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Amphibians
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Gururaja K. V. Sameer Ali Vishnu D. Mukhri
Frogs and toads, wakes up our memory lane with noisy calls (of hopping little creatures) as it dusks during monsoon. In their life cycle, young ones (tadpole or larvae) live in water and the adults live on land. Due to this dual life style they are called amphibians (in Greek amphibios means dual life). Amphibians are the first vertebrates to leave ancestral aquatic life style to venture terrestrial mode. Among the vertebrates, amphibians are the only class to have free-living tadpole stage and adult stage. All the living amphibians throughout the world have been grouped in 3 orders, namely Apoda (Caecilians), Salientia (Frogs and Toads) and Caudata (Salamander and Newts).
Evolution
The amphibians first appeared on earth about 360 million years ago, i.e., during late Devonian period (refer geological time scale). They all have the common ancestry of Sarcopterygian bony fish. Modern amphibians (Lissamphibians), appeared during the Triassic period and they have survived even today. Triadobatrachus massinoti is the earliest Lissamphibian fossil available from Madagascar. Among the Modern amphibians, Anura and Gymnophiona appeared during the early Jurassic period and Caudata during middle Jurassic.
Most of the modern day amphibians lack scales on their skin, and it is usually smooth. Mucous and granular glands are present in the skin and they help in social behaviour (hedonic glands produce pheromones that are used in courtship), defence (secretions from parotid gland are highly toxic and lethal) and protection (skin secretions avoid microbial infection).
4. Tympanum 5. Forelimb 6. Hindlimb 7. Web
Skin
helps in oxygen uptake and release of carbon dioxide (permeable to gases) from
the surrounding environment. It is also permeable to water. Due to this permeable
nature of skin, water readily evaporates from the skin and dehydrates the amphibians
easily. Hence they tend to restrict their activity only to high humidity and
low wind periods to avoid evaporation stress. One can probably find more amphibians
in moist environments and they are active during nights (nocturnal). Amphibians
in the dry region tend to absorb water through skin from moist soils. Periodic
shedding of the skin is common in amphibians. Scientifically it is called Ecdysis.
Colour of the skin is produced by xanthopores, iridophores and melanophores
(colour bearing cell organelles). Amphibians can change their skin colour according
the surrounding environment.
Amphibians
use gills, lungs and skin for the respiratory purposes. Utility of gills for
respiration is prominent in tadpoles and larvae, where as lungs and skins are
used in adults. In amphibians, the heart is three chambered and it has one ventricle
(to pump blood to lungs and other parts of body) and two auricles (to receive
blood from the body and lungs). High viscous blood and trabeculae in the ventricle
limits mixing of oxygen rich and oxygen poor blood by creating separate channels
for them.
Amphibians
are ectotherms. They receive the energy externally from sun, to raise their
body temperature and to perform normal activity. Some times they bask in the
sun to get the energy directly or they lay on the rocks that are previously
heated with sunlight. Amphibians are also regarded as poikilotherms (cold blooded),
since their body temperature varies along with surrounding environment.
These characters have put both requirement and limitation to the amphibian population and also made them highly susceptible to minute changes in the environment.
Amphibians feed on any thing, which is smaller than its body size and mouth opening. Aquatic larval amphibians are herbivorous to omnivorous (anurans) or carnivores (salamanders and caecilians). Cannibalism (eating their own kind) is also observed in amphibians. The amphibians can differentiate between kin and non-kin, i.e., they have the ability of kin recognition. This is done with the help of chemical cues.
