EVOLUTIONARY PRESSURES IN AMPHIBIANS

EVOLUTIONARY PRESSURES IN AMPHIBIANS

The lives of the most amphibians is divided into freshwater and land. Thus they show adaptations to both environments. The amphibians are supported by the buoyant of the water. They exchange gases with the water. They face the same osmoregulatory problems as freshwater fishes. On the other hand amphibian support themselves again gravity on land. They exchange gases with the air.

EXTERNAL STRUCTURE AND LOCOMOTION

Skin

Function: Vertebrate skin protects them from microorganisms, ultraviolet Light, dessication and mechanical injury. The skin of amphibians is used in gas exchange, temperature regulation, absorption and storage of water.

Skin glands:Amphibian skin lacks scales, feathers or hairs. However, it is highly glandular. It secretions protect the body. These glands keep the skin moist and prevent it from drying. They also produce sticky secretions. These secretions help male to attach with female during mating. It also produces toxic chemicals that discourage the predators. The skin of many amphibians is smooth. But some epidermal thickenings produce warts and claws. It makes the skin sandpapery. The deposition of keratin or the formation of hard, bony areas produces these warts.

Coloration: Chromatophores are specialized cells in the epidermis and dermis of the skin. They are responsible for skin color and color changes. Cryptic coloration (warning color), aposematic coloration (matching with the habitat) and mimicry are common in amphibians.

Support and Movement

Water buoys  and supports aquatic animals. Their skeletons protect the internal organs and attach the muscles. It also prevents the body from collapsing during movement. However, there are different adaptations in terrestrial vertebrates. Their skeleton is modified to provide support against gravity. It is strong and it supports the powerful muscles.

Axia skeleton

1. Skull

The amphibian skull is flattened. It is relatively smaller. It has lesser bony elements than the skull of fishes. These changes lighten the skull. Thus body can support it easily. There are certain changes in jaw structure and musculature. Therefore, the terrestrial vertebrates can crush prey in the mouth.

2. Vertebral column

The vertebral column of amphibians provides support and flexibility on land. It supports the wight of the body between anterior and posterior paired appendage.

(a)Every vertebra has a supportive process called zygapophyses. It prevents the v wtebral column from twisting.

(b) The amphibians have a neck. The first vertebra is cervical vertebra. It moves against the back of the skull. It allows the head to nod vertically.

(c) The last trunk vertebra is a sacral vertebra. This vertebra attaches the pelvic girdle with the vertebral column.

(d) Sternum is present in the anterior entral trunk region. It supports the forelimbs and rotects internal organs. It is reduced cr absent in the Anura.

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Fig: Skeletons of Amphibians. (a) The salamander skeleton is divided into four regions: cervical, trunk, sacral and caudal. (b) Interlocking processes, called zygapophyses. (c) A frog skeleton shows adaptations for jumping

Appendicular skeleton

The exact origin of the bones of vertebrate appendages is not known. However, similarities are present in the structure of the bone. of the amphibian appendages and the bones of  the fins of ancient sarcopterygians fishes. It suggests homologies between these two. Joints are present at the shoulder, hip, elbow, knee, wrist and ankle. These joints allow freedom of movement. They also develop better contact between the body and the substrate.

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The pelvic girdle of amphibians consists of three bones. These are ilium. ischium and pubis. These bones firmly attach pelvic appendages with the vertebral column. These bones are important for support on land. Tetrapods depend on appendages for locomotion. They do not depend on body wall for locomotion. Therefore, the body wall musculature is reduced and appendicular musculature has become strong.

Mode of locomotion

1. Salamanders: They have unspecialized form of locomotion. It is like undulatory waves in fish. Terrestrial salamanders also move with the help of limb and body movements. They show alternate movement of appendages and muscle contractions. It bends the body into a curve. This curve moves the limb forward.

2. Caecilians: They show an accordion (musical instrument)-like movement. In this case, adjacent body parts push or pull forward at the same time.

