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Amphibians 1. Amphibians are ectothermic vertebrates. 2. Their skin lacks scales, hair, and feathers, and is either smooth (like a frog) or rough (like a toad). They are dependent upon moisture and subject to desiccation; their skin must remain moist to aid in breathing. 3. They lay eggs in water, which hatch into an intermediate life form (tadpole or larva) that usually breathes with gills, and change into the adult form that breathes air and can live outside water. 4. They have three-chambered hearts. 5. They lack claws on their toes. Frog Facts - Amphibians Review 1. Which is the chordate class considered an evidence of the transition of the vertebrates from the aquatic to the dry land environment? The amphibians are totally aquatic in the larval stage and partially terrestrial animals as adults and for these facts they are considered intermediate beings in the evolutionary passage of vertebrates from the aquatic to the dry land habitat. Amphibians are also the first tetrapod animals, i.e., the first with two pair of limbs, a typical feature of terrestrial vertebrates. The name “amphibian” comes from the double life (aquatic as larvae and partially terrestrial as adults) of these animals. Frog Facts Review - Image Diversity: amphibians 2. What are the amphibian features that make them dependent on water to survive? Permeable skin, body subject to dehydration, external fecundation, eggs without shells and larval stage with branchial respiration are features that make amphibians dependent on water to survive. 3. Do amphibians have direct development? In amphibians the embryonic development is indirect (there is a larval stage). 4. How different are the respiration in fishes and the respiration in adult amphibians? In fishes gas exchange is done by direct contact of water with the branchiae (gills). Gases gain and exit the circulation through the gills. In adult amphibians gas exchange is done through the moist and permeable skin (cutaneous respiration) and also through the lungs, a set of tiny airway terminations associated to a highly vascularized tissue specialized in gas exchange. The axolotl is an exotic amphibian found in Mexico that lives in water and “breathes” through gills even as an adult. Frog Facts Review - Image Diversity: axolotl amphibian skin 5. How is respiration performed by the larva of amphibians? The larva of amphibians has exclusively branchial respiration. This is one of the reasons why it depends on water to survive. 6. How different is the amphibian heart from the fish heart? The fish heart has only two chambers, an atrium and a ventricle, and the blood that comes to it is purely venous. In amphibians there are three heart chambers (a second atrium is present) and there is arterial blood coming from the lungs; in these animals the heart has two atria (one that gets blood from the body and other that gets blood from the lungs) and one ventricle; arterial blood mixes with venous blood within the ventricle which in turn pumps the blood to the lungs and to the systemic circulation. Frog Facts Review - Image Diversity: amphibian heart 7. How is excretion done in amphibians? Adult amphibians have kidneys that filter blood. Nitrogen waste is excreted as urea (so amphibians are ureotelic beings). The larvae, aquatic, excrete ammonia. Frog Facts Review - Image Diversity: urea molecule ammonia molecule 8. Is fecundation in amphibians external or internal? In this aspect are amphibians evolutionarily proximal to fishes or to reptiles? In the majority of the amphibian species fecundation is external. This feature is common to bony fishes too and it shows that the reproductive system and the embryonic development of amphibians are a heritage from osteichthyes. Curiously although having external fecundation amphibian male and female copulate to stimulate the liberation of sperm and egg cells. This phenomenon does not characterize internal fecundation since the gametes unite in water. 9. Why is the occurrence of eyelids in amphibians in comparison to their absence in fishes an adaptation to terrestrial life? Eyelids associated to lacrimal glands protect and keep eyes lubricated against damage from the great luminosity of terrestrial environments. Fishes do not have eyelids since their eyes are in constant contact with the fluid medium. 10. What are the problems that vertebrates needed to solve to adapt to the terrestrial environment since they came from the aquatic habitat? How does evolution solved those problems? The main problems vertebrates coming from water needed to solve to adapt to the terrestrial environment were the following: the problem to avoid dehydration; the problem of elimination of wastes in a medium where water is less available; the problem of protection against nocent solar radiation; the problem of gamete locomotion in the environment for fecundation; the problem of gas exchange, earlier done by direct contact of water with gills; the problem of body support, since it was water that played this role in fishes. Solutions for the dehydration problem: thicker and impermeable skin, to lose less water, or moist and permeable skin, like in amphibians. Solution for the excretion problem: excretion of urea (also excreted by chondrichthyes) or uric acid, substances that need less water to be dissolved. Solutions for the problem of protection against radiation: skin pigments that filter harmful radiation, feathers, hair or carapaces. Solution for the gamete movement problem: internal fecundation (except for most amphibians, that have external fecundation). Solution for the gas exchange problem: appearing of airways and lungs. Solution for the body support problem: further development of muscular and bony structures, like limbs and claws. 11. Amphibian identity card. How are amphibians characterized according to examples of representing beings, basic morphology, skin, respiration, circulation, nitrogen waste, thermal control and types of reproduction? Examples of representing beings: frogs, toads, salamanders. Basic morphology: two pairs of limbs, eyelids, hydrodynamic larvae. Skin: moist and permeable, mucous glands. Respiration: cutaneous and pulmonary, branchial in larval stage. Circulation: closed, incomplete, heart with three chambers without interventricular septum. Nitrogen waste: urea. Thermal control: heterothermic. Types of reproduction: sexual, water dependant, external fecundation and aquatic larval stage. |