Amphibians: Evolution and Adaptations

• Most amphibians begin life as aquatic organisms and then live on in terrestrial environments as adults.

Essential Questions

• What key adaptations enabled animals to transition to land?
• What traits define amphibians?
• What distinguishes the different groups of amphibians?

Vocabulary

• Metamorphosis: A transformation or dramatic change
• Cloaca: A chamber receiving waste and reproductive cells from intestines
• Nictitating Membrane: a protective clear eyelid
• Tympanic Membrane: The external eardrum
• Ectotherm: Animals that obtain body heat from the external environment

Evolution of Tetrapods: The Move to Land

• Tetrapods, which include amphibians, reptiles, birds, and mammals, are four-legged vertebrates that appeared 360 million years ago.
• Major physical challenges in the water-to-land transition:
  • Air provides less buoyancy against gravity.
  • Oxygen is easier to obtain from the air than from the water
  • Flucuations in temperature can be wild and extreme
  • The lateral line system doesn't work well for detecting sound

Adaptations for Life on Land

• Development of Limbs: Strengthened skeletal and muscular systems.
• Lung Development: Enabled efficient oxygen extraction from the air
• New Behaviors: Like migration
• Temperature Adaptation: Development of behaviors and physical adaptations for extreme temperatures.
• Ears: Enable the detection of sound waves in the air.
• Land has a variety of habitats for animals.
• With proper adaptations, animals can live in tropical rain forests, temperate forests, grasslands, deserts, taiga, and tundra.

Characteristics of Amphibians

• Most amphibians start in aquatic environments.
• They start as limbless, gill-breathing tadpoles and undergo a daily metamorphosis.
• Hind legs form and grow, forelimbs sprout as the tail shortens.
• Lungs replace gills, completing the transformation into adult frogs.
• Adult amphibians have four legs, moist skin without scales, a double-loop circulatory system, and aquatic larvae.
• Gas exchange occurs through both their skin and lungs.

Feeding and Digestion

• Frog larvae are typically herbivores, while salamander larvae tend to be carnivores.
• Both groups become predators as adults.
• Amphibians eat a variety of invertebrates and small vertebrates.
• Some salamanders and legless amphibians use their jaws to catch prey.
• Food travels from the mouth, from the esophagus to the stomach, where digestion begins.
• In the small intestine, food receives enzymes from the pancreas.
• Digested food is absorbed into the bloodstream.
• The food moves from the small intestine to the large intestine before waste is eliminated.
• The intestines end in a chamber called the cloaca, which receives waste and reproductive cells before expelling them from the body
• The kidneys filter waste from the blood
• Amphibians that live in water get rid of wastes with ammonia.
• Amphibians that live on land excrete urea.
• In the liver, ammonia becomes urea
• Unlike ammonia, urea is stored in the urinary bladder until it eventually leaves the body through the cloaca.
• Larvae exchange gases through their skin and gills
• Adults do so with lungs, thin, moist skin, and mouth cavities.
• Also, amphibians use a double loop circulatory system

Double-Loop Circulatory System

• Oxygen-poor blood moves from the heart to the lungs and skin to get oxygen.
• Oxygen filled blood moves from the lungs and skin back to the heart.
• Oxygen-filled blood travels through vessels to the body.
• Oxygen diffuses into cells in the second loop.
• An amphibian's heart contains three chambers.
• The right atrium receives oxygen-poor blood from the body.
• The left atrium receives oxygen-filled blood from the lungs.
• The ventricle remains undivided

Brain and Senses

• Amphibians use their sight to locate and capture prey at high speeds and to escape predators.
• Frogs have useful nictitating membranes, clear eyelids that protect the eye.
• Frogs use their tympanic membranes, or eardrums, to hear high-pitched sounds and amplify sounds from the vocal cords and amphibians are ectotherms.
• They depend on external sources for body heat.
• In most species, fertilization is external
• Shell-less eggs must be laid and fertilized in water
• Tadpoles hatch and undergo metamorphosis from a fishlike animal to an air-breathing one.
• Modern amphibians are classified into three orders

Amphibian Classification

• Order Anura includes frogs and toads.
• Order Caudata includes salamanders and newts.
• Order Gymnophiona includes caecilians.

Frogs vs Toads

• Frogs can do longer, more powerful jumps than toads because of their legs
• Frogs have smooth moist skin, while toads have bumpy, dry skin.
• Both frogs and roads need to be near water for reproduction.
• Also, toads live farther away from water than frogs..
• Toads, unlike frogs, have glands near the back of their heads that release a foul-tasting poison to deter predators from eating them

Salamanders vs Newts

• Salamanders and newts both have long, slim bodies with necks and tails.
• Salamanders have four legs; thin, moist skin; and lay eggs in water
• Salamander larvae have gills, and look like small salamanders
• Salamanders must stay near water.
• They live in moist areas, such as under logs or in leaf litter.
• They eat worms, frog eggs, and insects.
• Newts are aquatic throughout their lives

Caecilians

• Unlike all the other amphibians, caecilians have no legs.
• They are wormlike animals.
• They burrow in the soil and feed on worms.
• Skin covers their eyes, and they are pretty nearsighted.
• They use internal fertilization and lay eggs in moist soil near water.
• Caecilians reside in tropical forests.