3. Anurans: The long hind limbs and the pelvic girdle of anurans are modified for jumping. The dorsal bone of the pelvis (the ilium) extends anterior. It is attached to the vertebral column. Their urostyle extends posterior and attaches to the pelvis. These skeletal modifications stiffen the posterior half of the anrans. Long hind limbs and powerful muscles are used for jumping efficiently. Pectoral girdle is attached to the skull and vertebral column by elastic connective tissues and muscles. These connective  tissues are used as shock absorbers for landing on the forelimbs.

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Fig: Salamander Locomotion

NUTRITION AND THE DIGESTIVE SYSTEM

Types of food

Most adult amphibians are carnivores. They feed on different invertebrates. Some anurans have more diverse diet. For example, a bull frog eats small mammals, birds and other anurans. The prey size and availability determine the type of diet. Most larvae are herbivorous. They feed on algae and other plant matter. Most amphibians locate their prey by, sight. They simply wait for prey to pass by it. Olfaction plays an important role in prey detection in aquatic salamanders and caecilians.

Mechanism of feeding

Many salamanders are unspecialized in their feeding methods. They use their jaws to capture prey. Anurans and plethodontid salamanders use their tongue and jaws in flip and grab feeding mechanism. A true tongue is first seen in amphibians. The amphibian tongue is attached at the anterior margin of the jaw. It folds back over the floor of the mouth. Mucous and buccal glands are present on the tip of the tongue. They release sticky secretions. When prey comes within range, an amphibian flicks out its tongue. The tongue turns over, and the lower jaw is depressed. The head tilts on its cervical vertebra. The tip of the tongue traps the prey. Then tongue and prey are licked back inside the mouth. All of this happens in 0.05 to 0.15 second. The amphibian holds the prey by pressing it against teeth on the roof of the mouth. The tongue and other muscles of the mouth push food towards the esophagus. The eyes sink downward during swallowing. They also push the food towards the esophagus.

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CIRCULATION

GAS EXCHANGE AND TEMPERATURE REGULATION

Circulatory System

The circulatory system of amphibians is adapted for both aquatic and terrestrial habitats. The separation of pulmonary and systemic circuits is less efficient in amphibians than lung fishes. The atrium is partially divided in urodeles. It is completely divided in anurans. The ventricle has no septum. A spiral valve is present in the conus arteriosus. It directs the blood into pulmonary and systemic circuits.

Blood circulation to lungs: The exchange of gases takes place through the skin and lungs in amphibians. Therefore, blood entering the right side of the heart is also oxygenated. All gas exchange occurs through the skin and other moist surfaces when amphibians are in water. Therefore, blood in right atrium has a higher oxygen concentration than left atrium. Left atrium receives blood from the lungs. Therefore, blood vessels leading to the lungs constrict. It reduces blood flow to the lungs for conserving energy. The hibernating frogs and salamanders use this mechanism during their hibernation in winter.

Aortic arches: Adult amphibians have lesser aortic arches than fishes. The corms arteriosus give rises to three blood vessels:

1. Carotid artery (aortic arch III): It supplies blood to the head.

2. Systemic artery (aortic arch IV): It supplies blood the body.

3. Pulmonary artery (aortic arch VI): It carries blood to lungs.

Lymphatic system: The amphibians have a well developed lymphatic system. It is composed of blind ending vessels. It filters fluids, proteins and ion from capillaries in  tissue spaces and returns them to the circulatory system. The lymphatic snstem also transports water absorbed across the skin. The amphibians have contractile vessels called lymphatic hearts. These hearts pump fluid through tile lymphatic system. Lymphatic system between the body wall muscles and the skin transport and store water. This water is absorbed through the skin.

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Gas Exchange

Air contains 20 times more oxygen than water. Therefore, terrestrial animals spend less energy for gas-exchange than aquatic animals. But the exchanges of gases require moist surfaces. Thus terrestrial animal loss water during exposure of respiratory surfaces to air. There are three types of respirations in amphibians:

1. Cutaneous respiration: The skin of amphibians is kept moist. Amphibian skin is richly supplied with capillaries. Thus their skin functions as a respiratory organ. Gas exchange through the skin is called cutaneous respiration.

