The following points highlight the top six characteristics of amphibians. The characteristics are: 1. External Features 2. Internal Features 3. Fertilization 4. Development 5. Chromosome Number 6. Physiological Adaptive Features.
Characteristic # 1. External Features:
a. The amphibians are ectothermic vertebrates.
b. Skin is naked, moist and highly glandular.
c. Dermal scales are present in some members of apodans (e.g., Uraeotyphlidae, Typhlonectidae and Caeciliidae).
d. Head distinct, trunk elongated, neck and tail may be present or absent (e.g., Anura).
e. Body with 2 pairs of pentadactylous limbs (except posterior pair in Sirenidae and total absent in Gymnophiona).
f. Fore limbs are provided with 4 digits and hind limbs with 5 digits. Digits are clawless except African toad, Xenopus and in larval forms of an Asiatic urodele, Onychodactylus.
g. The eyelids are present in terrestrial amphibians. The tympanic membrane or tympanum is absent in urodeles, apodans and in some anurans (e.g., Bombinator). The tympanum is prominent in most of anurans.
h. Lateral line system is present in larvae of air-breathing forms, and in the per-ennibranchiate urodeles (e.g., Necturus, Proteus and Siren), which helps to detect the vibrations of water.
i. Median fins are usually present in the larvae but not supported by fin-rays.
j. Head with a pair of nostrils leading into buccal cavity.
Characteristic # 2. Internal Features:
(a) Soft parts:
i. The tongue is flat, broad and muscular and more or less immobile in aquatic forms.
ii. The gut ends into cloaca.
iii. Respiration is performed by lungs in most adult amphibians. In the larval stages, and in the per-ennibranchiate urodeles the external gills help in respiration. In some salamanders (Salamandra atra and all plethodontids) where lungs are absent, reparation is performed exclusively by skin and pharynx.
iv. RBC are large, nucleated and oval. RBC of salamanders amongst vertebrates are largest in size. The RBC of Proteus measures about 58 pm in diameter.
v. Heart is 3 chambered with 2 auricles and an undivided ventricle. One sinus venosus and one conus arteriosus are present. In Siren and Necturus a septum divides the ventricle into right and left chambers. Within the cavity of conus arteriosus a longitudinal spiral valve of endothelial tissue divides the lumen of conus arteriosus incompletely and separates the circulation of well oxygenated and less oxygenated blood.
vi. Kidney is opisthonephric type. Modern amphibians possess a single archinephric duct which drains both the kidneys and the gonads.
vii. In female adult amphibians the archinephric duct is lost, the products of the ovary are carried by the oviduct.
viii. A ventral cloacal urinary bladder is present in amphibians, and the bladder has the capacity for water resorption.
ix. The brain is unspecialized, mainly in urodeles. The cerebral hemispheres are separate from each other as compared with fishes. The corpus striatum is small. The pineal body is well- developed in anurans. The cerebellum is rudimentary type, so the amphibians are sluggish in nature.
x. 10 pairs of cranial nerves are present.
xi. Organ of Jacobson or Vomeronasal organ, an organ of olfaction, is present in most amphibians.
(b) Hard parts:
i. Double occipital condyles of exoccipitals are present in the skull.
ii. Jaw suspension is autostylic type.
iii. Close otic notch is present.
iv. Post temporal fossa and ectopterygoid are present.
v. The tympanum, if present, connected with the inner ear through a rod-like stapes or columella (hyomandibula of fish), helps in sound transmission. An extra bone with the ear, called auricular operculum, is present.
vi. The mandible is composed of a single coronoid medially and 3-dermal elements.
vii. Pedicellate type of teeth are unique in all modern amphibians (except in a group of salamanders of the genus Siren, and two genera of the frogs, Phyllobates and Ceratophrys). Pedicellate teeth are whose base and crown composed of dentine, and are separated by a narrow zone of un-calcified dentine or fibrous connective tissue.
viii. The vertebral column is largely bony and the flexibility of the vertebral column is lost to give more strength and supports the weight of the body on land.
ix. The pectoral girdle is freed from the skull and does not articulate directly with the vertebral column.
x. Pelvic girdle is connected to the vertebral column by a single sacral vertebra and the hind limbs joined to the pelvic girdle are used for propulsion.
xi. The sacral region includes a single vertebra.
xii. Most of the extant amphibians have lost their ribs.
xiii. True sternum is absent.
xiv. Vertebrae are amphicoelous type in apodans, mostly procoelous and opisthocoelous in urodeles, and superficial procoelous type in most anurans.
Characteristic # 3. Fertilization:
There is a great diversity of reproductive modes that do not correspond very much in their taxonomy. Fertilization is mostly external in some urodeles and in anurans, and mostly internal in apodans and in some urodeles (Ambystomatidae).
Characteristic # 4. Development:
a. Mostly are oviparous, and a few are viviparous e.g., some members of apodans Scolecomorphidae, some members of Caeciliidae and Typhlonectidae, some species of Nectophrynoides (Bufonidae and in some species of salamanders).
b. Mostly amphibians lay eggs in moist microhabitats or in water, and for metamorphosis water is necessary.
c. Eggs are large and yolk is moderate amount – mesolecithal.
d. Cleavage is holoblastic but unequal.
e. Amnion and ailantois are absent (anamniotic).
f. An aquatic larval stage (tadpole) with external gill is present.
g. In most amphibians, metamorphosis takes place in many forms.
h. A peculiar phenomenon — paedogenesis (sexual maturity in the larva) or neoteny (retention of larval characters) occurs in some urodeles.
Characteristic # 5. Chromosome Number:
In Apoda, the lowest chromosome number is 20 in Chthonerpeton indistinctum (Typhlonectidae) and the highest number is found in Ichthyophis which is 42 in number. In Urodela, the lowest chromosome number in Taricha is 22, and in Onychodactylus japonicus and Ranoden sibiricus, the number is 66.
In Anura the lowest and the highest chromosome numbers have been recorded in the family Ranidae. The lowest number has been recorded in Arthroleptis which is 14 and the highest number has also been recorded in Astylosternus diadematus which is 54.
Characteristic # 6. Physiological Adaptive Features:
Skin for respiration and Water conservation:
The moist, glandular skin is permeable to water, thus facilitates the exchange of oxygen and carbon-dioxide. Evaporation of water vapour through the skin has restricted to the amphibians in wet and moist environments except the desert frog of Australia, Chiroleptis platycephalus, which aestivates under mud by adopting a special way, such as the ability to hold the large amount of urine in the urinary bladder by the loss of glomeruli in the kidneys.
The body fluids of aquatic amphibians are hypertonic, that is higher in concentration to the freshwater. Water constantly enters within the body through the gills, oral membrane and moist permeable skin and some water enters inside the body with food. Amphibians do not drink or scarcely drink water. The water balance of the body is maintained by the production of copious urine.
The terrestrial amphibians like most anurans live mainly in moist climate. The urine of these anurans is hypo-osmotic in relation to the blood plasma. So anurans actively resorb solutes from the renal tubular fluid and also resorb water from the urine, to maintain the balance of water within the body. The fishes and amphibians are called as Anamniota because of the lacking of amnion and ailantois around the eggs.
Extinct and living amphibians have a worldwide distribution. Living amphibians are absent from Antarctica and some oceanic islands.
Duellman and Trueb (1986), Halliday and Adler (1986) have reported 4015 species which are classified into 398 genera in 34 families.