Transcript Phylum Chordata

Phylum Chordata
 The
chordates are a group of particular
interest to us as we belong to it, being
members of the subphylum Vertebrata.
 The
chordates include all of the
vertebrates (fish, amphibians, reptiles,
mammals and birds), but also two nonvertebrate subphyla the Urochordata and
the Cephalohordata.
Phylum Chordata

The chordates were in the 19th century
considered to have been derived from
protostome ancestors (the annelid, mollusc,
arthropod group).

However, a better understanding of embryology
shows that chordates are deuterostomes and
the invertebrates they are most closely related to
are Echinodermata and the Hemichordata.
Figure 23.02
Figure 23.03
Characteristics of the Chordata
 Chordates




are:
bilaterally symmetrical
triploblastic
have a well developed coelom
have a complete digestive system
Five distinctive characteristics of
the chordates

Five distinctive characteristics separate the
chordates from all other phyla:






Notochord
Single, dorsal, tubular nerve cord
Pharyngeal pouches or slits
Endostyle
Postanal tail
Not all of these characteristics are apparent in
adult organisms and may appear only in the
embryonic or larval stages.
Notochord
 Notochord:
the notochord is a flexible,
rodlike structure. It extends the length of
the body and is an anchor point for
muscles.
 The
notochord bends without shortening
so it permits the animal to undulate.
Figure 23.01
Notochord
 In
nonvertetbrates and the jawless
vertebrates the notochord is present
throughout life.
 However,
in the jawed vertebrates it is
replaced by the vertebral column; the
remnants of the notochord being found in
the intervertebral disks.
Single, dorsal, tubular nerve
cord

In most invertebrates the nerve cord, if present,
is ventral to the gut.

In chordates, in contrast, the nerve cord is dorsal
to the gut and notochord. The nerve cord
passes through the neural arches of the
vertebrae, which protect it.

The nerve cord is enlarged in vertebrates into a
brain, which is surrounded by a bony or
cartilaginous cranium.
Pharyngeal pouches and slits

Pharyngeal slits occur in aquatic chordates and
lead from the pharyngeal cavity to the outside.

The pharyngeal slits are used as a filter feeding
device in protochordates (i.e., Urochordata
(Tunicates)) and Cephalochordata (lancelets
e.g. Amphioxus).

Water containing food is drawn in through the
mouth by cilia and exits via the pharyngeal slits
where the particles are trapped in mucus.
Figure 23.09b
Amphioxus
Pharyngeal pouches and slits
 In
vertebrates the pharyngeal arches have
been modified into gills by the addition of a
rich blood supply and thin gas permeable
walls.
 The
contraction of muscles in the pharynx
drive water through the gills.
Pharyngeal pouches and slits
 In
amniotes an opening may not form and
rather than slits only grooves called
pharyngeal pouches develop.
 In
tetrapods these pouches give rise
during development to a variety of
structures including the middle ear cavity,
eustachian tube, and tonsils.
Endostyle or thyroid gland
 The
endostyle is found in protochordates
and in lamprey larvae. It is located on the
floor of the pharynx and secretes mucus,
which is used to trap particles.
 The
endostyle works with the pharyngeal
slits in filter feeding.
Endostyle or thyroid gland
 Some
cells in the endostyle secrete
iodinated proteins and are homologous
with iodinated-hormone secreting thyroid
gland, which is found in adult lampreys
and vertebrates.
Postanal tail

The postanal tail, some musculataure and the
notochord enable larval tunicates and
amphioxus to swim.

The postanal tail evolved to allow organisms to
swim and its efficiency has been enhanced by
the addition of fins. The postanal tail is present
only in vestigial form in humans (the coccyx)
although tails as a whole are widespread
amoing vertebrates.
Figure 23.09b
Amphioxus
Classification of the Chordata
 There



are three subphyla in the Chordata:
Subphylum Urochordata: tunicates
Subphylum Cephalochordata: lancelets
Subphylum Vertebrata: fish, amphibians,
reptiles, birds, mammals, etc.
Subphylum Urochordata

The Urochordata (“tunicates” named for the
tough tunic that surrounds the adult) look like
most unpromising candidates to be chordates
and relatives of the vertebrates.

