Amphibians are vertebrates defined by their ability to live in both aquatic and terrestrial environments. The frog life cycle involves two profoundly different forms: the tadpole and the adult frog. The tadpole is the aquatic, juvenile stage, while the frog is the mature, terrestrial or semi-aquatic form. These stages are specialized for survival in distinct habitats.
Primary Physical Differences
The most striking difference between a tadpole and a frog is their body plan, which reflects their respective environments. The tadpole is built for a purely aquatic existence, featuring a streamlined, fish-like body with a rounded head and a long, laterally compressed tail. This tail, complete with fins, is the sole means of propulsion through water. Tadpoles lack limbs, possessing small, rasping mouthparts adapted for scraping food off surfaces.
In contrast, the adult frog has a stout, tailless body supported by four powerful limbs. The hind limbs are long and muscular, designed for jumping on land and strong swimming. The tadpole’s tail is completely resorbed during development, leaving behind a reduced structure called the urostyle. The tadpole’s small mouth is replaced by a wide gape and a specialized tongue in the adult, which also develops prominent, bulging eyes and a visible eardrum (tympanum).
Shifts in Lifestyle and Diet
The drastic changes in morphology support a fundamental shift in how the animal lives and what it consumes. As a tadpole, the animal is fully water-bound, relying on internal gills to extract dissolved oxygen. It possesses the lateral line sensory system, which detects vibrations and pressure changes in the aquatic environment. The tadpole’s diet is primarily herbivorous, consisting largely of algae and decaying plant matter, requiring a long, coiled intestine for efficient digestion.
The adult frog is a semi-aquatic animal capable of spending time on land. The transition to a terrestrial lifestyle requires a change in respiration; gills are replaced by lungs, allowing the frog to breathe air through buccal pumping. It supplements this with cutaneous respiration, absorbing oxygen through its moist skin. The adult frog is a carnivore, preying on insects, spiders, and other small invertebrates, necessitating a dramatic shortening of the digestive tract.
The Metamorphosis Process
The transformation from tadpole to frog, known as metamorphosis, is a rapid, systemic biological event driven by hormonal signaling. The primary trigger is an increase in thyroid hormones, specifically thyroxine, produced by the thyroid gland. These hormones circulate through the bloodstream, initiating sequential changes across organ systems.
During this phase, the tail is broken down and absorbed by the body through a controlled process called apoptosis. The organic material from the tail tissue is recycled, providing proteins and energy to fuel the development of new structures, such as the limbs. The hind limbs begin to grow first, followed by the forelimbs. Simultaneously, the digestive system shortens, larval horny jaws are replaced by bony structures, and the gills disappear as the lungs fully develop, preparing the animal for life on land.