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Cutaneous respiration can take place both in water and on land. This ability allows a frog to hibernate in winter. In salamanders, 30 to 90% of gas exchange occurs through the skin.

2. Buccopharyngeal respiration: Gas exchange can also take place through moist surfaces of the mouth and pharynx. It is called buccopharyngeaI respiration. It is only 1 to 7% of total gas exchange.

3. Pulmonary respiration: Most amphibians possess lungs. Lungs are absent in plethodontid salamanders. The lungs of salamanders are relatively simple sacs. The lungs of salamanders are subdivided into chambers. It increases the surface area for gas exchange. Pulmonary ventilation occurs by a buccal pump mechanism. Muscles of the mouth and pharynx create a positive pressure. This pressure forces air into the lungs.

Ratios of different methods of gas exchanges: Cutaneous and buccopharyngeal respiration have a disadvantage. Their percentage in the respirations is very small. The quantity of gas exchanged across these surfaces cannot be increased with increase in metabolic rate. However, lungs compensate this shortcoming. More gas exchange takes place through lungs with the increase of environmental temperature and activity. At 50 C, 70% of gas exchange occurs through the skin and mouth of a frog. At 25° C, the gas exchanged through skin and mouth remains the same. But pulmonary respiration increases. Now die exchange through skin and mouth is only about 30% of total oxygen exchange.

Gill: Amphibian larvae and some adults respire by external gills. Cartilaginous rods are present between embryonic pharyngeal slits. These rods support three pairs of gills. The gills are reabsorbed and pharyngeal slits are closed during metamorphosis and lungs become functional.

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Temperature Regulation

Amphibians are ectothermic. They depend on external heat sources to maintain body temperature. Water has powerful heat-absorbing properties. Therefore, it quickly absorbs heat from aquatic amphibians. Thus their temperature becomes equal to the temperature of water. But their body temperatures can differ from the environment on land.

Temperature regulation is mainly behavioral. Amphibians have different adaptations for regulation of temperature:

1. They cool their body by evaporative heat loss.

2. Many amphibians are nocturnal.

3. They remain in cooler burrows or under moist leaf litter during the hottest part of the day.

4. Amphibians warm themselves by basking in the sun or on warm surfaces. Body temperatures may rise 100 degree centigrade above the air temperature. Metabolic reactions are increased with the increase in body temperature. Thus heat also -reases the functions of digestive system. Therefore, basking after a meal is mmon. It increases the growth. and the fat deposition Fat deposition is necessary r periods of dormancy.

The daily and the seasonal environmental temperatures of amphibians fluctuate widely. Therefore, amphibians have wide range of tolerance of temperature. Critical temperature for salamanders lie between 2 and 27oC. Critical temperature for some anurans is between 3 and 41° C.

NERVOUS AND SENSORY FUNCTIONS

Brain

Nervous system of amphibians is similar to that of other vertebrates. The brain of adult vertebrates develops from three embryological subdivisions. The brain of amphibians is divided into three parts:

1. Forebrain: It contains olfactory centers. It also has regions that control color change visceral functions.

2. Midbrain: It contains a region called the optic tectum. Optic tectum collects sensory i 111wmation and initiates motor responses. The midbrain also processes visual sensory information.

3. Hindbrain: It functions in motor coordination. It regulates heart rate and the respiration.

Serve organs

(i) Nerve endings

Sensory receptors are distributed over the skin in many amphibians. Some of these are bare nerve endings. These nerve endings respond to heat, cold, and pain.

(ii) Lateralline system:

Th also have lateral-line system like fishes. A lateral line system is present in all aquatic larvae, aquatic adult salamanders and some adult anurans. Lateral-line organs are present singly or in small groups. They are distributed in the lateral and dorsolateral surfaces of the body and on head. These receptors respond to low frequency vibrations in the water. Lateral line receptors are less important on land.


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(iii Chemoreceptors

Chemoreception is an important sense in many amphibians. Chemoreceptors are present in the nasal epithelium, in the mouth, on the tongue, and over the skin. Olfaction is used in late recognition. It can detect toxic chemicals.

