least 14 frog species have declined or disappeared from rainforest areas of Queensland, Australia. The causes of the declines in Queensland’s upland rain-forest are still unclear. Recent work by Berger et al., (1998), however, indicates the possible involvement of a fungal pathogen in the declines.
No definite reason is considered for the decline of amphibian population, though different local reasons are forwarded in different countries for the declines of population. A few years ago, acid rain, UV-B and parasites were the focusing points. The evidence now suggests that chemical contaminants should now be considered the most likely than the UV-B and parasites.
The US National Science Foundation (NSF) organised a workshop on amphibian declines in Washington DC, 28th and 29th May, 1998. Several speakers reviewed the latest information on the geography of amphibian declines.
It is clear from the reports that amphibians are continuing to decline worldwide, for a variety of reasons. The potential causes for declines are UV-B radiation, deformities, toxins, viruses, chytrid fungi, diseases in salamanders, climate changes arid immunology.
Lastly we can say that herpetofauna represent a major part of our natural heritage. If these animals are in trouble, we are also in trouble. Amphibians and reptiles are the bio-indicators of the environmental pollution. If they decline and ultimately disappear, we need to make amends. What happens to herpetofauna is a sign of happening to other wild life and may be even to us.
FROGLOG, the bimonthly newsletter of the Declining Amphibian populations at the Department of Biology of The Open University. This news-letter helps to receive any news on amphibian declines at free cost.
In 1995 the Declining Amphibian Populations Task Force established an amphibian conservation forum, Amphibian Decline, on the internet. Subscribers of the forum may receive all e-mail sent to Amphibian Decline and may also send information that will be automatically distributed to other subscribers.
In 1989, the First World Congress of Herpetology was held in England and in a week-long discussion, it was known that amphibian populations that were once abundant, has become rare. The events that were isolated instances, gradually spread as a global pattern. In February, 1990, the scientists those were concerned about the vanishing amphibians, met at the West Coast Centre of the National Academy of Sciences.
In this conference it was known that amphibian populations were disappearing in different countries and often there was no apparent reason. Following that meeting an international effort was initiated to find out the causes of declines of amphibian populations by the Declining Amphibian Populations Task Force (DAPTF) of the Species Survival Commission (SSC) under the World Conservation Union (IUCN).
Now this effort is being conducted by the voluntary efforts of concerned herpetologists. Regional Working Groups of the Task Force are monitoring the status of the amphibian populations in their areas.
At first different methods were adopted to monitor the declining amphibian populations, so a book — Measuring and Monitoring Biological Diversity : Standard Methods for Amphibians, published by Smithsonian University Press, Washington D. C. which contained standard methods for surveying declining populations.
In India several organisations are doing a good job in creating awareness among biologists and common people for conservation of amphibians.
Froglog — the Newsletter of the Declining Amphibian populations Task Force — South Asia, the regional network of the Declining Amphibian populations Task Force, SSC, IUCN, is being edited by Sanjay Molur and Sushil Dutta, and published by Zoo Outreach Organisation and Conservation Breeding Specialist Group, India from PB 1683, 79 Bharathi Colony, Peelamedu, Coimbatore, Tamil Nadu, India.