Fishes and Amphibians
Modern Fishes
• The term fish refers to any member of one of
three general categories of vertebrates:
Agnatha (jawless fishes), Chondrichthyes
(cartilaginous fishes), and Osteichthyes
(bony fishes).
• The great diversity of fishes found today reflect
various adaptations that enable fishes to live in
the oceans and fresh waters around the world.
• Fishes vary in size from whale sharks longer
than a moving van to gobies no larger than
your fingernail.
Characteristics of Modern Fishes
• Despite the variation seen among fishes, all share
certain key characteristics.
1. Gills. Fishes normally obtain oxygen from the
oxygen gas dissolved in the water around them.
They do this by pumping a great deal of water
through their mouths and over their gills.
2. Single-loop Blood Circulation. Blood is pumped
from the heart to the capillaries in the gills. From
the gills, blood passes to the rest of the body and
then returns to the heart. (Lungfishes, which have
a double-loop circulation, are an exception.)
Characteristics of Modern Fishes
3. Vertebral Column (backbone). All fishes have an
internal skeleton made of either cartilage or bone,
with a vertebral column surrounding the spinal
cord. The brain is fully encased within a protective
covering called the skull or
cranium.
Gills
• If you look closely at the face of a swimming
fish, you will notice that as it swims, the fish
continuously opens and closes its mouth,
as if it were trying
to eat the water.
• What looks like eating
is actually breathing.
• The major respiratory
organ of a fish is the gill.
Gills
• Gills are made up of rows of gill filaments –
fingerlike projections through which gases
enter and leave the blood.
• The gill filaments hang like curtains between
a fish’s mouth and cheeks.
• At the rear of the cheek cavity is an opening
called a gill slit.
• When a fish ‘swallows,’
water is forced over
the gills and out
through the gill slits.
Gills
• In countercurrent flow, water passes over the
gills in one direction as blood flows in the
opposite direction through capillaries in the gills.
• Countercurrent flow ensures that oxygen diffuses
into the blood over the entire length of the
capillaries in the gills.
• Due to this arrangement, the gills of bony fishes
are extremely efficient respiratory organs.
• Fish gills can extract up to 85% of the oxygen in
the water passing over them.
Countercurrent Gill Flow
Circulation of Blood
• Chordates that were ancestral to the
vertebrates had a simple tubular “heart.”
• This structure was a little more than a
specialized zone of one artery that had more
muscle tissue than the other arteries had.
• When a tubular heart contracts, blood is
pushed in both directions and circulation is
not very efficient.
Circulation of Blood
• For an organism with gills, such as a fish, a
simple tubular heart is not an adequate pump.
• The tiny capillaries in a fish’s gills create
resistance to the flow of blood, so a stronger
pump is needed to overcome this resistance.
• In fishes, the tubular heart of early chordates
has been replaced with a simple chamber-pump
heart.
• The chamber-pump heart can be thought of as a
tube with four chambers in a row.
Circulation
of Blood
Circulation of Blood
1. Sinus Venosus. This collection chamber acts to
reduce the resistance of blood flow into the
heart.
2. Atrium. Blood from the sinus venosus fills this
large chamber, which has thin, muscular walls.
3. Ventricle. The third chamber is a thick-walled
pump with enough muscle tissue to contract
strongly, forcing blood to flow through the gills
and eventually to the rest of the body.
4. Conus Arteriosus. This chamber is a second
pump that smoothes the pulsations and adds still
more force.
Kidneys
• Vertebrates have a body that is about 2/3 water,
and most will die if the amount of water in their
body falls much lower than this.
• Minimizing dehydration (water loss) has been a
key evolutionary challenge facing all vertebrates.
• Even some fishes must cope with the problem of
water loss.
– The ion (salt) concentration of sea water is 3 times
that of the tissues of a marine bony fish.
– As a result, these fishes lose water to the
environment through osmosis.
– To make up for the water they lose, marine bony
fishes drink sea water and actively pump excess
ions out of their body.
Kidneys
• Freshwater fishes have the opposite problem.
– Because their bodies contain more ions than the
surrounding water, they tend to take in water
through osmosis.
– The additional water dilutes their body salts, so
freshwater fishes regain salts by actively taking
them in from their environment.
• Although the gills play a
major role in maintaining
a fish’s salt and water
balance, another key
element is a pair of kidneys.
Kidneys
• Kidneys are organs made up of thousands of
nephrons.
• Nephrons are tubelike units that regulate the body’s
salt and water balance and remove metabolic wastes
from the blood.
• Excess water and bodily wastes leave the kidneys in
the form of a fluid called urine.
• Marine fishes excrete small amounts of urine and rid
their bodies of ammonia largely through their gills.
