Amphibians: Characteristics, Classification & Examples

Modern amphibians are classified into three main orders based on their distinct physical characteristics:

• Order Anura (or Salientia): This group includes frogs and toads. They are characterised by the absence of a tail in their adult form ('a' meaning without, 'ura' meaning tail), long hind limbs for jumping, and a short, stout body. Examples include Rana (Frog), Bufo (Toad), and Hyla (Tree frog).
• Order Urodela (or Caudata): This group includes salamanders and newts. They possess a distinct tail throughout their life ('uro' meaning tail, 'dela' meaning visible), a lizard-like body, and typically have two pairs of well-developed limbs of nearly equal size. An example is Salamandra (Salamander).
• Order Gymnophiona (or Apoda): This group includes the limbless, burrowing amphibians known as caecilians. They are worm-like in appearance, lack limbs ('a' meaning without, 'poda' meaning feet), and have rudimentary eyes covered by skin. An example is Ichthyophis (Limbless amphibian).

The dual life of amphibians is a remarkable evolutionary adaptation that allows them to exploit both aquatic and terrestrial environments. The significance lies in reducing competition and maximising resource utilisation. The aquatic larval stage (like a tadpole) is typically herbivorous and breathes through gills, perfectly suited for life in water. After metamorphosis, the terrestrial adult develops lungs, limbs, and often becomes carnivorous. This transition allows the species to access different food sources and habitats during different life stages, enhancing its overall survival and reproductive success.

Metamorphosis in a frog is a profound transformation from an aquatic larva (tadpole) to a terrestrial adult. The process is regulated by the hormone thyroxine and involves several stages:

• Egg: A female frog lays eggs in a clutch in water.
• Tadpole (Larva): The egg hatches into a tadpole, which is fully aquatic. It has a long tail for swimming, breathes through external gills, and feeds on algae.
• Limb Development: Hind limbs begin to appear first, followed by the front limbs. Internally, the gills start to get replaced by developing lungs.
• Froglet: The tadpole now has limbs and lungs but still retains its tail. It starts to test its lungs by coming to the surface for air. The tail begins to shorten as its nutrients are absorbed by the body.
• Adult Frog: The tail completely disappears, the digestive system adapts for a carnivorous diet, and the frog becomes a fully formed, air-breathing adult capable of living on land.

While both are ectothermic vertebrates, amphibians and reptiles have several key differences:

• Skin: Amphibians have moist, glandular, and permeable skin without scales. Reptiles have dry, scaly skin that prevents water loss.
• Respiration: Amphibians can breathe through their skin (cutaneous respiration) in addition to lungs. Reptiles respire exclusively through well-developed lungs.
• Reproduction: Amphibians lay soft, shell-less eggs in water and undergo metamorphosis. Reptiles lay amniotic eggs with a protective leathery or hard shell, usually on land, and their young are miniature versions of adults (no larval stage).
• Heart: Most amphibians have a three-chambered heart, while most reptiles have a three-chambered heart with a partially divided ventricle (crocodiles have a four-chambered heart).

The moist and permeable skin of amphibians is a critical multifunctional organ. Its primary importance lies in facilitating cutaneous respiration, where gas exchange (oxygen in, carbon dioxide out) occurs directly through the skin. This supplements their lung-based breathing and is essential, especially when they are underwater or hibernating. The mucous glands keep the skin moist, which is necessary for this gas exchange. However, this permeability also makes them highly sensitive to dehydration and environmental pollutants, restricting them to moist or aquatic habitats.

An amphibian's three-chambered heart consists of two atria and a single ventricle. This structure supports their ectothermic (cold-blooded) metabolism and dual life. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs and skin. Both empty into the single ventricle, where some mixing of blood occurs. While less efficient than a four-chambered heart, this system is adequate for their lower metabolic needs. A specialized spiral valve in the ventricle helps direct the more oxygenated blood to the brain and body and the less oxygenated blood to the lungs and skin for re-oxygenation, thus partially compensating for the mixing.

Amphibians are considered excellent bio-indicators because of their high sensitivity to environmental changes. Their permeable skin readily absorbs substances from their surroundings, making them vulnerable to water and air pollution. Furthermore, their dual life cycle means they are exposed to both aquatic and terrestrial pollutants. A decline in amphibian populations or an increase in deformities can signal deteriorating environmental conditions, such as chemical contamination, habitat destruction, climate change, or increased UV radiation, often long before other species are affected.