Adaptive features of aquatic and terrestrial toad
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Amphibians were the first vertebrates to venture out onto land. Early amphibians retained many fish-like characteristics but during the Carboniferous period amphibians diversified. Even today, Amphibians live a dual life. They are totally aquatic during their larval stage and partially terrestrial animals as adults. Because of this, they are considered intermediate organisms in the evolutionary passage of vertebrates from an aquatic to terrestrial. Amphibians are also the first tetrapod animals; that is, the first with two pairs of limbs, a typical feature of terrestrial vertebrates. The name “amphibian” comes from the double life (aquatic as larvae and partially terrestrial as adults) of these animals.
Common Amphibians
Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless Amphibia), Newt are some of the common frogs.
Salient Features
There are several features of amphibians that make them dependent on water to survive. These include a permeable skin; a body subject to dehydration, external fertilization, eggs without shells and a larval stage with branchial respiration.
Respiration
In the fishes, the gas exchange is carried out via the direct contact of water with the gills, while in adult amphibians; the gas exchange is carried out through their moist and permeable skin. This kind of respiration is called cutaneous respiration. Further, they also have lungs. During larval (tadpole) stage, they respire only through gills and this is the reason that frogs and other amphibians need water to survive.
Amphibian Heart
While the fish heart only has two chambers; amphibians have three chambers (two atrium and one ventricle).
Excretion
The adult amphibians have kidneys to excrete nitrogenous wastes as Urea. However, their larvae (Tadpole) are aquatic and excrete ammonia.
Reproduction in Amphibians
In most amphibians, fertilization is external. However, despite the external fertilization, amphibian males and females copulate to stimulate the release of sperm and egg cells. However, females release eggs in water and males also release the sperms in water only.
Amphibians to Higher Vertebrates: Adaptations to Terrestrial environments
Transition of vertebrates from aquatic environment to terrestrial environment needed to solve some problems. Firstly, they needed to avoid dehydration. This problem was solved by a thicker, impermeable skin which allows to less water. While Amphibians have semi-permeable skin which helps them in respiration also, higher vertebrates have impermeable skin. Secondly, they needed to eliminate waste with less amount of water available. We note here that Ammonia is highly soluble in water and essentially excretion of ammonia is feasible only in aquatic animals. When they are on ground, they need to save water. This was possible by excreting nitrogenous wastes as Urea or Uric Acid as they need less water to dissolve. In amphibians, while their larvae release ammonia (thus called Ammonotelic), the adult amphibians need economy of water and thus excrete Urea. In Birds, the system is even more efficient as they excrete Uric Acid which needs least water. Thirdly, they needed to protect themselves against the harmful solar radiation. This was done was skin pigments, feathers, hair, fur or whatever means to filter the harmful radiation. While amphibians have skin pigments, other higher vertebrates have other means for the same purpose. Fourthly, they needed to solve the problem of fertilization. Hitherto animals had a media (water) which allowed the mixing of male and female gametes. In amphibians also male and female release the gametes in water. However, in higher vertebrates this problem was solved by internal fertilization
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