• Freshwater fishes
excrete large amounts
of dilute urine.
Reproduction
• The sexes are separate in most fishes, and generally
fertilization takes place externally.
• In a process called spawning, male and female
gametes are released near one another in the water.
• A yolk sac within each egg contains nutrients the
developing embryo will need for growth.
Reproduction
• The yolk sac remains attached to the hatchling
fish but is quickly used up.
• Then, the growing fish must seek its own food.
• More likely than not, it will become food for some
larger animal.
• Many species of fish release large numbers of
eggs that are fertilized in a single spawning
season.
• This practice helps ensure that some individuals
will survive to maturity.
Reproduction
• The eggs of sharks, skates, and rays are
fertilized inside the female’s body.
• During mating, the male uses two organs
called claspers to insert sperm into the
female.
• In most species, the
eggs develop inside
the female and the
young are born alive.
• A few species of sharks lay eggs.
Jawless Fishes
• Perhaps the most unusual fishes found today are the
surviving jawless fishes, the lampreys and hagfishes.
• These primitive creatures have changed little over the
past 330 million years.
• Little is known about hagfishes because they live on the
ocean floor at depths as great as 1,700 m (about 1 mi).
• Lampreys are better understood and are found in both
salt and fresh water.
• All species of lampreys must return to fresh water
to reproduce, suggesting
that their ancestors
lived in fresh water.
Jawless Fishes
• Lampreys and hagfishes have scaleless, eel-like
bodies with multiple gill slits and unpaired fins.
• Their skeletons are made of cartilage, a strong
fibrous connective tissue, and both kinds of fishes
retain their notochord into adulthood.
• Hagfishes are scavengers of dead and dying
animals on the ocean bottom.
– Because of this behavior, they are sometimes called
the “vultures of the sea.”
• When threatened, a hagfish can produce huge
quantities of slime from its roughly 200 slime
glands.
Jawless Fishes
• Most lampreys are parasitic on other living fishes.
• A lamprey has a suction-cup-like structure around
its mouth that it uses to attach itself to its host.
• After attachment, the lamprey gouges out a
wound with its rough tongue, feeding on blood
and bits of flesh from the wound.
Cartilaginous Fishes
• Sharks, skates, and rays are cartilaginous fishes.
• Their skeletons are made of cartilage
strengthened by the mineral calcium carbonate.
• Calcium carbonate is deposited in the outer layers
of cartilage and forms a thin layer that reinforces
the cartilage.
• The result is
a very light
but strong
skeleton.
Cartilaginous Fishes
• The shark’s light, streamlined body allows it to
move quickly through the water in search of prey.
• The shark’s skin contains cone-shaped placoid
scales, which give the skin a rough texture.
• The shark’s scales and teeth are quite similar in
structure.
– This is because
the teeth
are actually
modified scales.
Teeth of Shark
• The shark’s teeth are arranged in
6 to 10 rows along the shark’s jaw.
• The teeth in front are pointed
and sharp and are used for
biting and cutting.
• Behind the front teeth, rows of
immature teeth are growing.
• When a front tooth breaks or is worn down,
a replacement tooth moves forward.
• One shark may use more than 20,000 teeth
during its lifetime.
• This system of tooth replacement guarantees that the
teeth being used are always new and sharp.
Cartilaginous Fishes
• Two smaller groups of cartilaginous fishes,
the skates and rays have flattened bodies
that are well adapted to life on the sea floor.
• Rays are usually less than 1 m (3.3 ft) long,
while skates are typically smaller.
– The giant manta ray may be up to 7 m (23 ft) wide.
• Most species of skates
and rays have flattened
teeth that are used
to crush their prey,
mainly small fishes
and invertebrates.
Bony Fishes
• Jawless and cartilaginous fishes are not as diverse
as bony fishes, which are the most numerous of
all the fishes.
• Bony fishes have a strong, internal skeleton made
completely of bone, bony fishes and have a series
of unique structural
adaptations that
contributed to
their success.
Lateral Line System
• Bony fishes have
a fully developed
lateral line system.
• The lateral line is
a sensory system
that extends along
each side of a
bony fish’s body.
• As moving water presses against the fish’s sides,
nerve impulses from ciliated sensory cells within
the lateral line permit the fish to perceive its
position and rate of movement.
Lateral Line System
• The lateral line system also enables a fish to
detect a motionless object by the movement of
water deflected by that object.
• The way that a fish detects an object with its
lateral line and the way that you hear music with
your inner ear are quite similar.
• Both processes share the same
basic mechanism, sensory cells
with cilia detect vibrations and
send this information
to the brain.
