Chapter 6: Amphibians: Life Between Two Worlds

Amphibians were the first vertebrates to colonize land, making the evolutionary transition from water roughly 375 million years ago (Devonian period). Their name — from Greek amphi (both) and bios (life) — reflects their dual existence: most require water for reproduction while spending much of adult life on land. Today, approximately 8,000 species are recognized, belonging to three orders.

The Three Orders of Amphibians

Order	Common Name	Limbs	Tail	Examples
Anura	Frogs & toads	4 (hindlimbs enlarged)	Absent (adults)	Rana, Bufo
Urodela	Salamanders & newts	4 (equal)	Present	Ambystoma, Salamandra
Gymnophiona	Caecilians	Absent (limbless)	Short	Caecilia, Dermophis

Metamorphosis in Frogs

Frog development is a classic example of metamorphosis:

1. Egg — laid in water, fertilized externally (in most species)
2. Tadpole (larva) — aquatic, herbivorous, gill-breathing, laterally compressed tail
3. Prometamorphosis — hind limbs bud; gills begin to regress
4. Climax — front limbs emerge, tail resorbs, lungs develop, mouth widens, diet shifts to carnivory
5. Adult — semi-terrestrial, carnivorous, pulmonary/cutaneous respiration

The hormone thyroxine drives metamorphic changes; its synthesis is triggered by prolactin-thyrotropin interactions.

Cutaneous Respiration

Amphibian skin is moist, thin, and heavily vascularized — functioning as a respiratory organ. In some species (e.g., Plethodon salamanders, which are lungless), skin is the sole site of gas exchange. This dependence on skin respiration explains why:

• Amphibians are sensitive to pollution and desiccation
• Many require humid microhabitats
• Amphibians are excellent bioindicators of environmental health

Amphibian Decline

Amphibians are the most threatened vertebrate group, with ~40% of species facing extinction risk. Key threats include:

• Chytridiomycosis: fungal disease (Batrachochytrium dendrobatidis) that disrupts skin ion transport; has caused the extinction of >90 frog species
• Habitat loss: wetland drainage, deforestation
• Introduced species: predatory fish, bullfrogs outcompeting native species
• Climate change: altered rainfall patterns affecting breeding pools
• UV-B radiation: increased UV damages developing eggs

Key Checklist

• I can name the three orders of amphibians and distinguish them by limb and tail characteristics
• I can describe the five stages of frog metamorphosis and identify the hormone responsible
• I can explain why amphibians are sensitive environmental indicators and list the major threats they face