Amphibians

• Amphibians


  • They have a "double life" - they live both in water and on land
  • They include frogs, toads, salamanders, and caecilians
  • They were the first vertebrate group to make the transition onto land
• Adaptations for land life in amphibians

  • Increase in the number of blood vessels supplying the respiratory organs
  • Development of a pumping mechanism to get air into and out of lungs
  • The ear became more important as a sense organ
  • Need lacrymal glands to keep eyes moist
  • Need a new method of prey capture as they can't use suction effectively
  • Moveable tongue now used to manipulate food in mouth
• Early amphibian


  • Ichthyostega
• One hypothesis of evolutionary relationships among the Tetrapods

  
• Body forms of amphibians

  • Salamanders: head-trunk-tail
  • Frogs: Fused head-trunk; no tail
  • Caecilians: long slender snake-like body; no limbs; no post-anal tail

  
• Skin of amphibians


  • Most have thin, moist, glandular skin without scales
  • Thinness of skin and vascularization allow it to be used for respiration if kept moist
  • Often have many glands including mucous glands and poison glands
  • Skin is often brightly colored due to chromatophores in dermis
  • Darkening of skin color controlled by the light-sensitive pineal eye

  
• Support and movement in amphibians

  • Stronger skeleton, mostly of bone, supports the body weight on land
  • Still move in a very 'fish-like' fashion
  • Strengthened rib cage and axial skeleton to support internal organs
  • Limbs with toes for easier land locomotion
  • Most modern amphibians have 4 toes on their forelimbs and 5 toes on their hindlimbs
  • Muscles are modified into "opposing pairs" to flex/extend or abduct/adduct limbs
  • Aquatic forms have fish-like undulating swimming motion
  • Gliding frogs have large webbed feet
  • Presence of a "neck" is a new structural innovation in land animals
• Feeding and digestion in amphibians

  • Most are predators (carnivores) as adults, but herbivores as larvae
  • Have long flexible tongues for capturing prey
  • Free end of tongue produces sticky secretions to adhere to prey
  • Food is swallowed whole, not chewed
• Respiration (gas exchange) in amphibians

  1. Cutaneous respiration (through the skin)

  2. Buccopharyngeal respiration (mouth)

  3. Pulmonary (lungs)

  4. Larvae and some adults use external gills
• Pulmonary (lung) respiration in amphibians

  • Very simple lungs; essentially hollow air sacs
  • Nostrils are used for breathing as well as chemosensory
  • No diaphragm (breathing muscle), so amphibians must gulp air to force it into the lungs
• Cutaneous (skin) respiration in amphibians

  • The thinness of skin and blood vessels present allows it to be used as a respiratory surface
  • Even when lungs are used for oxygen, most carbon dioxide is lost through the skin
• Circulatory system of amphibians

  • Have a 3 chambered heart with 2 atria and 1 ventricle
  • Have two complete circuits of blood flow - pulmonary circuit and systemic circuit
• Ventilation in amphibians
• Diagrammatic representation of an anuran circulatory system
• Nervous system and senses of amphibians


  • Vertebrate brain is made up of 4 distinct functional areas: cerebrum, cerebellum, diencephalon, and brain stem
• Evolutionary pressures on the nervous and sensory functions of amphibians
• Sensory receptors in amphibians

  • Vision: sight feeders, binocular vision, eye structure and function, nictitating membrane
  • Hearing: air-borne vibrations detected by tympanic membrane and stapes, substrate-borne vibrations transmitted through front appendages and pectoral girdle
• Amphibian eye structure and function
• Excretion and osmoregulation in amphibians

  • Kidneys and dorsal body wall used for excretion
  • Excrete ammonia and urea
  • In water, they excrete excess water and conserve ions
  • On land, they conserve water with behaviors that prevent loss, like being nocturnal, body posture, cocoons, and cutaneous absorption
• Water conservation by anurans
• Reproduction, development, and metamorphosis in amphibians

  • Dioecious with usually external fertilization
  • Development usually in water
  • Courtship involves olfactory and visual cues in salamanders, tactile and auditory cues in anurans
  • Metamorphosis controlled by neurosecretions, involves major changes in anurans but relatively minor changes in salamanders and caecilians
• Amplexus in anurans
• Anuran vocalization
• Parental care of young amphibians
• Events of metamorphosis in the California Red-Legged Frog

  
• Threats and conservation of amphibians

  • One-third of amphibian species are threatened with extinction
  • Threats include clear-cutting forests, development, chytrid fungus, and climate change
  • Conservation measures include population monitoring, collecting restrictions, wetland conservation, and climate-change reversal
• Evolutionary changes that allowed sarcopterygians to move onto land
• Relationship between ancestral amphibians and early amniotes