(iv) Eyes

The amphibians are primarily sight feeders. Therefore, vision is the most important sense them. There are number of adaptations in the eyes of amphibians for terrestrial environments.

(a)  The eyes of some amphibians are on the front of the head. It forms the binocular vision and well-developed image. This image is necessary for capturing prey.

(b) Other amphibians like some salamanders have smaller lateral eyes. They do not form binocular vision.

Structure of eye

The  lower eyelid is movable in amphibians. It cleans and protects the eye. Its transparent part is called the nictitating membrane. The eyeball retracts into the orbit of the skull and the nictitating membrane covers the          cornea. Amphibians also have orbital glands.

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These glands lubricate and wash the eye. Eyelid and glands keep the eye tree from dust. The lens is large and rounded. It is present in the back of cornea. A fold of epithelium called the iris surrounds the lens. The iris can dilate or constrict and control the size of the pupil.

Accommodation of eye

The bending (refracting) of light rays at a focal point on the retina is called focusing or accommodation. Light waves moves from air into the cornea. These waves are refracted due to change in density between the two media. The lens increase refraction. The eye of amphibian can focus on distant objects at rest. But the protractor lentis muscle move the lens forward for focusing near objects. Receptors called rods and cones are present in the retina. Cones detect co lours. Thus amphibians can distinguish between some wavelengths of light with the help of cones. Amphibians have complex neuronal interconnections in the retina. Therefore, amphibian can distinguish between flying insect prey,  shadow of predator and background movements.

4. Ears

The auditory system of amphibians is adapted for life on land. It transmits both substrate borne vibrations and airborne vibrations. The ears of anurans consist of a tympanic membrane, a middle ear and an inner car. :

(i)  Tympanic membrane: The tympanic membrane is a piece of integument. It stretches over a cartilaginous ring. This ring receives airborne vibrations. It transmits vibrations to the middle ear.

(ii) Middle ear: It is a chamber beneath the tympanic membrane. A bone of middle ear  called the stapes (columella) touch the tympanic membrane. Stapes transmits the vibrations of the tympanic membrane into the inner ear. Ear receives two types of vibrations:

(a) High- frequency (1,000 to 5,000 Hz): These are air borne vibrations. These are transmitted to the inner ear though tympanic membrane.

(b) Low-frequency (100 to 1,000 Hz): These are substrate borne vibrations. These are transmitted through the front appendages and the pectoral girdle. These waves finally enter into the inner ear through operculum.

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Control of sound frequency: The anuran can lock operculum and stapes with the help of muscles. Thus they can screen out high or low-frequency sounds. The anurans use low and high frequency sounds in different situations. For example, mating calls are higt-frequency sounds. Thus it is used only during breeding season. The low-frequency sounds are used for warning ofpredators.

Ear in salamander: Salamanders lack a tympanic membrane and middle ear. They live in streams, ponds, caves, and beneath leaf litter. They have no mating calls. They hear only low-frequency vibrations. These vibrations are transmitted through the substrate and skull to the stapes and inner ear.

Equilibrium: The inner ear of amphibians has semicircular canals. These canals detect rotational movements.

EXCRETION AND OSMOREGULATION

Excretion

The kidneys of amphibians are present on the sides attached to the dorsal wall of the body cavity. A duct opens in cloaca. The cloaca has ventral outgrowth called urinary bladder. Urinary bladder stores urine. There are following adaptations in the amphibians for excretions:

1. Aquatic amphibians: The nitrogenous wastes of amphibians are ammonia or urea. The freshwater amphibians excrete ammonia. It is the immediate end product of protein metabolism. Therefore, they do not spend energy on converting ammonia into other compound. The ammonia diffuses into the surrounding water. Therefore, it does not produced toxic effect.

2. Terrestrial amphibians: Amphibians that spend more time on land excrete urea. Urea is produced from ammonia in liver. Urea is less toxic than ammonia. But it still requires large quantities of water for its excretion. Urea can be stored in the urinary bladder. Some amphibians excrete ammonia in water and urea on land.

