The challenges of living in water
 All
vertebrates inhabit one or other of two
fluid media: air and water.
 These
differ greatly in their physical
characteristics.
Air vs. water

Density: water is 800 times denser than air.

Because water is dense, aquatic animals don’t
need strong weight-bearing skeletons. Gravity
has little impact on their body structure.

In contrast, gravity is a constant challenge for
terrestrial animals.
Air vs. water

Viscosity: water is 18 times more viscous than
air. Viscosity measures how easily a fluid moves
across a surface.

Because of this difference aquatic animals have
to be much more streamlined than those that live
in the air.

Because air flows easily, tidal ventilation is
possible in lungs. In water, it is difficult and very
rare.
Air vs. water

Oxygen content: Oxygen makes up about
20.9% of the volume of air (209ml of O2 in a liter
of air). Water is never more than 50ml per liter
and is often 10ml or less.

Low O2 content is another reason fish don’t use
tidal ventilation. Because of the low O2 content
of water, fish gills have evolved to be very
efficient at extracting O2.
Air vs. water

Heat Capacity: The specific heat of water
(amount of heat needed to change the
temperature of one gram of water by one
degree) is 3400 times greater than that of air.

Thus, water resists temperature change. It
heats and cools slowly. Hence an aquatic
animal has a more stable thermal environment
than an air-living one.
Air vs. water
 Heat
Conductivity: Water conducts heat
almost 24 times as quickly as air.
 Because
water is such a good conductor
there is little variation in temperature within
a body of water. If water gets too hot, a
fish must go to deeper water and that may
not always be possible.
Air vs. water
 Electrical
conductivity: water is an
electrical conductor, but air is not (except
at high voltages).
 Electricity
therefore can be (and is) used
by aquatic animals to detect other animals
and also as a weapon.
Pressure effects
 Water
is much denser than air and
pressure changes with increasing depth
are very important to fishes.
 Every
10m increase in depth in water
increases the pressure experienced by 1
atmosphere. Thus, a fish at 100m
experiences 10 atmospheres of pressure.
Pressure effects
 Because
of pressure effects on the use of
gas-filled structures as buoyancy aids, fish
have had to evolve a variety of
adaptations to remain at or near neutral
buoyancy.
 In
contrast, in air, extreme changes in
altitude are required before significant
effects of reduced air pressure are felt.
Obtaining oxygen in water: Gills

Fish exchange oxygen and carbon dioxide
through the use of gills.

The gills of teleost fish (the largest group of bony
ray-finned fish) are enclosed in pockets of the
pharynx behind the mouth (the opercular
cavities).

A flap of tissue (the operculum) protects the gills
and also maintains the streamlining of the body.
Gills
 Within
the opercular cavity are a series of
gill arches and from each gill arch project
two sets of gill filaments.
 On
each gill filament are numerous, small
and thin-walled projections called
secondary lamellae. Gas exchange takes
place at the secondary lamellae.
Oxygen-poor
blood
Gill arch
Oxygen-rich
blood
Lamella
Blood
vessel
Gill
arch
Water
flow
Operculum
O2
Water flow
over lamellae
showing % O2
Figure 42.21
Gill
filaments
Blood flow
through capillaries
in lamellae
showing % O2
Countercurrent exchange
Gills

Water flow is one way through the gills.

Flaps just within the mouth and at the margins of
the operculae prevent backflow.

Many fish (especially less active ones or resting
ones) depend on the pumping action of the
mouth and opercular cavities (called buccal
pumping) to maintain a steady flow of water
across the gills
Ram Ventilation

For fast swimming predatory fish buccal
pumping would be inadequate to supply their
gas exchange needs.

In fishes such as tuna, mackeral and
swordfishes the ability to pump water has been
reduced or lost.

Instead these fish depend on ram ventilation.
They swim with their mouths open which creates
a steady flow of water across the gills.
Northern Bluefin Tuna www.nytimes.com
http://www.glaucus.org.uk/mackerel.jpg
Counter current exchange
 The
one-way flow of water across the gills
is exploited by the fish to maximize oxygen
extraction.
 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.
Figure 24.29
16.25
Counter current exchange

Because the direction of blood flow is opposite
the direction of the flow of water there is always
an oxygen gradient between the water and the
blood. Hence, oxygen always flows from the
water into the blood.

