Class Chondrichthyes: cartilaginous fishes

1
Class Chondrichthyes cartilaginous fishes

• The class Chondrichthyes has two subclasses
• Elasmobranchii, which includes the sharks and
  rays.
• Holocephali the chimaeras ratfish and
  ghostfish.

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16.1
3
16.2
4
Class Chondrichthyes

• Modern Chondricthyes include the sharks, rays and
  Chimeras.
• 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.
• There are just under 1000 living species, all of
  which have cartilaginous skeletons, even though
  they are descended from ancestors that had bone.

5
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.
• The oldest unambiguous Chondrichthyans are found
  in the Early Devonian although there are older
  fossils of scales.

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Fossil history of Chondrichtyes

• One of the best known extinct genera is
  Cladoselache a pelagic marine predator from the
  Devonian.
• It was shark-like in appearance. About 2 meters
  long with a large gape and three-pronged teeth.
  As in modern sharks the teeth were arranged on a
  ligamentous band in a whorl-shaped arrangement.

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Fossil history of Chondrichtyes

• Cladoselache had two dorsal fins, each preceded
  by a large spine.
• It also possessed paired pelvic and pectoral fins
  as in modern sharks, but the fins were much more
  broad based than in later sharks.
• The tail was symmetrical externally, but
  internally asymmetrical with the notochord
  extending into the upper lobe of the tail.

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Cladoselache picture
Cladoselache
http//www.dinosoria.com/poissons/cladoselache_03.
jpg
9
Fossil history of Chondrichtyes

• Cladoselaches skin had few scales found on the
  fins and around the eyes.
• In addition, Cladoselache lacked the rostrum of
  modern sharks.

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Fossil history of Chondrichtyes

• A contemporary genus of Cladoselache was
  Xenacanthus a freshwater shark.
• A bottom dweller with robust fins and a heavily
  calcified skeleton.
• Xenacanths appeared in the Devonian and died out
  in the Triassic.

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www.toyen.uio.no/ .../montre/english/x508.htm
Xenacanthus
http//dinosaurcollector.files.wordpress.com/2008/
11/kaiyodo-xenacanthus.jpg
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Fossil history of Chondrichtyes

• In the Carboniferous sharks with modifications to
  feeding and locomotor structures arose.
• An example is Hybodus of the late Triassic. It
  had heterodont dentition. Anterior teeth had
  sharp cusps for piercing and slashing softer
  foods. Posterior teeth were flattened presumably
  for crushing crustaceans and mollusks.

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Fossil history of Chondrichtyes

• Hybodus also had pelvic and pelagic fins
  supported by a narrow base made up of lengths of
  cartilage.
• The narrow base (as in modern sharks) allowed the
  fin to be rotated.
• Hybodus also had an anal fin and a true
  heterocercal tail.

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Fossil history of Chondrichtyes

• The elasmbranch heterocercal tail contains
  numerous radial skeletal elements which make it
  flexible.
• Its shape can be controlled by intrinsic muscles.
• When undulated from side to side the tail because
  of its shape generates both forward and upward
  thrust, which counteracts a sharks natural
  tendency to sink.

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Fossil history of Chondrichtyes

• Hybodus resembled its Cladoselache-like ancestors
  by retaining dorsal fin spines and its terminal
  mouth.

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Hybodus
http//www.resourcemodels.org/hybodus1.jpg
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Extant radiation of Chondrichthyes

• By the Jurassic sharks of modern appearance had
  evolved. Several genera from that era are still
  extant.
• The most distinctive feature of modern sharks is
  the rostrum or snout that overhangs the mouth.
• Less prominent, but also of major importance was
  the evolution of solid calcified vertebrae.
• Finally, the teeth are covered with thicker more
  complex enamel than in earlier sharks.

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Thresher Shark
http//dsc.discovery.com/sharks/shark-types/thresh
er-shark.jpg
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Class Chondrichthyes

• About 1000 living species divided into two
  distinct groups
• Neoselachii also known as elasmobranchs
  (sharks, skates and rays) about 950 species.
• Holocephalii (ratfishes). About 33 species.