The largest group, the ascidians or sea squirts
(Class Ascidiacea) as adults are marine, sessile,
filter feeding organisms that live either solitarily
or in colonies.
Ciona intestinalis
(a solitary sea squirt)
0147.jpg
Synoicum pulmonaria a colonial sea squirt
Ascidians
 Adult
ascidians lack a notochord and there
is only a single ganglion in place of the
dorsal nerve cord.
 Of
the five characteristics of chordates
adults possess only two: pharyngeal gill
slits and an endostyle, both of which they
use in filter feeding.
Ascidians
 The
adult sea squirt draws water in
through an incurrent siphon and pushes it
back out an excurrent one.
 Food
particles are filtered out in the
pharyngeal slits with mucus from the
endostyle used to trap particles.
Figure 23.04
15.4
Larval Ascidian
 Even
though the adult ascidian hardly
resembles a chordate its larva does.
 Larval
ascidians are very small and
tadpole-like and possess all five chordate
characteristics previously outlined.
Young larval ascidian
Larval Ascidian
 The
larval ascidians role is to disperse and
to achieve this it is free swimming.
However, it has only a short larval life
(minutes to a couple of days) and does not
feed during this time.
 Instead
it searches for a place to settle
and then attaches and metamorphoses
into an adult.
Ascidian metamorphosis
 During
metamorphosis the notochord
disappears, the nerve cord is reduced to a
single nerve ganglion and a couple of
nerves.
Figure 23.06
15.5
Other Urochordate classes
 Besides
the ascidians there are two other
classes of the Urochordata: the Larvacea
and Thaliacea.
 Both
are small, transparent planktonic
forms. Thaliaceans are cylindrical or
spindle shaped whereas larvaceans are
tadpolelike and resemble an ascidian
larva.
Garstang’s hypothesis of chordate
larval evolution
 In
the 1920’s it was proposed that the
vertebrates were derived from an
ancestral ascidian that retained its
characteristics into adulthood (the process
by which juvenile characteristics are
retained into adulthood is referred to as
paedomorphosis).
Figure 23.12
Garstang’s hypothesis of chordate
larval evolution
 Garstang’s
hypothesis is supported by
embryological evidence, but more recently
molecular analyses have suggested that
sessile ascidians are a derived form and
that the free living larvaceans are more
likely to be the closest relatives.
Subphylum Cephalochordata

The cephalochordates are the lancelets, which
are small (3-7 cm long) laterally compressed
fishlike animals that inhabit sandy sediments of
coastal waters. They lack a distinct head and
have no cranium.

They are commonly referred to as Amphioxus as
this was the original genus name. There are 29
species, five of which occur in North American
coastal waters.
Figure 23.09b
Amphioxus
 Amphioxus
 Water
is a filter feeder.
enters the mouth and then passes
through the pharyngeal slits, where food is
trapped in mucus. Cilia then move the
food to the gut.
Figure 23.09a
Amphioxus
Amphioxus

Amphioxus is interesting because it displays the
basic chordate characteristics in a simple and
obvious form because of its transparency.

Amphioxus is considered to be the closest living
relative of the vertebrates because it shares
several characteristics with vertebrates that
Urochordates do not possess.
Amphioxus characteristics
shared with vertebrates
 Characteristics
amphioxus shares with
vertebrates include:



Segmented myomeres (muscle blocks)
Dorsal and ventral aortas
Branchial (gill) arches (blood vessels running
over the gills).
Amphioxus characteristics not
shared with vertebrates
 Amphioxus
however lacks several
characteristics that biologists think the
ancestor of vertebrates possessed. These
include:




Tripartite brain (with forebrain, midbrain and
hindbrain) protected by a cranium
Chambered heart
Muscular gut and pharynx
List continues on next slide
Amphioxus characteristics not
shared with vertebrates


Various special sensory organs (eyes,
chemical and pressure receptors)
Neural crest (ectodermal cells that are found
on the embryonic neural tube and are
engaged in the formation of the cranium, tooth
dentine, some endocrine glands and
Schwann cells, provide myelin insulation to
nerve cells).
Subphylum Vertebrata

The vertebrates are a large and diverse group
including the fishes and tetrapods.