Gills are very efficient and can extract up to 85%
of the dissolved oxygen in the water.
Counter current exchange

All counter current exchangers work on the
principle of maintaining a concentration gradient
along the length of the structure.

In the gill, blood entering the lamellae is
deoxygenated and it encounters water that has
had much of its oxygen removed. However, the
concentration gradient ensures the water gives
up oxygen to the blood.
Counter current exchange
 As
the blood flows through the lamellae its
oxygen concentration increases, but
because of the countercurrent
arrangement, it is always encountering
water with a higher oxygen content than is
in the blood so the blood continues to gain
oxygen until it is saturated.
Counter current exchange

Countercurrent exchangers are widespread
among vertebrates.

For example, they are found in the flippers of
whales (to reduce heat loss from the body), in
the lungs of birds (to maximize oxygen
extraction), in the salt glands of seabirds (to
concentrate salt) and as we will see shortly in
the swim bladder of fishes (to maintain high gas
pressure in the swim bladder).
How fish obtain oxygen from the air
 Some
fish that live in water with low
oxygen content cannot obtain enough
oxygen to survive using their gills alone.
 These
fish supplement their oxygen intake
by using lungs or other accessory
respiratory structures.
 Tropical
Asian anabantid fish (which
include the common pet fish tetras and
gouramis) have vascularized chambers in
the rear of the head called labyrinths.
 The
fish gulp air at the surface and it is
transferred to the labyrinth where gas
exchange takes place.
Pearl Gourami
http://www.thekrib.com/Fish/gourami.jpg
Tetra. http://animal-world.com/encyclo/fresh/characins/
images/SerpaeTetraWFCh_C2418.jpg
Lungs
 Lungs
obviously are most associated with
tetrapods, but they evolved in fish millions
of years before the first tetrapods evolved.
 In
fact lungs have evolved independently
multiple times in different lineages of fish.
Lungs
 Embryonically,
lungs develop as outpocketings of the pharyngeal region of the
gut.
 In lungfishes and tetrapods lungs develop
from the ventral surface of the gut.
 However, in gars (a primitive bony fish)
lungs develop on the dorsal surface as is
also true in teleosts.
South American Lungfish
http://www.ucmp.berkeley.edu/vertebrates/sarco/lungfish1.jpg
Australian Lungfish
http://animals.nationalgeographic.com/staticfiles/NGS/
Shared/StaticFiles/animals/images/primary/gar.jpg
Longnose gar
http://www.biokids.umich.edu/files/12296/gar_large.jpg
.
Lungs

Increased surface area increases the efficiency of
lungs.

Ridges and pockets in the wall of the lung
increase surface area and these alveolar lungs
are found in lungfishes and tetrapods (both
groups also have paired lungs).

Gars in contrast have a single alveolar lung
whereas bichirs (a group of African air-breathing
fish) have paired non-alveolar lungs, but one lobe
is smaller than the other.
Armored Bichir
http://www.aquarticles.com/images/Gallo/Armoured%20bichir%202.gif
http://www.fbas.co.uk/Bichir.jpg
Swim bladder

Teleosts 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.

Swim bladders are found mainly in fish that occur in the
upper 200m of the water column.

The swim bladder’s wall is impermeable to gases, but
can expand a lot.
Swim bladder
A
gas-filled bladder is affected by depth
changes so the fish must be able to add
and remove gas to remain neutrally
buoyant.
 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, goldfish) gulp or release
air by opening a pneumatic duct that connects to
the esophagus. These fishes are referred to as
physostomous (Greek phys = bladder, stom =
mouth)

