Lecture 31: Chordata IV: Amphibians

Early tetrapods

transitions from water to land; invasion of a physically hazardous environment

1. water content of body

   • animals are mostly water in composition

2. density and support

   • air is 1000x les buoyant and 50 times less viscous than water

   • skeleton is weight bearing

3. climatic fluctuations (temperature)

   • greater temperature fluctuactions in air

4. oxygen concentration

   • oxygen is 20 times more abundant in air and diffuses more rapidly through air

Early tetrapod evolution

• arose from a Sarcopterygian ancestor

• bony fishes diversified to include freshwater forms by Devonian period (400 mya)

• freshwater environments are unstable, prone to evaporation and depletion of oxygen

• possessed characteristics that evolved in an aquatic habitat which permitted some degree of terrestriality

  • air-filled cavity connected to pharynx (swim bladder/lungs)

  • internal nares for chemoreception (connecting outside to inside)

  • strong paired fins (support movement on land)

• one such transition provided the ancestral lineage of all tetrapod vertebrates

Transitional form (Tiktaalik)

• intermediate between lobe-finned fish and tetrapod

• fins had basic wrist bones and simple fingers, an indication that they were weight bearing

• broad snout with nostrils and eyes on top of the head

Class Amphibia

• 4 limbs with skeletal frame

• lungs and internal nostrils

• ears, eyes, and nasal cavity modified for sensory reception in air

• three-chambered heart

• metamorphosis

• thin, moist skin (most use cutaneous respiration)

• ectotherms with a low metabolic rate

• cool and wet environments

Caecilians

• limbless and tail less

• mostly fossorial (burrowing)

• small eyes or blind

• breathe through the skin and a single lung (left lung is reduced)

• fertilization occurs internally

  • most bear live young

Salamanders

• trailed amphibians

• generally 4 limbs at right angles to body

• considerable variation in modes of respiration

  • external gills, lungs, skin, both, or neither at various stages of life

• all carnivores

• mostly internal fertilization (hellbenders: external)

  • some aquatic species lay eggs and undergo metamorphosis

  • terrestrial species direct development

Frogs and toads

• no tail in adults, present in larvae

• specialized for jumping

• males set up territories and use sound for courtship

• may be indicator species for environmental changes

Skin of frogs and toads

• moist with soft keratin

• mucous and poison glands

• chromatophores

Skeleton of frogs and toads

• well-developed endoskeleton

• stiff vertebral column

• frogs are short-bodied (9 vertebrae)

• powerful muscles

Respiration in frogs and toads

• skin, buccal breathing, lungs

• positive-pressure breathers: fill lungs by forcing air into them

• vocal cords for sound production (vocal sac)

Circulation of frogs and toads

• closed system

• 3-chambered heart

• pulmonary and systemic circuits (partly separate)

Feeding of frogs and toads

• carnivorous

• protrusible tongue

• teeth prevent prey escape

Sensory organs of frogs and toads

• olfactory epithelium

• modified eyes with nictitating membrane

• ear developed for airborne sounds

Reproduction of frogs and toads

temperate species

• external fertilization by amplexus: both release sperm and egg externally

• eggs laid in water

• hindlegs appear during metamorphosis

• lungs develop, gills resorbed

tropical

• lay eggs (floating foam, leaves, dump burrows)

• parental care

• direct development in some (no larval stage or metamorphosis)

Metamorphosis for frogs

thyroid hormone (thyroxine) triggers metamorphosis