Vertebrates possess the basic chordate
characteristics, but also a number of novel
homologous structures.

An alternative name for the group Craniata is
actually a better descriptor for the entire group
because all members possess a cranium, but
some jawless fishes lack vertebrae.
Important developments of the
Vertebrates

Musculoskeletal system. Vertebrates possess
an endoskeleton, which is much more
economical in materials than the exoskeleton of
invertebrates.

It forms a jointed scaffolding for the attachment
of muscles. Initially the endoskeleton probably
was cartilaginous (it still is in jawless fishes and
sharks) and later became bony in many groups.
Important developments of the
Vertebrates
 Bone
is stronger than cartilage, which
makes it a better material to use for
muscle attachment in places where
mechanical stress may be high.
 Bone
may have evolved initially as a
means of storing minerals and was later
adapted for use in the skeleton.
Important developments of the
Vertebrates

Various aspects of vertebrate physiology have
been upgraded also to meet increased
metabolic demands.

For example the pharynx, which was used for
filter feeding in primitive chordates has had
muscles added that make it a powerful water
pumping organ.

With the conversion of the pharyngeal slits to
highly vascularized gills the pharynx has
become specialized for gas exchange.
Important developments of the
Vertebrates

The ancestors of vertebrates switched from filter
feeding to more active feeding, which required
movement and the ability to sense the
environment in detail.

With these changes came the need for a control
center to process information. The anterior end
of the nerve cord consequently became
enlarged into a brain.
Important developments of the
Vertebrates
 The
vertebrate brain in fact developed into
a tripartite brain (with a forebrain,
midbrain, and hindbrain) that was
enclosed within a protective cranium of
bone or cartilage.
Important developments of the
Vertebrates
 Sense
organs have also become highly
developed among the vertebrates.
 These
include complex eyes, pressure
receptors, taste and smell receptors,
lateral line receptors for detecting water
vibrations, and electroreceptors that detect
electrical currents.
Important developments of the
Vertebrates
 The
development of the head in
vertebrates with its array of sense organs
appears to have been driven by the
evolution of new embryonic tissues that
give rise to cells that play an important role
in the formation of sensory structures.
Important developments of the
Vertebrates
 A factor
that may have played a major role
in the evolution of the vertebrates is the
duplication of Hox genes.
 Hox
genes play a major role in embryonic
development and vertebrates have four
copies, whereas invertebrates and
amphioxus have only one.
Important developments of the
Vertebrates
 The
duplication of the Hox genes appears
to have occurred around the time
vertebrates originated and it may be that
this gene duplication freed up copies of
these genes, which control development,
to generate more complex animals.
Early vertebrate ancestors
 Fossils
of early chordates are scarce, but
a few are known including Pikaia from the
Burgess Shale (approx 580 mya) that
appears to be an early cephalochordate
and has a notochord and segmented
muscles.
Figure 23.10
15.8
Pikaia
Early vertebrate ancestors
 Another
fossil from China Haikouella
lanceolata about 525mya.
 This
fossil has a notochord, pharynx, and
a dorsal nerve cord which are chordate
characters, but also pharyngeal muscles,
eyes, a head, gills and a brain which are
vertebrate traits.
Haikouella lanceolata
Haikouella lanceolata
Jawless early vertebrates
 A wide
variety of armored jawless fishes
called ostracoderms are known from the
Ordovician (approximately 490-440 mya)
up to near the end of the Devonian period
(about 360 mya).
 These
fish in many cases lack paired fins
and so probably were not precision
swimmers.
Figure 23.14
15.10
Ostracoderms
Jawless early vertebrates
 The
ostracoderms were heavily armored
and jawless with narrow, fixed mouths.
They appear to have been mainly filter
feeders that used their pharyngeal
muscles to pump water.
 Ultimately,
the ostracoderms were
outcompeted by fish that possessed the
next big evolutionary development: jaws.
Early jawed vertebrates

The origin of jaws was a hugely significant event
in the evolution of the vertebrates and the
success of the Gnathostomes [the jawed
vertebrates, “jaw mouth”] is obvious.