Osmoregulation

Osmoregulation is a biggest problem of the amphibians. They must remove excess water and conserve essential ions. Amphibian kidneys produce large quantities of hypotonic urine. Their skin and walls of the urinary bladder transport Na, Cl and other ions into the blood.

The amphibians conserve water on land. Adult amphibians do not drink water. Their skin is also not impermeable like other tetrapods. Their kidneys are unable to produce hypertonic urine. Instead, amphibians loss water by their behavior. They show following type of behaviours:

1. Nocturnal amphibians: They do not come out in desiccating conditions. Many terrestrial amphibians are nocturnal. The go to high humidity area during day times. These areas are present under stones, in logs , leaf mulch or burrows. They come out at night and absorb the lost water through skin.

2. Diurnal amphibians: They live in areas of high humidity. They rehydrate themselves by entering the water.

3. Reduction in surface area: Many amphibians reduce exposed surface area of body to air. It reduces loss of water by evaporation. They curl their bodies and tails into tight coils. They bring their limbs close to their bodies. Many individuals come close to each other in groups. It reduces overall surface area.

4. Protective covering: Some amphibians have protective coverings. It reduces the water loss. Their skin has some hardened regions. These regions are resistant to water loss. They close the mouth of burrows with these hardened regions of skin. It maintains high humidity in the burrow.

5. Cocoon formation: Some amphibians form cocoons. It covers the body during long periods of dormancy and reduces the loss of water. Cocoons are made from outer rs of the skin. This layer of skin detaches and become parchment like. These cocoons open only at the flares or the mouth. Cocoon can reduce water loss by 20 to 50%.

6. Rehydration: The skin is also most important structure for rehydration. The amphibian flattens its body on moist surfaces. It absorbs water. The permeability, vascularization, and epidermal layering of skin promote water reabsorption. Minute channels increase surface area. These areas spread water over surfaces which are not directly exposed to water.

7. Storage of water: Amphibians can also temporarily store water. Water is stored in urinary bladder and lymph sacs. This water is absorbed to replace the loss of water by evaporation. Amphibians living in very dry environments can store water equal to 35% of their total body weight.

REPRODUCTION, DEVELOPMENT, AND METAMORPHOSIS

Amphibians are dioeeious. Their ovaries and testes are located near the dorsal body wall. Fertilization is external. The developing eggs lack any resistant coverings. Therefore, development takes place in moist habitats in water. A few anurans have terrestrial nests.

These nests are covered with foam. This foam reduces the loss of water. Sometimes these nests are placed near water. In a few species, larval stages are passed in the egg membranes. The immatures hatch into an adult like body.

Fertilization and development

External fertilization is less common in salamanders. Only 10% salamanders have external fertilization. Remaining salamanders develop spermatophores and fertilization is internal. Eggs are deposited in soil or water. Or they may be retained in the oviduct during development.

All caecilians have internal fertilization. 75% caecilians have internal development. Tadpole larvae are formed during development of amphibians. Amphibian tadpole larva is different from the adults. It has different mode of respiration, form of locomotion and diet. These differences reduce competition between adults and larvae.

Breeding behaviour

Amphibian: Internal factors like hormones and external factors determine the timing of reproductive activities. Temperature is most important environmental factor in temperate region. It induces physiological changes in the amphibians. These changes control the breeding and breeding periods. Breeding occurs in spring and summer. In tropical regions, rainy seasons induce breeding in amphibians. The individuals locate breeding sites and identify potential mates by courtship behaviorIt also prepares the individuals for reproduction. It also ensures that eggs are fertilized and deposited at proper locations.

 Salamander: Salamanders use olfactory and visual signs for courtship and mating. The anurans use male vocalizations and tactile signs. Many species congregate in one location during breeding activity. Male vocalizations are species specific. These are used for initial attraction and contact between mates. Then tactile cues become more important. The male mounts on the female. The male grasps the female with the help of his forelimbs around her waist. The male is dorsal to the female. This positioning is called amplexus. It lasts from I to 24 hours. The male releases sperm and the female releases eggs during amplexus.