Gill Cover
• Most bony fishes have a hard plate, an operculum
that covers the gills on each side of the head.
• Movements of certain muscles and of the opercula
permit a bony fish to draw water over the gills,
which enables the fish to take in oxygen.
Gill Cover
• Through opening and closing the operculum,
most bony fishes can move water over their gills
while remaining stationary in the water.
• A bony fish doesn’t have to swim forward with its
mouth open to move water over its gills.
• This ability to respire without swimming enables
a bony fish to conserve
energy that can be
spent chasing after
prey and escaping
from predators.
Swim Bladder
• The density of the fish body is slightly greater
than that of sea water.
• Bony fishes contain a special gas sac called a swim
bladder.
• By adjusting the gas content of the swim bladder,
bony fishes can regulate
their buoyancy.
• As the swim bladder
fills, the fish rises,
and as it empties,
the fish sinks.
Swim Bladder
• The swim bladder of early bony fishes was
connected to their throat, and they
gulped air to fill it.
• The swim bladder of modern bony fishes
does not have a direct passage to the mouth.
• Instead, gas is exchanged between the
bloodstream and the swim bladder.
• This permits the fish
to maintain or change
its depth in the water.
Ray-Finned Bony Fishes
• Ray-finned bony fishes comprise the vast
majority of living fishes.
• Their fins are supported by bony structures
called rays.
• Teleosts, such as the yellow perch are the most
advanced of the ray-finned bony fishes.
• Teleosts have highly mobile fins, very thin
scales, and completely symmetrical tails.
• About 95% of all living fish species are teleosts.
Ray-Finned Bony Fishes
Ray-Finned Bony Fishes
Lobe-Finned Bony fishes
• Only several species of lobe-finned fishes survive today.
• One species is the coelacanth and the other six species
are all lungfishes.
– Coelacanth can reach up to nearly 3 m (9.8 ft in length).
• The lobe-finned fishes have paired fins that are
structurally very different from the fins of ray-finned
fishes.
• In many lobe-finned fishes, each fin consists of a long,
fleshy, muscular lobe that is supported by a central core of
bones.
• These bones are connected by joints, like the joints
between the bones in your hand.
• Bony rays are found only at the tips of each lobed fin.
• Muscles within each lobe can move the bony rays
independently of each other.
Lobe-Finned Bony fishes
• Scientists have debated whether the direct
ancestor of amphibians was a coelacanth or a
lungfish.
• Recent evidence has led biologists to believe
that it was neither.
• The ancestor of the amphibians most likely
was a third type of lobe-finned fish that is
now extinct.
Amphibians
• Class amphibia contains three
orders of living amphibians:
order Anura (frogs and toads),
order Urodela (salamanders
and newts), and order
Apods (caecilians).
• Most amphibians share five key characteristics.
1. Legs. The evolution of legs was an important
adaptation for living on land. Frogs, toads,
salamanders, and newts have four legs.
Caecilians lost their legs during the evolutionary
course of adapting to a burrowing existence.
Amphibians
2. Lungs. Although larval amphibians have gills,
most adult amphibians breathe with a pair of
lungs. Lungless salamanders are an exception.
3. Double-loop circulation. Two large veins called
pulmonary veins return oxygen-rich blood from
the lungs to the
heart. The blood
is then pumped
to the tissues at
a much higher
pressure than in
the fish heart.
Amphibians
4. Partially divided heart. The atrium of the
amphibian heart is divided into left and right
sides, but the ventricle is not. A mixture of
oxygen-rich and oxygen-poor blood is delivered
to the amphibian’s body tissues.
5. Cutaneous respiration. Most amphibians
supplement their oxygen intake by respiring
directly through their moist skin. Cutaneous
respiration (‘skin breathing’) limits the
maximum body size of amphibians because it is
efficient only when there is a high ratio of skin
surface area to body volume.
Amphibian Lungs
• Although air contains about 20 times as much
oxygen as sea water, gills cannot function as
respiratory organs when out of water.
• One of the major challenges that faced the first
land vertebrates was that of obtaining oxygen
from air.
• The evolutionary solution to
this challenge was the lung.
Amphibian Lungs
• A lung is an internal, baglike respiratory organ
that allows oxygen and carbon dioxide to be
exchanged between the air and the
bloodstream.
• The amount of oxygen a lung can absorb
depends on its internal surface area.
• The greater the surface area, the greater the
amount of oxygen that can be absorbed.
• In amphibians, the lungs are sacs with folds on
their inner membrane that increase their surface
area.
Amphibian Lungs
• Which each breath, fresh oxygen-rich air is drawn
into the lungs.
• There it mixes with a small volume of air that has
already given up most of its oxygen.