The first jawed vertebrates were the placoderms
haevily armored fish which arose in the early
Devonian (about 400mya) and possessed not
only jaws, but paired pelvic and pectoral fins that
gave them much better control while swimming.
Figure 23.17
15.13
Early jawed fishes of the Devonian (400 mya).
Jaws

Jaws arose by modification of the first cartilaginous gill
arches, which aid in gill support and ventilation.

It is believed that selection favored enlargement of these
gill arches and the development of new muscles that
enabled them to be moved and so pump water more
efficiently.

Once enlarged and equipped with muscles it would
have been quite easy for the arches to have been
modified into jaws.
Figure 23.16
15.12
Note resemeblance between upper jaw (palatoquadrate cartilage) and lower jaw
(Meckel’s cartilage) and gill supports immediately behind in this Carboniferous shark
Living fishes
 The
living fishes (not a monophyletic
group) include:




the jawless fishes (e.g. lampeys),
cartilaginous fishes (e.g. sharks and rays),
bony, ray-finned fishes (most of the bony
fishes such as trout, perch, pike, carp, etc)
and
the bony, lobe-finned fishes (e.g. lungfishes,
coelacanth).
Figure 24.01
16.1
Figure 24.02
16.2
Living jawless fishes
 There
are a little more than 100 species of
living jawless fishes or Agnathans (the
term agnathan does not represent a
monophyletic group).
 These
belong to two classes the Myxini
(hagfishes) and the Cephalaspidomorphi
(lampreys).
Characteristics of agnathans
 Lack
jaws (duh!)
 Keratinized plates and teeth used for
rasping
 Vertebrae absent or reduced
 Notochord present
 Dorsal nerve cord and brain
 Sense organs include taste, smell,
hearing, vision.
Hagfishes: class Myxini

Hagfishes are a marine group of primarily
scavengers.

They use their keen sense of smell to find dead
or dying fish and invertebrates and rasp off flesh
using their toothed tongue.

As they lack jaws, they gain leverage by knotting
themselves and bracing themselves against
whatever they’re pulling.
Figure 24.03
16.3
Hagfishes

Hagfishes are unusual in that they have body
fluids, which are in osmotic equilibrium with the
surrounding sea. This is unknown in other
vertebrates, but common in invertebrates.

They are also unusual in having a low pressure
circulatory system that has three accessory
hearts in addition to a main heart.
Hagfishes
 Hagfishes
have a remarkable (and
revolting) ability to generate enormous
quantities of slime, which they do to
defend themselves from predators.
 A single
slime.
individual can fill a bucket with
Lampreys: Class
Cephalaspidomorphi

Lampreys occur in both marine and fresh waters
and about half of all species are ectoparasites of
fish (the others are non-feeding as adults and
live only a few months).

Lampreys spawn in streams and the larvae
(ammocoetes) live and grow as filter feeders in
the stream for 3-7 years before maturing into an
adult. Feeding adults live a year or so before
spawning and dying.
Figure 24.05
16.5
Lampreys

Parasitic lampreys have a sucker-like mouth with
which they attach to fish and rasp away at them
with their keratinized teeth.

The lamprey produces an anticoagulant as it
feeds to maintain blood flow. When it is full the
lamprey detaches, but the open wound on the
fish may kill it. At best the wound is unsightly
and largely destroys the fish’s commercial value.
Sea lamprey close up of sucker and teeth
Figure 24.06
16.4
Figure 24.04
Introduced sea lampreys

Landlocked sea lampreys made their way into
the Great Lakes around 1918 and caused the
complete collapse of the lake trout fishery by the
1950’s.

Lamprey numbers fell as their prey base
collapsed and control efforts were introduced.
Trout numbers have since recovered somewhat,
but wounding rates are still high.
Sea lampreys in Lake Champlain

Lake Champlain also has large populations of
sea lampreys which spawn in the creeks that
empty into the lake.