More derived teleosts (physoclistic; Greek clist =
closed) have discarded the pneumatic duct and
instead secrete gas into the swim bladder using
a gas gland.
Gas gland
 When
arterial blood arrives at the gas
gland it enters a layer of tissue called the
secretory epithelium and here lactic acid is
released.
 This decreases pH which causes oxygen
to be released by the hemoglobin because
the hemoglobin’s oxygen affinity (Bohr
effect) and oxygen capacity (Root effect)
are reduced.
Gas gland
 The
release of oxygen 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.
Rete mirabile
 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 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.
Ovale
 To
release gas from the swimbladder, fish
use a structure called the ovale.
 The
ovale is a muscular valve that
connects the swim bladder to a capillary
bed. When the ovale is opened the high
pressure of oxygen in the swimbladder
causes it to diffuse into the capillary bed
and enter the blood stream.
Deep Sea fishes
 Many
deep sea bony fishes deposit oils
and lipids in the gas bladder. Others have
lost the gas bladder entirely.
 Fish
that migrate over a large vertical
distance tend to depend more on oils for
buoyancy than gas because oils are
incompressible and thus unaffected by
pressure changes.
Buoyancy in sharks
 Cartilaginous
fish do not possess a swim
bladder. To compensate they store large
amounts of low density oils in their
enlarged livers (which may represent 25%
of their body mass).
 Sharks
also have high concentrations of
urea in their blood, which also reduces
their buoyancy.
Buoyancy in sharks

Pelagic sharks have the largest livers and
contain the most oil. Liver tissue has an
average density of 0.95 g/ml

With the liver removed the tissue density of a
shark is about 1.06-1.09 g/ml (water is 1g/ml),
but with the liver included average density falls
to approximately 1.007 g/ml.

A 460 kg tiger shark thus has an effective weight
of only about 3.5 kg.
Large liver of a great white shark
Buoyancy in sharks
 Bottom-dwelling
sharks such as nurse
sharks can afford to be more negatively
buoyant.
 They
have smaller livers and there is less
oil deposited in the liver.
Nurse sharks
http://www.kidzone.ws/sharks/photos/
Tiger Shark
Diving mammals
 For
air-breathing vertebrates lungs and
pressure pose a different series of
challenges.
 Air
contained within the lungs becomes
pressurized with increasing water depth.
 Under
high pressure nitrogen in the lungs
is forced into the blood stream.
Diving mammals
 As
an animal ascends the pressure falls
and the nitrogen gas comes out of
solution.
 If
the animal comes up too quickly,
bubbles of nitrogen may form in the
tissues causing decompression sickness
(“the bends”).
Diving mammals

To avoid this problem, diving mammals breathe
out before they dive and their thoracic cavity
actually collapses at about 150m depth which
forces air out of the lungs.

To avoid an accumulation of nitrogen over time
whales and seals do not perform multiple long
dives, but alternate deep dives with surface time,
which allows nitrogen to leave the blood stream.
Vision in water
 Air
and water have different refractive
indices so light bends as it passes through
an air water boundary.
 Thus,
to an observer on land, an object in
water appears closer than it really is.
Vision in water
 The
corneas of terrestrial and aquatic
organisms both have a refractive index of
about 1.37.
 For
terrestrial vertebrates light is bent
when it passes through the air-cornea
interface. Thus, the cornea can play a
major role in focusing.
Vision in water

For aquatic vertebrates there is too little
difference in the refractive indices of water and
the cornea for the cornea to assist in focusing.

In fish, therefore, the lens is largely responsible
for focusing.

As a result, fish have spherical lenses with a
high refractive index and the whole lens is
moved in and out to focus.
Vision in water
 Light
is absorbed by water and disappears
with depth. It also can be scattered by
suspended particles. Thus, vision is often
of limited use.
 Hence,
fish must often depend on other
senses.
Other sensory systems in water

For fish a distinction between taste and smell is
pointless. Chemoreception is a better term.

In fish chemoreceptors often occur over the
entire body and very low concentrations can be
detected.

Both sharks and salmon can detect odors at
concentrations of less than 1 part in a billion and
home in on the source of a smell.
Touch

In terrestrial vertebrates in the inner ear, hair
cells play a major role in hearing.

Similar clusters of these cells in fish form
neuromast organs that are distributed over the
head and body.

In jawed fishes (and amphibian larvae) the
neuromast organs are often arranged in one or
more canals (the lateral line system) that runs
along the side of the body.
Lateral line system

The fluid-filled canals of the lateral line system
are open to the outside.

