30-3 Amphibians
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30-3 Amphibians
What Is an Amphibian?
What Is an Amphibian?
Amphibian means “double life,” emphasizing that
these animals live both in water and on land.
An amphibian is a vertebrate that, with exceptions:
• lives in water as a larva and on land as an adult
• breathes with lungs as an adult
• has moist skin that contains mucous glands
• lacks scales and claws
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30-3 Amphibians
Evolution of Amphibians
Evolution of Amphibians
The first amphibians appeared in the late Devonian
Period, about 360 million years ago, and probably
resembled lobe-finned fishes (like the coelacanth).
The transition from water to land required that the
terrestrial vertebrates had to:
• breathe air
• protect themselves and eggs from drying out
• support themselves against the pull of gravity
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30-3 Amphibians
Evolution of Amphibians
Early amphibians evolved adaptations that helped
them live at least part of their lives out of water.
Bones in the limbs and limb girdles of amphibians
became stronger, permitting more efficient
movement.
Lungs and breathing tubes enabled amphibians to
breathe air.
The sternum, or breastbone, formed a bony shield to
support and protect internal organs, especially the
lungs.
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30-3 Amphibians
Evolution of Amphibians
Amphibian Adaptations
Lungs
Pelvic Girdle
Leg Bones:
The legs of a land
vertebrate must be
strong enough to hold
its weight.
Skin: The skin and the lining of
the mouth cavity of many adult
amphibians are thin and richly
supplied with blood vessels.
Watery mucus is secreted by
glands in the skin.
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Evolution of Amphibians
In many adult amphibians, the internal surfaces of the
lungs are richly supplied with blood vessels and folds
that increase surface area.
Soon after they first appeared, amphibians
underwent a major adaptive radiation.
Some ancient amphibians were huge! Eogyrinus is
thought to have been about 5 meters long.
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30-3 Amphibians
Evolution of Amphibians
Amphibians become dominant in the warm, swampy
fern forests of the Carboniferous Period (360-290
mya). It is often called the Age of Amphibians.
Climate changes caused many of their low, swampy
habitats to disappear, and most were extinct by the
end of the Permian Period, about 245 mya.
Only 3 orders survive today- frogs & toads,
salamanders, and caecilians.
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30-3 Amphibians
Form and Function in Amphibians
Form and Function in Amphibians
The class Amphibia is relatively small and diverse.
Feeding
Tadpoles are typically filter feeders or herbivores that
graze on algae.
Their intestines’ long coiled structure helps break down
hard-to-digest plant material and are usually filled with
food.
The feeding apparatus and digestive tract of adults are
meat-eating structures, complete with a much shorter
intestine.
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30-3 Amphibians
Form and Function in Amphibians
Adult amphibians are almost entirely carnivorous.
They will eat practically anything they can catch and
swallow.
Legless amphibians can only snap their jaws open
and shut to catch prey.
Many salamanders and frogs have long, sticky
tongues specialized to capture insects.
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30-3 Amphibians
Form and Function in Amphibians
Mouth
In a frog’s digestive
system, food slides down
the esophagus into the
stomach.
Esophagus
Stomach
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30-3 Amphibians
Form and Function in Amphibians
The breakdown of food
begins in the stomach
and continues in the
small intestine.
Small intestine
Stomach
There digestive enzymes
are manufactured and
food is absorbed.
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Form and Function in Amphibians
The liver, pancreas, and
gallbladder are
connected by tubes to Gallbladder
the intestines and
secrete substances that
aid in digestion.
Liver
Pancreas
The small intestine leads
to the large intestine, or
colon.
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30-3 Amphibians
Form and Function in Amphibians
At the end of the colon is a
muscular cavity called the
cloaca, through which
digestive wastes, urine, and
eggs or sperm leave the body.
Large
intestine
(colon)
Cloaca
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Form and Function in Amphibians
Respiration
In most larval amphibians, gas exchange occurs
through the skin and the gills.
Adult amphibians typically respire using lungs, but
some gas exchange occurs through the skin and the
lining of the mouth.
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Form and Function in Amphibians
Circulation
In frogs and other adult amphibians, the circulatory
system forms a double loop.
The first loop carries oxygen-poor blood from the
heart to the lungs and skin, and takes oxygen-rich
blood from the lungs and skin back to the heart.
The second loop transports oxygen-rich blood from
the heart to the rest of the body, and carries oxygenpoor blood from the body back to the heart.
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Form and Function in Amphibians
Amphibian Circulation
Heart
and Excretion
Lung
The amphibian heart
has three separate
chambers:
•left atrium
•right atrium
•ventricle
Kidney
Ureter
Urinary
bladder
Cloaca
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Form and Function in Amphibians
When the atria contract, they empty their blood into
the ventricle.