Until recently, lampreys were believed to have
been introduced into Lake Champlain, but
genetic analyses indicate the population was
established perhaps as much as 11,500 years
ago by lampreys that migrated up the St.
Lawrence.
Sea lampreys in Lake Champlain
 As
is the case elsewhere there has been a
campaign to control lamprey numbers
primarily by using lampricides in steams.
 Controls
do reduce lamprey wounding
rates and after control rates have fallen
from 60-70 wounds per 100 fish examined
to as low as 30 wounds/fish.
Class Chondrichthyes:
cartilaginous fishes
 The
class Chondrichthyes has two
subclasses:


Elasmobranchii, which includes the sharks
and rays.
Holocephali: the chimaeras: ratfish and
ghostfish.
Figure 24.12
Two species of ray
Figure 24.co
Hammerhead
Shark
Class Chondrichthyes
 The
Chondrichthyes are an ancient group
that although not as diverse as the bony
fishes have persisted largely unchanged
for hundreds of millions of years.
 There
are about 850 living species, all of
which have cartilaginous skeletons, even
though they are descended from
ancestors that had bone.
Class Chondrichthyes
 The
Chondrichthyes’ well-developed jaws,
highly developed sense organs, powerful
swimming ability and streamlined shape
have enabled them to thrive as marine
predators for more than 350 million years,
as other groups have come and gone.
Hammerhead sharks
Two skates
Great White Shark
Whale shark
Figure 24.07
Diversity of sharks
Sharks
 Sharks
represent a little less than half of
the elasmobranchs and most are
specialized predators.
 The
largest species is the whale shark,
which is a plankton feeder, but most of the
others are predators of fish, marine
mammals, crustaceans and whatever else
they can catch.
Sharks
 Sharks
are very well streamlined, but are
heavier than water (because they lack a
swim bladder) and sink if not swimming
forward.
 Sharks
increase their buoyancy by having
a large oil-filled liver that reduces their
density, but not enough to prevent them
from sinking.
Large liver of a great white shark
Sharks
 Sharks
have an asymmetrical heterocercal
tail and the vertebral column extends into
the dorsal lobe.
 The
tail provides both lift and thrust, while
the large flat pectoral fins also provide lift
to keep the head up.
Figure 24.08
16.6
Sharks

Sharks have skin covered in dermal placoid
scales, which are small tooth-like structures
(with enamel, dentine and pulp just like real
teeth).

These scales give sharkskin a tough, leathery
and abrasive feel.

The scales are modified in the mouth to produce
the rows of replaceable teeth characteristic of
sharks.
Figure 24.18
16.15
Figure 24.09
Sand tiger shark (note multiple rows of teeth)
Sharks
 Sharks
use a variety of senses to track
detect prey. They have highly developed
olfactory senses and can detect minute
quantities of blood in the water.
 They
are also able to detect vibrations in
the water using a lateral line system.
Lateral line system
 The
lateral line system consists of a series
of fluid-filled canals that open to the
outside.
 Inside
in the canals are sensory cells
called neuromasts that are very sensitive
to vibrations in the water
Figure 24.10
Organs of Lorenzini
 Sharks
are also able to detect the faint
bioelectric fields that surround all animals.
This allows them to locate prey buried in
sand or sense prey at night.
 The
bioelectric detectors are called
ampullary organs of Lorenzini and are
found in the sharks head.
Reproduction
 Reproduction
in all Chondrichthyes is
internal and the male uses modified pelvic
fins called claspers to insert sperm.
 The
presence or absence of claspers
makes it easy to distinguish male from
females.
Great white shark claspers
Reproduction
 All
skates and some sharks are oviparous
and lay eggs soon after fertilization.
 Other
sharks are ovoviviparous and the
eggs develop within the mothers body and
hatch either in her or just after being
released from her.
Egg case of cat shark
Embryo of deep sea cat shark.
There is a very large yolk sac to
support the embryo’s growth.
Reproduction
 The
remaining sharks are viviparous and
the offspring are nourished by a placenta,
unfertilized eggs or smaller siblings.
Skates and rays
 More
than half of all elasmobranchs are
skates and rays.
 They
have characteristically dorsoventrally
flattened bodies and greatly enlarged
pectoral fins, which they swim with using a
wavelike motion.
Blue spotted ray
Manta Ray
Skates and rays
 The
spiracles are much larger in rays than
in sharks because water for the gills enters
exclusively through them as the mouth is
usually buried in the sand.
Skates and rays
 Skates
and rays are usually well
camouflaged and sit on the bottom. A few
species are dangerous because of their
sharp and barbed tail (stingrays) or
because they can generate severe electric
shocks (electric rays).
 Their
teeth are for crushing prey and they
mainly feed on molluscs and crustaceans.
Subclass Holocephali: Chimaeras