The neuromasts are located inside in the canals
and are very sensitive to vibrations in the water.

The hair cells in the neuromasts have cilia
embedded in a gelatinous structure (the cupula).
When the cupula is displaced the cilia bend and
a nerve impulse is triggered.
Figure 24.10
Lateral line system
 The
neuromast cells can detect water
currents of as little as 0.025mm/sec.
 Because
neuromast cells are distributed
across the body, differences in arrival time
of pressure waves can be used to locate
the source of a disturbance (e.g. an insect
on the surface of the water).
Electroreception
 Many
fishes, especially sharks can detect
electric fields.
 By
detecting the faint bioelectric fields that
surround all animals sharks can locate
prey buried in sand or sense prey at night.
Organs of Lorenzini
 The
bioelectric detectors are called
ampullary organs of Lorenzini and are
found in the shark’s head.
 In
rays they are also on the pectoral fins.
 The
receptor is connected to a surface
pore by a canal that is filled with an
electrically conductive gel.
Organs of Lorenzini
 Because
the canal runs quite deep under
the epidermis the sensory cell can detect
when there is a difference between the
electrical potential in the surrounding
tissue and in the distant pore opening.
 The
electroreceptors are very sensitive
and can detect minor changes in the
electrical field around the shark.
Figure 24.10
Organs of Lorenzini

The threshold for detection is less than 0.01
microvolts per cm, which is comparable to the
best commercially available voltmeters.

The electrical activity sharks detect is due to
muscle contraction, the firing of motor nerves
and also potential differences due to chemical
differences between organisms and their
surroundings.
Electric shark repellents
 http://www.youtube.com/watch?v=5BpFnb
fbjSA
Regulation of the internal
environment
 Organisms
are not impermeable and
aquatic vertebrates face considerable
challenges in regulating their internal
environments.
 Fish
in freshwater environments face the
problem of being flooded with water,
whereas those is seawater can be drained
of water.
Kidneys
 Kidneys
play a central role in regulating
the internal environment.
 The
functional unit of the kidney is the
nephron (of which there are usually
thousands to millions) each of which
produces urine.
Kidneys

The blood is first filtered through a cluster of
capillaries called the glomerulus to produce a
non-selective filtrate.

The filtrate is then processed so that essential
metabolites (amino acids, glucose, etc.) and
water are retrieved.

The final fluid, which may differ greatly in its
composition depending on circumstances, is
urine, which is voided to the outside.
Marine fishes
 Marine
bony fishes are hypoosmotic to sea
water and lose water by osmosis and gain
salt by both diffusion and from food they
eat.
fishes balance water loss by drinking
seawater and actively excrete salt through
their gills. They produce little urine.
 These
Gain of water and
salt ions from food
and by drinking
seawater
Excretion of
salt ions
from gills
Osmotic water loss
through gills and other parts
of body surface
Excretion of salt ions
and small amounts
of water in scanty
urine from kidneys
(a) Osmoregulation in a saltwater fish
Freshwater fishes

Freshwater animals constantly take in water from
their hypoosmotic environment

They lose salts by diffusion.

Freshwater animals maintain water balance by
excreting large amounts of dilute urine

Salts lost by diffusion are replaced by foods and
uptake across the gills
Osmotic water gain
through gills and other parts
of body surface
Uptake of
water and some
ions in food
Uptake of
salt ions
by gills
Excretion of
large amounts of
water in dilute
urine from kidneys
(b) Osmoregulation in a freshwater fish
Nitrogen excretion

Proteins and nucleic acids both contain nitrogen.

When these substances are metabolized they
are broken down to ammonia. Ammonia is very
soluble in water, but also toxic and must be
excreted quickly.

Because ammonia can be lost through the gills
easily most fish excrete ammonia.
Nitrogen excretion
 In
vertebrates nitrogen is also excreted as
urea and uric acid.
 Both
are less toxic than ammonia.
Nitrogen excretion

Urea is produced from ammonia and has two
advantages. It increases the osmotic
concentration of the blood so in the sea
dehydration is reduced because the
concentration gradient between the blood and
seawater is reduced.

