Transcript Document 7199114

Chapter 42: Amphibians
42-1 Origin and Evolution of Amphibians
42-2 Characteristics of Amphibians
42-3 Reproduction of Amphibians
42-1 Origin and Evolution 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).
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(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).
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(1) Tadpole
• Swimming, TAILED larvae (adults ~ tailless) results from EXTERNALLY
fertilized eggs LAID in water.
Critical Thinking
(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?
(B) Order Urodela (e.g., salamanders and newts)
• ELONGATED body and tail with four equally sized limbs; MOST urodelans
are capable of INTERNAL fertilization.
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(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.
42-2 Characteristics of Amphibians
I. External Covering
• Skin (moist ~ PERMEABLE to gases and water) provides :
(1) Protection (2) Respiration.
(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.
Critical Thinking
(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?
(3) Columella
• A SMALL BONE extends between tympanic membrane and inner ear.
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42-3 Reproduction in Amphibians
I. Life Cycle
• Evolved from LOBE-finned fish BUT were forced to REMAIN dependent
on AQUATIC habitats to complete life cycle.
(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.
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Critical Thinking
(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.
(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.
Critical Thinking
(4) How does the parental care of a female gastric-brooding help her
offspring survive? How might her behavior reduce her likelihood of
survival?
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.
Critical Thinking
(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?
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.