Chapter 25Chapter 25

AmphibiaAmphibia

Devonian periodDevonian 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 lobe-finned 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 modificationsDevelopment 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 crudeEarly 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 modification of the appendicular skeleton into the tetrapod

body planbody plan– modifications to the axial skeletonmodifications to the axial skeleton– development of lungsdevelopment of lungs– dual circulatory systemdual circulatory system

• In vertebrates such as amphibians, reptiles, birds, and mammals, blood flows through two different paths or circuits. One to the lungs (pulmonary)lungs (pulmonary) the other to the rest of the body (systemic)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 girdlesSkeletal girdles connected the left & right components of the forelimbs/hindlimbs to each other

• 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.

Body

• 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:Males: paired testes pass sperm through the kidney and out the ureters.

• Females:Females: produce gelatinous eggs that pass down the oviduct before being released through the cloaca.

• In both malesmales and femalesfemales: the fat body is assumed to assist in meeting the metabolic demands of the reproductive season.

• In salamanders, the males produce a spermatophorespermatophore passed to the female, who stores the sperm in a seminal receptacleseminal receptacle until the eggs are fertilized internally.

Figure 25.06

• Frogs are oviparous, and fertilization for the most part is external. – Oviparous: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 metamorphosismetamorphosis into an adult begins. Salamanders don't have as large a change betweenlarva and adult, and both stages have legs.

Figure 25.26

Figure 25.08

Paedomorphosis

Figure 25.10

Figure 25.10a

Permanently gilled aquatic formPermanently gilled aquatic form

Figure 25.10b

May remain permanently gilled or if its pond habitat May remain permanently gilled or if its pond habitat evaporates, may metamorphose to terrestrial form that loses evaporates, may metamorphose to terrestrial form that loses gills and breathes by lungs. gills and breathes by lungs.

(not usually this color – albinos used for lab experiments)(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