Amphibians The First Terrestrial Vertebrates

1. AmphibiansThe First Terrestrial Vertebrates Alexander True, Ryan Adams, Michael Sinfield-Albets Toler

2. General Characteristics • Amphibians are present on all continents except for Antarctica. • 3,000 modern day species. • This is a remnant of a once diverse group. • Belong to three orders: • Caudata (salamanders), Anura (frogs and toads), Gymnophiona (caecilians)

3. Evolutionary Perspective • Amphibians are animals that either move back and forth between water and land or live one of their stages of life in water and another on land. • Amphibians are tetrapods“Four-foot” • The presence of four muscular limbs and feet with toes and fingers

4. Phylogenetic Relationships • Fossil record provides evidence of many extinct taxa and no one knows what animal was the first stegocephalian. • Taxonomists agree that amphibians are monophyletic and closely related to reptiles, birds, and mammals.

5. Order Caudata • This order consists of salamanders. • Means “tail-bearing” • Characteristics: • Posses a tail • Have two pairs of unspecialized legs • External gills • Habitat: • Live in moist forest-floor litter • Spend most of life in water • Reproduction: • Internal fertilization • Produce aquatic larvae • Larvae are similar to adults but smaller

7. Order Gymnophiona • This order consists of caecilians. • Means “naked like a snake” • Characteristics: • Worm-like • Appear segmented because of skin folds that overlie separations between muscle bundles. • Skin over eyes (most likely blind) • Habitat: • Burrow into the ground • Reproduction: • fertilization is internal • Can have aquatic larvae or embryotic larvae that develop on land • Young are miniature adults • Feeding: • Eat soil and worms when they burrow

9. Anura • This order consists of frogs and toads. • Means “without tail” • About 4,000 species • Characteristics: • Long muscular hind limbs • Webbed feet • Habitat: • Generally moist environments (can really be found almost anywhere) • Reproduction: • External fertilization • Undergoes metamorphosis from tadpole to adult

10. Anura Feeding • They mostly eat small insects and some plants. • They utilize a sticky tongue, located on their lower jaw, that is extended from their mouth as a sort of sticky lasso to catch their prey. • Most frogs sit and wait for their prey. Their skin colors and textures perfectly camouflages them in their surroundings.

12. External Structure and Locomotion • The skin of amphibians functions in defense, gas exchange, temperature regulation, and absorption and storage of water. • Skin is highly glandular, these glands keep the skin moist to prevent drying. • The glands also produce a sticky secretion that helps them cling to mates during sexual activities. • The glands produce toxic chemicals that discourage predators. • The bone structure is set up in a way that makes terrestrial locomotion possible. • They have joints at the shoulder, hip, elbow, knee, wrist, and ankle allow freedom of movement and better contact with the substrate. • Each type of Amphibian has a distinct form of movement Salamanders move like a fish swims through water, a Caecilian has accordion movement like a snake, and Frogs have strong hind limbs specialized for jumping.

13. Locomotion Continued • The terrestrial skeleton is modified to provide support against gravity and it must be strong enough to support the relatively powerful muscle that propel terrestrial vertebrates across land. • The amphibian skull is flattened, smaller, and has fewer bony elements than the skull of fishes. • This lightens the skull so it can be supported out of water. • Changes in jaw structure and musculature allow terrestrial vertebrates to crush pray held in the mouth.

14. Locomotion Continued Continued • Salamanders move in a way that is reminiscent of the undulatorywaves that pass along the body of a fish. • Terrestrial salamanders also move by a pattern of muscle contractions that throw the body into a curve to advance the stride of a limb. • Caecilians have an accordion-like movement where adjacent body parts push or pull forward at the same time. • The long hind-limbs and pelvic girdle of anurans are modified for jumping. • The dorsal bone of the pelvis (ilium) extends anteriorly and securely attaches to the vertebral column, and the urostyle extends posteriorly and attaches to the pelvis.

15. Nutrition and Digestion System • Adult amphibians are carnivores that feed on a wide variety of invertebrates. • However, some anurans are more diverse. For example, some bullfrogs on small mammals, birds, and other anurans. • Main factor that determines what amphibians eat is prey size and availability. • Most larvae are herbivorous and feed on algae and other plant matter. • Most amphibians locate prey via sight and simply wait for prey to pass by. • Olfaction plays an important role in prey detection by aquatic salamanders and caecilians.

16. Nutrition and Digestive System Continued • Most salamanders use just their jaws to capture prey but anurans and plethodontid salamanders use their tongues and jaws. • A true tongue is first seen in amphibians. • The anuran tongue attaches at the anterior margin of the jaw and folds back over the floor of the mouth. • Mucous and buccal glands on the tip of the tongue exude sticky secretions. • When prey comes within range, an anuran flicks out its tongue and uses it as a sort of sticky lasso to capture its prey.

