Chapter 42: Amphibians

42-1 Origin and Evolution of Amphibians42-2 Characteristics of Amphibians

42-3 Reproduction of Amphibians

I. Adaptation to Land (~ from LOBE-finned fishes, 370 m.y.a.)• Ancestors LEFT water to escape PREDATION and COMPETITION, AND to access to new TERRESTRIAL resources (i.e., FOOD).

42-1 Origin and Evolution of Amphibians

QuickTime™ and a decompressor

are needed to see this picture.

(A) Characteristics of Early Amphibians (~ Crossopterygian ANCESTOR)• Forelimbs ~ homologous to PECTORAL fins, while hindlimbs ~ homologous to PELVIC limbs.

NOTE: Ichthyostega (BEST evidence of an EARLY amphibian) had a LONG tail FIN, lateral lines on HEAD, sharp TEETH for FISH, and developed senses for AIRBORNE scents and sounds.

(B) Diversification of Amphibians (~ 360-286 m.y.a.)• DIVERGENT evolution split amphibians into 3 evolutionary lines, (Orders Anura, Urodela, and Apoda).

II. Modern Amphibians

• In all THREE orders, traits SHARED include…

(1) Metamorphosis between aquatic larval stage (GILLS) to terrestrial adult stage (LUNGS).

(2) Moist, thin skin WITHOUT scales.

(3) Feet WITHOUT claws, and are often WEBBED.

(4) Gills, lungs, and skin are all involved in GAS EXCHANGE (respiration).(5) Shell-less eggs laid in water (or moist habitats) and are usually fertilized EXTERNALLY.

(A) Order Anura “tail-less” (e.g., frogs (smooth skin), toads (bumpy skin))• Body adapted for JUMPING; short, COMPACT muscular body with RIGID spine and STRONG forelimbs (carnivore adults).

QuickTime™ and a decompressor

are needed to see this picture.

(1) Tadpole• Swimming, TAILED larvae (adults ~ tailless) results from EXTERNALLY fertilized eggs LAID in water.

(1) There are usually many ecological differences between a frog and its tadpoles. Describe two of these differences and explain why such differences may have been favored by natural selection?

Critical Thinking

(B) Order Urodela (e.g., salamanders and newts)• ELONGATED body and tail with four equally sized limbs; MOST urodelans are capable of INTERNAL fertilization.

QuickTime™ and aH.264 decompressor

are needed to see this picture.

(C) Order Apoda (e.g., mysterious caecilians, BURROWING amphibians)• Legless amphibians (like snakes?), small eyes (nearly BLIND), and rely on a CHEMOSENSORY TENTACLE located on head to FIND prey.

I. External Covering• Skin (moist ~ PERMEABLE to gases and water) provides :

(1) Protection (2) Respiration.

42-2 Characteristics of Amphibians

(1) Mucous Glands (an adaptation to a TERRESTRIAL niche)• LUBRICANT keeps skin MOIST in air (Slime MAY ALSO be a poisonous OR slippery deterrent).

II. Internal Anatomy (VERTEBRAE of spine INTERLOCK rigid support)• WEIGHT of body is TRANSFERRED to limbs by PECTORAL and PELVIC GIRDLES, UROSTYLE adaptations for LAND.

(A) Heart and Circulatory System (3-chambered DOUBLE circulatory loop)• Advantage over SINGLE-loop of FISHES—faster BLOOD FLOW to body q(2 pumped POPULATIONS of blood instead of ONE)

(1) Pulmonary Circulation (1st loop)• Carries deoxygenated blood (db) from HEART to LUNGS, then returns oxygenated blood (ob) BACK to HEART.

(2) Systemic Circulation (2nd loop)• Carries oxygenated blood (ob) from HEART to BODY, then returns deoxygenated blood (db) BACK to HEART.

(B) Respiration• Larval amphibians (GILLS and skin), Adult amphibians (skin and LUNGS).

(1) Pulmonary Respiration (NOSTRILS control AIR FLOW DIRECTION)

• 4 stage cycle involved with FROG’S “positive-pressure breathing”

(ONE) Air is drawn into MOUTH by LOWERING floor of mouth cavity.

(TWO) Drawn-in air is then pumped into lungs by RAISING floor of mouth cavity and CLOSING nostrils.

(THREE) Muscles and elasticity of lungs FORCE air back out to MOUTH, and floor of mouth DROPS.

(FOUR) Nostril OPENS, and floor of mouth RISES to force air OUT.

(2) Cutaneous Respiration (via skin)• MOIST, thin layer of skin allows GAS EXCHANGE; carried out with gills (larva) or lungs (adult).