Chimaeras are a small group (about 35 species)
of deep sea cartilaginous fishes known
commonly as ratfish or ghostfish.

They have a large head, plate-like grinding
teeth, a cover over the gills and lack both a
spiracle and stomach.

The tail is thin and not much use in swimming.
Instead chimaeras depend on flapping their
pectoral fins for much of their movement.
Male spotted ratfish
Bony fishes: Osteichthyes
 The
term osteichthyes does not describe a
monophyletic group, but is a term of
convenience to describe the fishes whose
skeletons are made of bone that replaces
cartilage during embryonic development.
 There
are two classes the Actinopterygii
(the ray-finned fishes) and the
Sarcopterygii (the lobe-finned fishes)
General characteristics of bony fish
 Skeleton
made of bone of endochondral
origin (derived from cartilage).
 Paired and median fins supported by
dermal rays.
 Respiration mainly by gills. Gills covered
with operculum.
 Swim bladder often present.
 Complex nervous, circulatory and
excretory systems present
Class Actinopterygii (ray-finned
fishes)

This is by far the larger of the two living classes
of fishes with more than 27,000 species.

Ancestral ray finned fishes in the Devonian were
small and heavily armored with ganoid scales
(thick, bony non-overlapping, relatively inflexible
scales) and heterocercal tails (shaped like that
of modern sharks).
Figure 24.18
Chondrosteans
 A few
relic species (the chondrosteans)
still possess such characteristics.
 These
include sturgeon, paddlefish and
the African bichir.
Figure 24.19
Teleosts
 The
vast majority of modern fishes are
teleosts.
 They
have replaced the heavy armored
scales of their ancestors with much lighter
more flexible scales that overlap each
other and also have evolved homocercal
symmetrical tails.
Figure 24.15
Swim bladder

Teleosts also have evolved extremely fine
control over their buoyancy and can remain
neutrally buoyant, which provides large energy
savings.

Most pelagic teleosts have a swim bladder,
which evolved from paired lungs of Devonian
fishes.

Gas can be secreted into or removed from the
swim bladder so that the fish remains at neutral
buoyancy.
Swim bladder

Some fishes (e.g. trout) can gulp or release air
by opening a pneumatic duct that connects to
the esophagus.

More advanced teleosts have discarded the
pneumatic duct and instead secrete gas into the
swim bladder using a gas gland or absorb it
through a highly vascularized part of the swim
bladder called the ovale.
Gas gland
 When
arterial blood arrives at the swim
bladder lactic acid is released by the gas
gland, which causes oxygen to be
released by hemoglobin.
 This
raises the partial pressure of oxygen
in the blood above that in the swim
bladder and so the oxygen flows into the
swim bladder.
Rete mirabile

In deep sea fish a very high gas pressure must
be maintained to resist the pressure of the water.

For example, at 2000 meters gas at a pressure
of 200 atmospheres (more than the oxygen
pressure in fully charged steel cylinder) must be
maintained in the swim bladder even though the
oxygen pressure in the fish’s blood is only 0.2
atmospheres (oxygen pressure at sea level).
Rete mirabile

Why doesn’t the oxygen in the swim bladder flow
out into the blood?

Because of a structure called a rete mirabile
(miraculous net), which stops this loss.