The ventricle then contracts, pumping blood out to a
single, large blood vessel that divides and branches
off into smaller blood vessels.
Because of the pattern of branching, most oxygenpoor blood goes to the lungs and most oxygen-rich
blood goes to the rest of the body.
However, there is some mixing of oxygen-rich and
poor blood.
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30-3 Amphibians
Form and Function in Amphibians
To body,
lungs and
skin
Amphibian
Heart
From
Body
To body,
lungs and
skin
From
Lungs
Left
atrium
Right
atrium
Ventricle
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30-3 Amphibians
Form and Function in Amphibians
Excretion
Amphibians have kidneys that filter wastes from the
blood.
Urine, the nitrogenous waste product, travels through
tubes called ureters into the cloaca.
Urine is then passed directly to the outside, or
temporarily stored in a small urinary bladder just
above the cloaca.
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Form and Function in Amphibians
Reproduction
In a few species, including most salamanders, eggs
are fertilized internally.
In most species of amphibians, the female lays eggs
in water, then the male fertilizes them externally.
The male climbs onto the female’s back and
squeezes. In response, the female releases up to 200
eggs that the male then fertilizes with sperm.
After fertilization, frog eggs are encased in a sticky,
transparent jelly.
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Form and Function in Amphibians
The jelly attaches the egg mass to underwater plants
and makes the eggs difficult for predators to grasp.
The yolks of the eggs nourish the embryos until they
hatch into larvae that are commonly called tadpoles.
Most species abandon their eggs after they lay them.
A few amphibians take care of both eggs and young.
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Form and Function in Amphibians
Frog Metamorphosis
Adult
Frog
Fertilized
eggs
Young
Frog
Tadpoles
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30-3 Amphibians
Form and Function in Amphibians
Adults are typically ready to breed in about 1-2 years.
Frog eggs are laid in water and undergo external
fertilization.
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Form and Function in Amphibians
The fertilized eggs hatch into tadpoles a few days to
several weeks later.
Fertilized eggs
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30-3 Amphibians
Form and Function in Amphibians
Young frog
Tadpoles gradually
grow limbs, lose their
tails and gills, and
become meat-eaters
as they develop into
terrestrial adults.
Tadpole
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30-3 Amphibians
Form and Function in Amphibians
Movement
Amphibian larvae move by wiggling their bodies and
using a flattened tail for propulsion.
Adult salamanders walk or run by throwing their
bodies into S-shaped curves and using their legs to
push backward against the ground.
Frogs and toads, have well-developed hind limbs that
enable them to jump long distances.
Some amphibians, like tree frogs, have disks on their
toes that serve as suction cups for climbing.
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Form and Function in Amphibians
Response
The brain of an amphibian has the same basic parts
as that of a fish, having a well developed nervous and
sensory system.
An amphibian's eyes are large and can move in their
sockets. They are protected from damage and kept
moist by a transparent nictitating membrane.
This moveable membrane is located inside the
regular eyelid and can be closed over the eye.
Frogs have keen vision that enables them to spot and
respond to moving insects, but probably do not see
color as well as fishes do.
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Form and Function in Amphibians
Amphibians hear through tympanic membranes, or
eardrums, located on each side of the head.
Many amphibian larvae and adults have lateral line
systems that detect water movement.
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30-3 Amphibians
Groups of Amphibians
Groups of Amphibians
The three groups of amphibians alive today are:
• salamanders
• frogs and toads
• caecilians
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Groups of Amphibians
Salamanders & Newts
Order Urodela
Salamanders and newts have long bodies and tails.
Most have four legs.
Both adults and larvae are carnivores.
Adults usually live in moist woods, where they tunnel
under rocks and rotting logs.
Mud puppies keep their gills and live in water all their
lives.
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Groups of Amphibians
Frogs and Toads
Order Anura
Frogs and toads have the ability to jump.
Frogs tend to have long legs and make lengthy jumps.
Toads have relatively short legs and are limited to
short hops.
Frogs are generally more closely tied to water than
toads.
Toads often live in moist woods and even in deserts.
Adult frogs and toads lack tails.
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Groups of Amphibians
Caecilians
Order Apoda
Caecilians are legless animals that live in water or
burrow in moist soil or sediment.
Caecilians feed on small invertebrates such as
termites.
Many have fishlike scales embedded in their skin.
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Ecology of Amphibians
Ecology of Amphibians
Amphibians have adaptations that protect them from
predators such as:
•skin colorings/markings to blend into surroundings
•skin glands that ooze unpleasant-tasting and
poisonous toxins
For the past several decades, the number of living
species has been decreasing, and scientists do not yet
know the cause.
Possible causes: decreasing habitat, depletion of the
ozone layer, acid rain, water pollution, fungal infection,
introduced aquatic predators, and increasing human
populations.
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