• Because of this mixing, the respiratory efficiency of
lungs is much less than that of gills.
• Because there is much more oxygen in air than
there is in the water, lungs do not
have to be as efficient as gills.
• Many amphibians also obtain
oxygen through their thin, moist skin.
Double-Loop Circulation
• As amphibians evolved and became active
on land, their circulatory system changed,
resulting in a second circulatory loop.
• This change
allowed more
oxygen to be
delivered to
their muscles.
Double-Loop Circulation
• Amphibians have a pair of blood vessels not
found in fishes, the pulmonary veins.
• The pulmonary veins carry oxygen-rich blood
from the amphibian’s lungs to its heart.
• The heart pumps the oxygen-rich
blood to the rest of the body.
• The advantage of this arrangement
is that oxygen-rich blood can be
pumped to the amphibian’s tissues
as a much higher pressure than it
can in fishes.
Circulation of Blood
• Not only did the path of circulation in
amphibians change, but several important
changes occurred in the heart.
• The sinus venosus continues to deliver oxygenpoor blood from the body to the right side of
the heart.
• Oxygen-rich blood
from the lungs
enters the left side
of the heart directly.
Circulation of Blood
• A dividing wall known as the septum separates the
amphibian atrium into right and left halves.
• The septum prevents the complete mixing of oxygen-rich
and oxygen-poor blood as each enters the heart.
• Both types of blood empty into a single ventricle,
where some mixing of blood occurs.
• Due to the anatomy of the ventricle, the two streams
of blood remain somewhat separate.
• Oxygen-rich blood tends to
stay on the side that directs
blood toward the body.
• Oxygen-poor blood tends to
stay on the side that directs
blood toward the lungs.
Circulation of Blood
• A number of amphibians have a spiral valve that
divides the conus arteriosus.
• The spiral valve also helps to keep the two streams
of blood separate as they leave the heart.
Frogs and Toads
• The order Anura is made up of
frogs and toads that live in
environments ranging from
deserts to rain forests, valleys to
mountains, and ponds to puddles.
• Adult anurans are
carnivorous, eating
a wide variety
of small prey.
• Some species have a
sticky tongue that they
extend rapidly to catch
their prey.
Frogs and Toads
• The frog body is adapted for jumping, with its long
muscular legs providing the power.
• Toads are very similar to frogs but have squat bodies
and shorter legs.
• The toad’s skin is not smooth like that of a frog but is
covered with bumps.
Reproduction in Frogs
• Like most living amphibians, frogs depend on the
presence of water to complete their life cycle.
• The female releases her eggs into the water and a
male’s sperm fertilizes them externally.
• After a few days, the fertilized eggs hatch into a
swimming, fishlike larval form called tadpoles.
• Tadpoles breathe with gills and feed mostly on algae.
• After a period of growth, the body of the tadpole
changes into that of an adult frog.
• The rate at which tadpoles develop depends on the
species and the availability of food.
• This process of physical change is called
metamorphosis.
Reproduction in Frogs
Salamanders
• Salamanders have elongated bodies,
long tails, and smooth, moist skin.
• There are about 369 species of
salamanders all belonging to
the order Urodela.
• They typically range from 10 cm
to .3 m (4 in to 1ft) in length.
• However, giant Asiatic salamanders
of the genus Andrias grow as long
as 1.5 m (5 ft) and weight up to 41 kg (90 lbs).
• Because salamanders need to keep their skin moist, most
are unable to remain away from water for long periods,
although some salamander species manage to live in dry
areas by remaining inactive during the day.
Salamander Reproduction
• Salamanders lay their eggs in water or in moist places.
• Fertilization is usually external.
• A few species of salamanders practice a type of internal
fertilization in which the female picks up a sperm packet
that has been deposited by the male and places it in her
cloaca.
• Unlike frog and
toad larvae,
salamander
larvae do not
undergo a
dramatic
metamorphosis.
Salamander Reproduction
• The young that hatch from salamander eggs
are carnivorous and resemble small versions
of the adults, except that the young usually
have gills.
• A few species of salamanders, such as the
North American mudpuppy and the Texas
spring salamander,
retain their external
gills as adults.
Caecilians
• Caecilians (order Apoda)
are a highly specialized
group of tropical, burrowing amphibians
with small, bony scales embedded in their skin.
• They feed on small invertebrates found in the soil.
• These legless, wormlike animals grow to about
.3 m (1 ft) long, although some species can be
up to 1.2 m (4 ft) long.
• During breeding, the male deposits sperm
directly into the female.
• Depending on the species, the female may bear live
young or lay eggs that develop externally.
• Caecilians are rarely seen, and scientists do not know
much about their behavior.