Amphibians
Overview
Amphibians (class Amphibia) are a class of vertebrates that, as the name suggests, live across two stages of life, water and land. Most lay eggs in water and spend a larval period breathing with gills, then undergo metamorphosis to become adults that breathe with lungs and skin. More than about 8,700 species are known worldwide, and because their skin is thin and highly permeable, they are extremely sensitive to environmental changes such as water quality, temperature, and ultraviolet radiation; for this reason they are used as representative indicator species of ecosystem health.
Main content
Classification
Extant amphibians are divided into three orders. First, order Gymnophiona (caecilians) has about 200 species, lacks legs, is snake-like, and lives in soil in tropical regions. Second, order Urodela (salamanders) has about 800 species, has a well-developed tail, and mostly inhabits water or humid forests. Third, order Anura (frogs) has more than about 7,500 species, accounting for over 88% of all amphibians, and adults lack a tail. In Korea, the Oriental fire-bellied toad (무당개구리), black-spotted pond frog (참개구리), Japanese tree frog (청개구리), gold-spotted pond frog (금개구리), Korean salamander (도롱뇽), Korean clawed salamander (꼬리치레도롱뇽), and four-toed salamander (네발가락도롱뇽) occur.
Morphology and physiology
Amphibian skin is smooth, rich in mucous and poison glands, and lacks scales, hair, and feathers. This thin skin enables cutaneous respiration, the direct exchange of oxygen and carbon dioxide, but at the same time has the duality of being vulnerable to water loss and penetration by pollutants. Adults breathe using lungs, the buccal mucosa, and skin together. The heart has two atria and one ventricle, a simple circulatory system in which oxygenated and deoxygenated blood partially mix. Because they are ectotherms, they cannot regulate their body temperature themselves, so their activity level varies greatly with temperature and humidity.
Life cycle and reproduction
Most species lay eggs in water, and hatched tadpoles breathe with gills and grow as herbivores. Thereafter, under the action of thyroid hormones, metamorphosis occurs: legs grow, the tail is resorbed, lungs develop, and the digestive tract is reorganized for carnivory. Some species undergo direct development, completing metamorphosis inside the egg and hatching as juveniles, and diverse parental care behaviors have evolved, such as poison frogs carrying eggs on their backs or raising them in the stomach. The breeding season is usually spring to summer, and male calls serve as key signals for species recognition and mating.
Ecological role
Amphibians inhabit various aquatic-terrestrial boundary areas such as wetlands, rice paddies, forest valleys, and alpine grasslands, and serve as intermediate links in the food chain. Adults prey on insects, spiders, earthworms, and other organisms, regulating pest populations, while larvae and adults are important food for snakes, birds, fish, and mammals. Therefore, declines in amphibian populations have cascading effects on both higher predators and lower prey organisms.
Threats and conservation
The International Union for Conservation of Nature (IUCN) classifies a substantial proportion (over 40%) of assessed amphibians as threatened species. The main causes are habitat fragmentation from wetland reclamation and forest destruction, water pollution including pesticides and heavy metals, mismatch in breeding timing due to climate change, and the spread of the fungal diseases chytridiomycosis (Bd) and salamander chytridiomycosis (Bsal). These two diseases are known to have caused global population collapses.
Recent trends
In 2024–2025, long-term observational studies continue to report that climate change is advancing amphibian spawning seasons and shifting suitable habitat ranges toward higher elevations. With advances in genomic analysis technology, research on metamorphosis-regulating genes and immune genes has become more active, and studies on Bd-resistant populations have also made progress. As surveys based on citizen science and acoustic monitoring spread, including iNaturalist and automated recording sensors for amphibian calls, species distribution data are rapidly accumulating. In Korea as well, the creation of alternative habitats and installation of ecological corridors for legally protected species such as the gold-spotted pond frog and the Boreal digging frog (맹꽁이) are increasing, and conservation projects linked with eco-friendly farming are expanding in recognition of the biodiversity value of rice-paddy ecosystems.
Related topics
- [[Frog]]
- [[Salamander]]
- [[Biodiversity]]
- [[Ectotherm]]
- [[Wetland]]
- [[Amphibian diseases]]