17. Circulation • The amphibian circulatory system shows remarkable adaptations for a life divided between aquatic and terrestrial habitats. • The pulmonary and systemic circuits are separated. • The atrium is partially divided in urodeles and completely divided in anuras. • There is no ventricle in the septum. • A spiral valve in the conusarteriosusor ventral aorta helps direct blood flow into the pulmonary and systemic circuits.

18. Gas Exchange • Because the skin of amphibians is moist and richly supplied with capillary beds, the skin can act as a respiratory organ. • Gas exchange across the skin is called cutaneous respiration and can occur in water and on land. • In salamanders, 30 to 90% of gas exchange occurs across the skin. • Gas exchange also occurs across the moist surfaces of the mouth or pharynx which is called buccopharyngeal respiration. • This accounts for 1 to 70% of total gas exchange. • Most amphibians, except plethedontid salamanders, posses lungs. • The lunges of salamanders are simple sacs and the lungs of anurans are subdivided which increases surface area for gas exchange. • The buccal pump mechanism allows pulmonary ventilation by creating positive pressure to force air into the lungs with the use of the muscles in the mouth and pharynx.

19. Temperature Regulation • Amphibians are cotothermic • They depend on external heat sources to maintain body temperature. • They are ectothermic because they are poorly insulated aquatic animals; so, regardless of how much metabolic heat it produces, it will lose heat as quickly as it is produced because of powerful heat-absorbing properties of the water.

20. Amphibian Nervous and Sensory Functions • The amphibian brain consists of three embryological subdivisions: • The forebrain contains olfactory center and regions that regulate color change and visceral functions. • The midbrain contains a region called the optic factum that assimilates sensory information and initiates motor responses, and also processes visual sensory information. • The hindbrain functions in motor coordination and in regulating heart rate and the mechanics of respiration. • Sensory receptors are widely distributed across the skin that respond to heat, cold and pain.

21. Amphibian Nervous and Sensory Functions Continued • Amphibians have singly or small grouping of lateral line systems that respond to low frequency vibrations in water. • They have chemoreceptors in the nasal epithelium and the lining of the mouth, on the tongue, and over the skin. • Olfaction is used in mate recognition as well as in detecting noxious chemicals and in locating food. • Vision is one of the most important senses in amphibians because they are primarily sight feeders. • Eyes are located at the front of the head, providing binocular vision and depth perception. • The lower eyelid is moveable and it cleanse and protects the eye.

22. Amphibian Excretion and Osmoregulation • The kidneys of amphibians lie on either side of the dorsal aorta on the dorsal wall of the body cavity. A duct leads to the cloaca, and a storage structure, the urinary bladder, is a ventral outgrowth of the cloaca. • Nitrogenous waste product comes in the forms of either ammonia or urea. • Freshwater amphibians excrete ammonia. • Land-based amphibians excrete urea that is produced from ammonia in the liver. • One of the biggest problems amphibians face is osmoregulation. • Facing the same osmoregulatory problems as freshwater fishes they must rid the body of excess water and conserve essential ions. • Amphibians must conserve water. • Many amphibians reduce evaporative water loss by reducing the amount of body surface exposed to air.

23. Amphibian Reproduction, Development and Metamorphosis • Amphibians are dioecious with ovaries and testes located near the dorsal body wall. • Fertilization is usually external (caecilians are an exception), and because the developing eggs lack any resistant covering, development is tied to moist habitats, usually water. • Some anurans have terrestrial nests that are kept moist by being enveloped in foam or by being located near the water and subjected to flooding. • Only about 10% of all salamanders have external fertilization while all others produce spermatopohores, and fertilization is internal. • 75% have internal development. • Metamorphosis is a series of abrupt structural, physiological, and behavioral changes that transform a larva into an adult. • A variety of environmental conditions, including crowding and food availability, influence the time required for metamorphosis.

24. Vocabulary • Cutaneous Respiration – Gas exchange across thin, moist surfaces of the skin. Also called cutaneous exchange of integumentary exchange. • Amplexus – The positioning of a male amphibian dorsal to a female amphibian, his forelimbs around her waist. During amplexus, the male releases sperm as the female releases eggs. • Buccal Pump – The mechanism by which lung ventilation occurs in amphibians. Muscles of the moth and pharynx create positive pressure to force air into the lungs. • Nicitating Membrane – The thin, transparent lower eyelid of amphibians and reptiles. • Tetrapods – A taxonomic designation that refers to extant amphibians, reptiles, birds and mammals and their closest common ancestor.