The primary characteristic of amphibians is their dual life cycle, beginning in aquatic environments and transitioning to terrestrial habitats as adults. Amphibians typically start as eggs laid in water, developing into larvae (like tadpoles) that breathe through gills. As they mature, they undergo metamorphosis, developing lungs for breathing air and limbs for movement on land. This unique life cycle allows them to thrive in both aquatic and terrestrial ecosystems. Additionally, amphibians usually have moist skin, which plays a critical role in respiration and is sensitive to environmental changes, making them indicators of ecosystem health.