Caecilian: Little is known about the breeding behavior in caecilian. Males have an intermittent organ (copulatory organ). It is a modification of the cloaca! wall. Therefore, fertilization is internal.

Vocalization

Sound production has reproductive function in male  anurans. Frogs  produces different calls:    
1. Advertisement calls:
It is used to attract females to breeding areas. It is also an announcement for other males the given territory is occupied. Advertisement calls are species specific.  These calls induce psychological and physiological changes in female. As a result the female get ready to breed.

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2. Receptive calls: If female get ready for breeding then it produces receptive calls.

3. Release call: Release call informs the partner that it is incapable of reproducing. Some male try in amplexus unresponsive females. Such females also give release calls. Sometimes, a male mistakenly identified other male as female. Thus otlit r male also produce release call.

4. Distress calls: These calls are not associated with reproduction. These calls are produced in response to pain or danger of a predator. The calls are much aloud. It frightens the predator to release the frog. The distress call of the South American jungle frog is as loud as a cat in distress.

Sound producing apparatus

The sound production apparatus of frogs consists of the larynx and its vocal cords. It is called laryngeal apparatus. It is well developed in males. They possess a vocal sac. Vocal sac is a diverticulum  from the lining of the buccal cavity. Lungs force air in vocal cords and cartilages of the larynx. This air produces vibrations in them. Muscles control the tension of the vocal cords and regulate the frequency of sound. Vocal sacs act as resonating structures. They increase the volume of the sound.

Advantages of vocalization

1. The amphibians live in widely dispersed habitats. Therefore, it is useful for them to attract mates from distant places.

2. Many species of frogs collected at the same pond for breeding. It becomes difficult for them to find their proper mate. Vocalizations help to attract their mate of same species.

Parental Care

Parental care increases the chance of development of an egg. But it requires large amount of amount from the parents. Mostly both parents care for the egg clutches. It is most common form of parental care in amphibians. It may be:

1. Maternal care: It takes place in species with internal fertilization, e.g. salamanders an caecilians.

2. Parernal care: It takes place in species with external fertilization, e.g. anurans. It involves aeration of aquatic eggs, cleaning and moistening of terrestrial eggs, protection of eggs from predators, or removal of dead and infected eggs.

Eggs re transported during development on land. Females of the genus Pipa carry eggs on their backs. Two species of Rheobatrachus were discovered in Australia. Rheobatrachus females brooded tadpoles in their stomachs. The young come out through mouths the female. But it is not known whether the female swallow egg or tadpole larvae. The stomachs of female expanded and till most of her body cavity during brooding. Thus the stomach stops producing digestive secretions. Viviparity and ovoviviparity occur in salamanders and caecilians.

Metamorphosis

Metamorphosis is a series of structural, physiological, and behavioral changes that transform a larva into an adult. A number of environmental conditions influence the time required for metamorphosis. These conditions are collections and availability of food. Metamorphosis is directly controlled of neurosecretions of hypothalamus, hormones of the anterior lobe of the pituitary gland and the thyroid gland.

1. Minor morphological changes take place during metamorphosis of caecilians and slamanders. Reproductive structures develop, gills are lost, and a caudal fin is lost.

2. Major changes take place during metamorphosis of tadpole into the small frog in the anuran. Limbs and lungs are developed. The tail is reabsorbed and the skin thickens.

Many changes take place in the head and digestive tract.

Paedomorphosis

A phenomenon in which larva becomes sexually mature while still showing larval characteristics is called paedomorphosis. The mechanism of metamorphosis explains paedomorphosis in amphibians. Paedomorphosis mostly takes place in some families of salamander. In other families, the occurrence of paedomorphosis is variable. It is influenced by environmental conditions. Two conditions promote paedomorphosis:

1. Some salamanders do not respond to thyroid hormones. Therefore, it becomes paedomorphic.

2. Some larvae do not produce the hormone necessary tbr metamorphosis. Therefore, they become paedomorphic.

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