Living fishes

1
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).

2
16.1
3
16.2
4
Living jawless fishes

• The living jawless fish once were included in the
  Agnatha along with ostracoderms because they
  lack the gnathostome characters of jaws and two
  sets of paired fins.
• Today it is apparanet that the extinct
  ostracoderms are more closely related to the
  gnathostomes than are the living agnathans.

5
Living agnathans

• There are slightly 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).

6
Characteristics of living 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.

7
Hagfishes class Myxini

• Hagfishes are a marine group of deep-sea,
  cold-water 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 theyre pulling.

8
Hagfishes

• Hagfishes feed using two horny plates located
  either side of their tongue that are covered in
  sharp tooth-like structures.
• When the tongue is everted the plates are spread
  apart and when the tongue is retracted the plates
  come together and mesh.

9
16.3
10
Hagfishes

• Hagfishes are considered the sister group of all
  vertebrates because they lack any trace of
  vertebrae.
• They also have many other primitive
  characteristics including simple kidneys and only
  one semicircular canal on each side of the head.

11
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.

12
Hagfishes

• Hagfishes have a remarkable (and revolting)
  ability to generate enormous quantities of slime,
  which they do to defend themselves from
  predators.
• A single individual can fill a bucket with slime.

13
Lampreys Class Cephalaspidomorphi

• Lampreys are similar in general size and shape to
  hagfishes, but are more closely related to
  gnathostomes than are hagfishes.
• Lampreys possess vertebral structures called
  arcualia, tiny cartilaginous skeletal elements
  that are homologous with the neural arches of
  vertebrates.

14
Lampreys

• Unlike hagfishes, lampreys possess large well
  developed eyes and have two semicircular canals.
• They also are not isosmotic. Instead
  well-developed kidneys and chloride cells in the
  gills regulate the concentration of body fluids
  and allow lampreys to live in a wide range of
  salinities.

15
Lampreys

• The lampreys mouth is located at the base of the
  oral hood (a fleshy suction cup lined with
  teeth).
• The oral hood allows the lamprey to latch on
  tight to its prey and once attached the lamprey
  is very hard to dislodge.

16
Lampreys

• 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.

17
16.5
18
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 fishs commercial value.

19
Sea lamprey close up of sucker and teeth
20
16.4
21
(No Transcript)
22
Lampreys

• Because attached lampreys cannot have a
  through-flow of water they have to ventilate
  their gills in a tidal fashion.
• Water is drawn in and pumped out of the gill
  slits, which is not very efficient, but is a
  necessary compromise.

23
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 1950s.
• 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.

24
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.

25
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.

26
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
  heavily armored fish which arose in the early
  Devonian (about 400mya).
• They also possessed paired pelvic and pectoral
  fins that gave them much better control while
  swimming.

27
15.13
Early jawed fishes of the Devonian (400 mya).
28
Evolution of Jaws

• Vertebrate jaws are made of cartilage derived
  from the neural crest, the same material as the
  gill arches (which support the gills).
• Jaws appear to have arisen by modification of the
  first cartilaginous gill arches, which aid in
  gill support and ventilation.

29
Evolution of Jaws

• The advantages of possessing jaws are obvious.
• However, structures must benefit the organism at
  all times or they will not be selected for.
• What use would a proto-jaw have been before being
  fully transformed?

30
Evolution of Jaws

• Mallatt (1996,1998) has suggested that jaws were
  originally important for gill ventilation, not
  grasping prey.
• Gnathostomes have much higher energy demands than
  agnathans. They also possess a series of
  powerful muscles in the pharynx. These muscles
  allow them to both pump water across the gills
  and suck water into the pharynx.

31
Evolution of Jaws

• It is likely that selection initially favored
  enlargement of the 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 relatively easy for the arches to have
  been modified into jaws.

32
Evolution of Jaws

• Being able to close the mouth would have enabled
  the muscles of the pharynx to squeeze water
  forcefully across the gills.
• Selection would have favored any change in gill
  arches and musculature that enhanced water
  movement over the gills.
• Thus, Mallatt suggested that the mandibular
  branchial arch enlarged into protojaws because it
  allowed the entrance to the pharynx to be rapidly
  opened and closed.

33
Evolution of Jaws

• Selection would have favored enlargement and
  strengthening of the mandibular arch to tolerate
  the forces exerted on it by the strong pharyngeal
  muscles.
• Once the proto-jaws can be rapidly closed they
  can also take on a grasping function and new
  selective forces would quickly have driven jaw
  elaboration.

34
15.12
Note resemblance between upper jaw
(palatoquadrate cartilage) and lower
jaw (Meckels cartilage) and gill supports
immediately behind in this Carboniferous shark
35
Evolution of Jaws

• Equipped with jaws for grabbing and holding prey
  and powerful pharyngeal muscles that could suck
  in prey gnathostomes could attack moving prey.
• An enormous diversification of gnathostomes
  followed.

36

• Four major groups of fish are present in the
  Devonian, two now extinct groups (the placoderms
  and acanthodians) and two living (the
  Chondrichthyians, sharks and relatives and the
  Osteichthyians, bony fishes).

37
Placoderms

• The Placoderms are armored fishes that appear to
  be basal to other gnathostomes.
• The oldest known are from the early Silurian.
  Large, heavy plates of dermal bone covered the
  front half of the body and small bony scales
  covered the rest.

38
http//universe-review.ca/I10-29-placoderm.jpg
http//tea.armadaproject.org/Images/deaton/deaton_
5placoderm.JPG.jpg
39
Placoderms

• Most placoderms did not possess true teeth
  (although late forms do, evolved independently of
  the other gnathostomes).
• Instead they had toothlike structures called
  tooth plates that were extensions of the dermal
  jawbones.