(C) Digestive System• Adults (carnivorous), larvae (often herbivorous), prey are swallowed WHOLE and while alive (teeth primarily used for grasping, NOT chewing).NOTE: Mouth, pharynx, esophagus, stomach, liver, gall bladder, and intestine are still present from FISH.

(1) Duodenum• UPPER portion of small intestine leading from STOMACH.

(2) Ileum• Coiled MIDDLE portion of small intestine, leads into LARGE intestine.

(3) Mesentery• Thin, translucent MEMBRANE holds small intestine in place.

(4) Vent (opening following the cloaca)• Indigestible material, WASTES, and even GAMETES are passed out through this OPENING.

(D) Excretory System (kidneys filter NITROGENOUS wastes from blood)• During metamorphosis, TADPOLES change from excreting AMMONIA (very toxic) to excreting UREA as ADULT FROGS.

(E) Nervous System• Larger olfactory lobes (than fish), optic lobes, cerebellum, cerebrum, medulla oblongata, cranial nerves, spinal cord, and peripheral nerves.

(F) Sense Organs (water AND land senses)• Lateral line system ~ retained in larvae BUT is lost into adulthood; smell, sight, and hearing are also WELL DEVELOPED.

(1) Nictitating Membrane• Transparent and moveable covers, protects and moistens amphibian EYES.

(2) Tympanic Membrane• Thin membrane sensitive to VIBRATIONS in air, used to detect sound with COLUMELLA.

(2) In the brains of amphibians, the largest parts are the olfactory lobes and the optic lobes, the centers of smell and sight. This is very important to amphibians in hunting prey as well as avoiding predation. In what other biological process may the capacity for HEARING be important? Explain why hearing must then be especially distinctive?

Critical Thinking

(3) Columella• A SMALL BONE extends between tympanic membrane and inner ear.

QuickTime™ and a decompressor

are needed to see this picture.

I. Life Cycle• Evolved from LOBE-finned fish BUT were forced to REMAIN dependent on AQUATIC habitats to complete life cycle.

42-3 Reproduction in Amphibians

(A) Courtship and Fertilization• MALE frogs emerge from hibernation during EARLY SPRING and migrate to ponds and streams to begin MATING SEASON.

(1) Amplexus (i.e., an embrace of frog courtship)• After successful courtship, male frog climbs ONTO female’s back grasping her in an encouragement to lay her EGGS for him to fertilize.

QuickTime™ and a decompressor

are needed to see this picture.

(3) Charles Darwin observed that frogs and toads are often absent from oceanic islands, such as the Galapagos Islands, even though they may be found on the nearby mainland. Darwin conducted some experiments showing that frogs’ eggs cannot tolerate exposure to salt water. What hypothesis do you suppose Darwin was trying to test? Explain.

Critical Thinking

(B) Metamorphosis (“THYROXINE-stimulated”, developmental hormone)• Tadpole lives on YOLK until mouth broadens as TEETH develop, LEGS emerge, TAILS and GILLS are replaced by a COMPACT body and LUNGS.

(4) How does the parental care of a female gastric-brooding help her offspring survive? How might her behavior reduce her likelihood of survival?

Critical Thinking

II. Parental Care (common among most amphibians)• EQUAL care by males OR females to PROTECT zygotes before hatching.

NOTE: Female GASTRIC-brooding frogs of Australia swallow their eggs which hatch and mature in the STOMACH. They are NOT digested because the stomach STOPS producing acid and digestive enzymes until the young pass through metamorphosis and are released.

(5) The female gastric-brooding frogs of Australia do NOT produce stomach acid or digestive enzymes while brooding their young in their stomachs until the tadpoles have completed metamorphosis and leave. If the mother frog does NOT eat during this period, from where does she get her energy? What other types of frogs may have to live off the same energy source during similar periods of fasting?

Critical Thinking

Extra Slides AND Answers for Critical Thinking Questions

(1) The young are protected from predators and from some environmental changes during the egg, larval, and metamorphic stages. The female may reduce her likelihood of survival because she cannot eat while her young live inside her.

(2) Adult amphibians often live in different habitats than their larvae. Most larvae are aquatic, while most adults are at least partially terrestrial. The larvae often eat different foods than the adults. Many larvae are herbivorous, while adults are carnivorous. The separation between larvae and adults may reduce competition.

(3) He was trying to test the hypothesis that the islands separated from the mainland before frogs evolved. Salt water would keep recently evolved frogs off the island.

(4) Females must be able to hear mating calls of males and know which calls are of her species.

(5) She must obtain energy from stored fat. Desert-dwelling frogs that burrow into the earth to preserve their moisture have to fast until it rains.