Urea is less toxic than ammonia so it can be
stored when there is too little water for the urea
to be excreted.
Nitrogen excretion

The production of urea was an important trait
that facilitated the invasion of the land.

Lobe-finned fishes however probably evolved
urea production because it reduced osmotic
dehydration.

Uric acid requires very little water for excretion
and is the main form of nitrogen waste in dry
environments.
Temperature regulation

Because of the high heat capacity and heat
conductivity of water it is difficult for organisms
to maintain a difference between their body
temperature and the surrounding water
temperature.
 In air, in contrast, it is comparatively easy to do
so.
 For fish, however, there is much less variation in
the temperature of water over time and many
fish live in water that hardly changes
temperature over a year.
Temperature regulation

Historically the terms poikilotherm (variable
heat) and homeotherm (same) were used to
categorize organisms into those whose body
temperatures varied over time or stayed
constant.
 However, often organisms don’t fit neatly into
these categories (e.g. hibernating mammals let
their temperatures fall)
 Ectotherm and Endotherm however are better
terms as they describe the sources of heat and
most organisms use a combination.
Temperature regulation of aquatic
vertebrates: fish
 Many
fishes display regional heterothermy
in which they keep the core of the body
much warmer (up to 15ºC) than the
surrounding water.
 In
some sharks, such as the mako and
great white, countercurrent heat
exchangers keep the core 5-10ºC warmer
than the water
Temperature regulation of aquatic
vertebrates: fish

Tuna have myoglobin rich swimming muscles
which produce a lot of heat and are kept at
about 30ºC, again by a rete system. Tuna and
sharks also use rete in the brain and eyes to
retain heat in those organs.

In billfishes (swordfish, marlin, sailfish) the
superior rectus muscle of the eye has evolved
into an exclusively heat generating structure that
keeps the brain and eye warm.
Swordfish, marlin, sailfish
Blue Marlin
http://www.fishingmaui.com/gamefish/blue_marlin_hawaii.jpg
Temperature regulation of aquatic
vertebrates: fish

Being able to keep portions of the body warm is a big
advantage to these fish.

Because of their high core temperature, tuna muscles
can work more efficiently and the fish can swim much
faster.

It also allows the fish to enter cold water that would
otherwise slow their body functions. Swordfish, which
diver deeper and spend more time in cold water, have
better heated organs than do marlin and sailfish which
spend less time in cold water.
Temperature regulation of aquatic
vertebrates: mammals

Because aquatic mammals breathe using lungs
they don’t risk losing heat through blood flow to
gills and can keep the whole body at a high
temperature.

Fully aquatic mammals such as cetaceans
(whales and dolphins) and seals use a thick
layer of blubber as insulation and countercurrent
heat exchangers limit heat loss from the flippers.
Temperature regulation of aquatic
vertebrates: mammals
 Semi-aquatic
mammals (e.g. beavers and
otters) use thick, water-repellant fur coats
to trap air, which is a good insulator.
 Similarly,
diving birds trap air in their
feathers for the same purpose.
Importance of body size

Because surface area increases as a square
function of a linear dimension, but volume
increases as a cube function, larger animals
have proportionally less surface area than
smaller animals of the same shape.

It is not surprising therefore that selection has
favored large body size in marine mammals and
birds.
Importance of body size
 Large
body size also plays a big role in
temperature regulation of leatherback
turtles (the largest of all marine turtles at
up to 850 kg).
 Leatherbacks
are the most specialized
turtles and have replaced their shell with a
a leathery body covering.
Leatherback Turtle
http://mvyps.org/~John_Nelson/01033434-000F4C52.3/Turtle_leatherback-jso1.jpg
Importance of body size

Leatherbacks are pelagic and range from Alaska and
Norway south to the tips of South Africa and South
America where water is often frigid.

The turtle’s large body size and heat exchangers in the
flippers however, enable the animal to maintain a body
temperature 18ºC higher than surrounding water.

Other species of turtles are no more than half the size of
leatherbacks and are confined to much warmer waters
because they cannot maintain a large temperature
difference between themselves and the environment.