The swim bladder is supplied with blood via an
artery. Before the artery reaches the swim
bladder it divides into an enormous number of
thin, parallel capillaries that run parallel to but
whose contents flow in the opposite direction to
a similar array of venous capillaries.
Figure 24.27a
Rete mirabile (below)
Figure 24.27b
Figure 24.27c
Rete mirabile
 Let
us assume the swim bladder contains
gas at 100 atmospheres. Venous blood
leaving the swim bladder thus contains
oxygen at that pressure.
 As
the venous capillary leaves the swim
bladder it runs parallel to incoming arterial
blood which contains blood with a slightly
lower partial pressure of oxygen.
Rete mirabile

Oxygen thus flows from the venous capillary to
the arterial capillary.

Along its entire length from the swim bladder the
gas pressure in the venous capillary is falling as
it gets further from the swim bladder, but the
pressure is always higher than that in the
parallel arterial capillary so gas always flows
from the venous capillary to the arterial capillary.

Thus the rete acts as a trap that keeps gas in
the swimbladder.
Respiration
 Fish
obtain oxygen using gills, which
consist of filaments covered with a thin
epidermal membrane that is repeatedly
pleated into thin, flat sheets of tissue
called lamellae.
 The
gills are found within the pharyngeal
cavity, which is covered with a flap called
the operculum.
Respiration
 The
operculum protects the gills and also
maintains the streamlining of the body.
 Water
enters the mouth and is pumped
across the gills by movements of the
pharynx and exits under the operculum.
Figure 24.29
16.25
Respiration
 The
lamellae of the gills are richly supplied
with blood, which flows in a countercurrent
direction to the flow of water maximizing
the amount of oxygen extracted.
 The
gills are very efficient and can extract
up to 85% of the dissolved oxygen in the
water.
Respiration
 Certain
highly active fish such as mackerel
with high metabolic rates cannot obtain
enough oxygen by pumping water through
their gills.
 Instead
they must swim forward constantly
in order to drive water through their mouth
and over the gills, a process called ram
ventilation
Lobe-finned fishes: Class
Sarcoptrygii
 Today
the sarcopterygians are a very
small group that includes only six species
of lungfishes and two species of
coelacanths.
 However,
all of the tetrapods (four-legged
vertebrates) are descended from a group
of sarcopterygian fishes known as the
rhipidistians.
Lungfishes
 There
are six species of lungfishes: one
South American, one Australian and four
African species.
 As
their name suggests, these fish, as all
sarcopterygians do, possess lungs and
can breathe air.
Lungfishes

The Australian lungfish can gulp air and survive
being in oxygen poor water, but cannot live out
of water.
 In contrast, the South American and African
species can survive out of water for long periods
of time.
 The African species live in seasonal steams and
ponds that dry out, but the lungfish survives by
burrowing into the mud and forming a cocoon in
which it survives until the water returns.
Figure 24.22
The discovery of living coelacanths

Coleacanths were believed to have been extinct
for perhaps 50 million years when one was
caught by a South African fishing boat in 1938.

The curator of a small museum, M. CourtneyLatimer, recognized the fish was unusual and
she brought it to the attention of the icthyologist
J.L.B. Smith who after some delay in arriving
identified the fish.
The discovery of living coelacanths

Unfortunately, the delay in arriving meant the fish had
badly decomposed and many important structures had
been lost.

Smith named the fish (Latimeria) in honor of CourtneyLatimer and then embarked on a 14-year quest to find
another coelacanth.

But it wasn’t until 1952 that a second was caught off the
Comoro Islands, north of Madagascar, which is where
the fish occur naturally (the 1938 fish apparently had
drifted far from its normal range).
Images from the rediscovery of the
Coelacanth off the Comoros 1952.
The discovery of living coelacanths
 In
1998 another population of Latimeria
[but a different species] was discovered off
Indonesia (10,000km east of the Comoros.
 Coelacanths
are large fish about 5 feet
long and when they swim they move their
pelvic and pectoral fins in the same
pattern that tetrapods walk.
Figure 24.23
16.20