29.3 Amphibians

Amphibian Life Cycle and Characteristics

Amphibians are vertebrates that live a "double life," splitting their time between water and land. The name itself comes from the Greek amphibios, meaning "living a double life." Most amphibians start as aquatic larvae, then undergo metamorphosis to become terrestrial or semi-terrestrial adults. This dual lifestyle requires specialized adaptations in breathing, reproduction, and body structure that set amphibians apart from all other vertebrate groups.

Amphibian vs. Vertebrate Life Cycles

Amphibians undergo metamorphosis, transitioning from an aquatic larval stage (e.g., a tadpole) to a terrestrial or semi-aquatic adult (e.g., a frog). Most other vertebrates skip this larval stage entirely. Mammals and birds, for instance, develop directly into their adult form.

A few key contrasts help clarify what makes amphibians different:

• Respiration across life stages: Amphibian larvae breathe through gills, while adults typically develop lungs and also absorb oxygen through their skin. Fish, by comparison, retain gills their entire lives.
• Eggs and fertilization: Amphibian eggs are laid in water, lack a protective shell, and rely on external fertilization. Reptiles and birds lay shelled eggs on land using internal fertilization. Most mammals (except monotremes like the platypus) give birth to live young.
• Habitat shifts: Amphibians are among the only vertebrates that routinely shift from a fully aquatic juvenile stage to a partly terrestrial adult stage.

Distinguishing Features of Amphibians

All amphibians share moist, permeable skin that lacks scales and plays an active role in gas exchange and water balance. Beyond that shared trait, the three living orders look quite different from one another.

• Salamanders (Order Caudata):
  • Elongated bodies with long tails retained into adulthood
  • Two pairs of limbs that are roughly equal in size
  • Smooth, moist skin
• Frogs and toads (Order Anura):
  • Short, tailless bodies with a broad head
  • Powerful hind legs adapted for jumping
  • Skin ranges from smooth (poison dart frogs) to warty (American toads)
• Caecilians (Order Gymnophiona):
  • Limbless, serpentine bodies with ring-like segments in the skin
  • Greatly reduced eyes and a unique sensory tentacle on the head used for chemoreception
  • Adapted for burrowing underground or living in aquatic sediments

Amphibian Physiology and Adaptations

Amphibians are ectothermic, meaning they rely on external heat sources to regulate body temperature. Their physiology reflects the demands of living in two very different environments.

• Cutaneous respiration: Because their skin is thin and moist, amphibians can exchange gases ($O_2$ and $CO_2$) directly through it. This supplements lung breathing in adults and is the primary mode of respiration in some lungless salamanders.
• Cloaca: Amphibians have a single opening, the cloaca, that serves the digestive, urinary, and reproductive systems.
• Three-chambered heart: Adults have a heart with two atria and one ventricle. This partially separates oxygenated and deoxygenated blood, which is less efficient than the four-chambered hearts of mammals and birds but sufficient for animals that also breathe through their skin.
• Neoteny: Some species retain larval features into adulthood. The axolotl is the classic example: it keeps its external gills and remains fully aquatic even as a sexually mature adult.

During metamorphosis, a tadpole undergoes dramatic changes: gills are resorbed, lungs develop, limbs grow, the tail (in frogs) is resorbed, the digestive system shifts from herbivorous to carnivorous, and the lateral line system is typically lost. These changes are driven largely by thyroid hormones.

Evolution and Ecology of Amphibians

Amphibians evolved from lobe-finned fish during the Devonian period, roughly 370 million years ago. Early amphibians like Ichthyostega were among the first tetrapods (four-limbed vertebrates), and the evolution of lungs and weight-bearing limbs allowed them to begin exploiting terrestrial habitats.

• During the Carboniferous period, humid climates and vast wetlands created ideal conditions, and amphibians diversified extensively.
• The Permian-Triassic extinction (~252 million years ago) sharply reduced amphibian diversity. Surviving lineages eventually gave rise to the three modern orders: Anura (frogs and toads), Caudata (salamanders), and Gymnophiona (caecilians).

Amphibians have evolved a range of survival strategies for different ecological niches:

1. Chemical defense: Poison dart frogs produce toxic alkaloids in their skin glands, advertising their toxicity with bright warning coloration (aposematism).
2. Prey capture: Many frogs have sticky, projectile tongues and large eyes positioned for wide-field vision, allowing them to snag fast-moving insects.
3. Camouflage: Species like the mossy frog use cryptic coloration and textured skin to blend into their surroundings.
4. Parental care: The Suriname toad embeds fertilized eggs into the skin of the female's back, where they develop until fully formed toadlets emerge.