40
Arthrodires

• More than half of all known placoderms are
  arthrodires (jointed necks).
• Arthrodires had modified joints between the head
  shield and trunk shield, which gave them an
  enormous gape and made them ferocious predators

41
http//www.palaeos.com/Vertebrates/Units/050Thelod
onti/Images/Gnathostomata1.jpg
Dunkleosteus upper Devonian. 10 meters long.
42
(No Transcript)
43
Placoderms

• Placoderms like ostracoderms declined rapidly in
  the mass extinctions of the late Devonian.
• A few forms survived for about 5 million years
  beyond the last ostracoderms, but the group was
  extinct by the end of the Devonian.

44
Acanthodians

• The other extinct group of fishes is the
  acanthodians, which appear closely related to the
  bony fishes.
• Acanthodians (from the Greek acantha meaning a
  spine) are named for the spines they had in front
  of their numerous fins (as many as six pairs in
  addition to the pelvic and pectoral pairs).

45
http//higheredbcs.wiley.com/legacy/college/levin/
0471697435/ chap_tut/images/nw0273-nn.jpg
Acanthodians
http//people.eku.edu/ritchisong/RITCHISO//Acantho
dian.gifo
46
Acanthodians

• Acanthodians had fusiform bodies and heterocercal
  tails and so were likely midwater fishes.
• Acanthodians became extinct in the early Permian.

47
Chapter 4. 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.

48
Air vs. water

• Density water is 800 times denser than air.
• Because water is dense, aquatic animals dont
  need strong weight bearing skeletons. Gravity
  has little impact on their body structure.
• In contrast, gravity is a constant challenge for
  terrestrial animals.

49
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.

50
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 dont use
  tidal ventilation. Because of the low O2 content
  of water, fish gills have evolved to be very
  efficient at extracting O2.

51
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.

52
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.

53
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.

54
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.

55
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.

56
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.

57
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.

58
(No Transcript)
59
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

60
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.

61
Northern Bluefin Tuna www.nytimes.com
http//www.glaucus.org.uk/mackerel.jpg
62
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.

63
16.25
64
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.

65
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.

66
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.

67
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).

68
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.

69

• 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.

70
Pearl Gourami http//www.thekrib.com/Fish/gourami
.jpg
Tetra. http//animal-world.com/encyclo/fresh/char
acins/ images/SerpaeTetraWFCh_C2418.jpg
71
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.

72
Lungs

• Embryonically, lungs develop as out-pocketings 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.

73
South American Lungfish http//www.ucmp.berkeley.e
du/vertebrates/sarco/lungfish1.jpg
74
Australian Lungfish
75
http//animals.nationalgeographic.com/staticfiles/
NGS/ Shared/StaticFiles/animals/images/primary/gar
.jpg
Longnose gar http//www.biokids.umich.edu/files/12
296/gar_large.jpg
76
.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.

77
Armored Bichir http//www.aquarticles.com/images/G
allo/Armoured20bichir202.gif
http//www.fbas.co.uk/Bichir.jpg
78
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 bladders wall is impermeable to gases,
  but can expand a lot.

79
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.

80
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.

81
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
  hemoglobins oxygen affinity (Bohr effect) and
  oxygen capacity (Root effect) are reduced.

82
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.

83
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 fishs blood is only 0.2
  atmospheres (oxygen pressure at sea level).

84
Rete mirabile

• Why doesnt 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.

85
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.

86
Rete mirabile (below)
87
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.

88
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.

89
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.

90
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.

91
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.

92
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.

93
Large liver of a great white shark
94
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.

95
Nurse sharks
http//www.kidzone.ws/sharks/photos/
Tiger Shark
96
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.

97
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).

98
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.

99
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.

100
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.

101
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 thus 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.

102
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.

103
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.

104
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.

105
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.

106
(No Transcript)
107
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).

108
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.

109
Organs of Lorenzini

• The bioelectric detectors are called ampullary
  organs of Lorenzini and are found in the sharks
  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.

110
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.

111
(No Transcript)
112
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.

113
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.

114
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.

115
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.

116
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.
• These fishes balance water loss by drinking
  seawater and actively excrete salt through their
  gills. They produce little urine.

117
(a) Osmoregulation in a saltwater fish
118
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

119
(b) Osmoregulation in a freshwater fish
120
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.

121
Nitrogen excretion

• In vertebrates nitrogen is also excreted as urea
  and uric acid.
• Both are less toxic than ammonia.

122
Nitrogen excretion

• Urea is produced from ammonia and has two
  advantages. It increases the osmotic
  concentration of the blood so marine waters
  dehydration is reduced.
• Urea is less toxic than ammonia so it can be so
  it can be stored when there is too little water
  for it the urea to be excreted.

123
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.

124
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.

125
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 dont 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.

126
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

127
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.

128
Swordfish, marlin, sailfish
Blue Marlin
http//www.fishingmaui.com/gamefish/blue_marlin_ha
waii.jpg
129
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 affect their body functions.
  Swordfish, which diver deeper and spend more time
  in cold water, have better heater organs than do
  marlin and sailfish which spend less time in cold
  water.

130
Temperature regulation of aquatic vertebrates
mammals

• Because aquatic mammals breathe using lungs they
  dont 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.

131
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.

132
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.

133
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.

134
Leatherback Turtle
http//mvyps.org/John_Nelson/01033434-000F4C52.3/
Turtle_leatherback-jso1.jpg
135
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 turtles 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.