Transcript Amphibians PowerPoint

Chapter 25
Amphibia
Devonian period
• Earth's climate was much warmer than it is today
• Was always the possibility that the body of water where a fish lived
could dry up/become oxygen depleted.
• Thus, an advantage to being able to move to a new location. The lobefinned fishes were able to support themselves on land using their fins;
allowing 1st vertebrate forays on land.
• Ancestral amphibians looked different from the 3,000+ living species.
– big boned
– looked more like large lungfish rather than the frogs/salamanders.
• Amphibians: evolved from freshwater fish
– because of that, are only found in terrestrial & freshwater
environments.
Development of limbs required only a few minor modifications
- Elongation of proximal elements into long-bones of limbs
- Proliferation of more distal elements into bones of feet
- Elaboration of pectoral and pelvic girdles (for anchoring the limbs)
Early locomotion was probably very crude
- Orientation of limb did not effectively allow body to be raised above substrate
- Limbs acted as pivot points
rather than for propulsion.
Modern salamanders still
move about in this way.
Figure 25.01a
Figure 25.01b
• The salamanders, frogs, and toads we see today are all that remains.
• Amphibians are an example of the vertebrate body plan in its
transition between the aquatic and terrestrial environments.
• First time we see that will become important features for the
completely terrestrial vertebrates that come later:
– modification of the appendicular skeleton into the tetrapod
body plan
– modifications to the axial skeleton
– development of lungs
– dual circulatory system
• In vertebrates such as amphibians, reptiles, birds, and mammals,
blood flows through two different paths or circuits.
One to the lungs (pulmonary) the other to the rest of the body (systemic).
• A fish-shaped body doesn't move well on land.
• Both axial and appendicular skeletons were modified to lift the body up and off
the ground permitting movement based on the tetrapod stance.
• Skeletal girdles connected the left & right
components of the forelimbs/hindlimbs to each other
Body
• Girdles were connected to the axial skeleton.
• Vertebrae were strengthened with overlapping bony extensions to support the
body suspended below.
– Advantage of this is best demonstrated in the frog hindlimb adapted for
jumping.
• The explosive force generated by the leg when the frog jumps is
transmitted from the legs through to the axial skeleton from which
hangs the rest of the body.
• Skin (and lungs in some amphibians) is an important respiratory surface and gas
exchange but its use is problematic.
– For gas exchange to occur, the surface must be kept moist and numerous glands
continually secrete fluid onto the surface of the skin.
– Because of the skins respiratory role, ancestral amphibians couldn't use scales or
other modifications of the integument (covering of an animal) for protection.
• Don't think that they were left completely defenseless; the same
glands that secrete moistening compounds have been modified as
poison glands that secrete a variety of toxic and distasteful
chemicals. Many amphibians are brightly colored, a warning for
would-be predators of their potential toxicity.
• Chromatophores produce the unique colors.
Pigment does not contract or expand – produced by streaming
cytoplasm.
Pigment Dispersed
Pigment Concentrated
• The respiratory role of the skin means that this leads to potential
osmoregulatory problems:
– Water being lost across the surface when on land
– Vital salts when submersed in freshwater.
• The kidneys produce a lot of dilute urine when in water
• Urine stored in a urinary bladder is an important source of water
when on land.
-Gas exchange occurs through lungs, gills,
and/or the skin
-Some salamanders have no lungs
(secondarily lost)
-Adults have double circulation
-Both a pulmonary and a systemic circuit
-Heart has three chambers
-Two atria and one ventricle
-Some mixing of oxygenated and deoxygenated blood occurs in
ventricle
Figure 25.21
• Despite all these changes, amphibians were unable to solve 1 problem.
– reproduction in the terrestrial environment (where the sperm, egg, or
developing embryo must be protected from desiccation).
• Amphibians must return to the aquatic environment to lay their eggs
and fertilize them.
• It is there that the developing embryo grows until it is ready to move
back on land.
• The reproductive system is closely linked with the ducts and tubes of
the kidney to form a urogenital system.
• Males: paired testes pass sperm through the kidney and out the
ureters.
• Females: produce gelatinous eggs that pass down the oviduct before
being released through the cloaca.
• In both males and females: the fat body is assumed to assist in
meeting the metabolic demands of the reproductive season.
• In salamanders, the males produce a spermatophore passed to the
female, who stores the sperm in a seminal receptacle until the eggs
are fertilized internally.
Figure 25.06
• Frogs are oviparous, and fertilization for the most part is external.
– Oviparous: When the eggs are laid by the female and the embryo
develops and hatches outside of the body of the female.
• A larval tadpole hatches from one of the fertilized eggs and begins to
swim and feed. As the tadpole grows, hindlimbs appear followed by
the forelimbs, and then the metamorphosis into an adult begins.
Salamanders don't have as large a change between
larva and adult, and both stages have legs.
Figure 25.26
Figure 25.08
Paedomorphosis
Figure 25.10
Figure 25.10a
Permanently gilled aquatic form
Figure 25.10b
May remain permanently gilled or if its pond habitat
evaporates, may metamorphose to terrestrial form that loses
gills and breathes by lungs.
(not usually this color – albinos used for lab experiments)
• Around the world amphibians are disappearing at alarming rates.
• Many amphibians are known as an indicator species
• Possible Causes:
– Global warming
– Changes in the ozone
– Viral infections
– Susceptibility to increasing levels of
environmental toxins.
• But the loss of the first vertebrate on land
certainly indicates that something drastic has changed
• The big question is how this change will affect the rest of the
vertebrates, including us.
Figure 25.16
Figure 25.12
Figure 25.12a
Figure 25.12b
Figure 25.13
Figure 25.14
Figure 25.15
Figure 25.18
Figure 25.25