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Neoselachii

• Neoselachii
• Galeomorpha about 279 species of sharks with an
  anal fin. 1m to perhaps 18m in length. Sand
  tigers, mackerel sharks, threshers, basking
  sharks, hornsharks, whale sharks, nurse sharks,
  mako, great white.
• Squalomorpha Not a monphyletic group. About
  124 species of deep sea sharks, dogfish, angel
  sharks. 15cm to 7m.
• Batoidea skates and rays. At least 534 species.
  Electric rays, Manta rays, stingrays, skates.
  1-6m and up to 6 m wide.

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Diversity of sharks
22
Hammerhead Shark
23
Great White Shark
Hammerhead sharks
Whale shark
Two skates
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Two species of ray
25
Spotted Ratfish http//www.elasmodiver.com/BCMarin
elife/images/Spotted-ratfish.jpg
26
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, as is the basking shark, but
  most of the others are predators of fish, marine
  mammals, crustaceans and whatever else they can
  catch.

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Basking Shark http//oursurprisingworld.com/wp-con
tent/uploads/ 2008/02/disgusting_fishes_7-basking-
shark.jpg
Whale shark
http//animals.nationalgeographic.com/ staticfiles
/NGS/ Shared/StaticFiles/animals/images/ primary/w
hale-shark-with-fish.jpg
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Sharks

• The extant sharks include at least two lineages
  and molecular studies suggest there may be
  several others included within these two.
• The squaloid sharks are smaller brained, mostly
  live in cold, deep water and include the dogfish,
  megamouth, and cookie-cutter sharks.

29
http//www.flmnh.ufl.edu/fish/Gallery/descript /Me
gamouth/cookie.JPG Cookie-cutter shark
http//vivaldi.zool.gu.se/Fiskfysiologi_2001/Cours
e_material/ Introduction_fish_evolution/Images/Coo
kie_cutters.GIF
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Sharks

• The galeoid sharks are the dominant carnivores of
  shallow, warm species rich parts of the ocean.
• They include hammerheads, tiger sharks,
  threshers, mackeral sharks, and the whale shark.

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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.

32
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.

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16.6
34
Sharks

• A typical shark is about 2m long, but they range
  in size from a few miniature forms that are 25 cm
  long up to perhaps 18m in length.
• Despite their range of sizes all modern sharks
  share a suite of characteristics.

35
Characteristics of sharks

• The cartilaginous vertebral centra of sharks are
  distinctive.
• Adjacent vertebrae have depressions in their
  faces into which fit spherical remnants of the
  notochord.
• This arrangement of a rigid vertebral column of
  calcified cartilage swivelling on bearings of
  notochord allows the axial skeleton to swing from
  side to side.

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Dorsal intercalary plate
37
Characteristics of sharks

• In addition to the neural and hemal arches in the
  vertebral column, which protect the spinal cord
  and blood vessels all sharks possess additional
  intercalary plates that provide extra protection
  to the nerve cord and blood vessels.

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Dorsal intercalary plate
39
Sharks

• Unlike earlier sharks, living species have their
  skin entirely 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 skin is also very
  streamlined.

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16.15
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Mako shark skin

• The shortfin mako shark is capable of swimming in
  brief bursts at speeds approahing 50mph (kph).
• Recent research has shown that its skin is able
  to reduce drag by bristling, which creates tiny
  depressions across the surface of the skin (like
  those on a golf ball).

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Shortfin mako Shark http//elasmodiver.com/images/
Shortfin-Mako-022.jpg
43
Mako shark skin

• The 200 micrometer long scales when held at 90
  degrees to the sharks body cause tiny vortices
  to form in between the scales.
• These vortices prevent a turbulent wake from
  forming, which would exert a backwards pull.
• (Lang et al. 2008. Bioinspiration and
  Biomimetrics New Scientist 15 Nov 2008, p.16)

44
Teeth

• The placoid scales are modified in the mouth to
  produce the rows of replaceable teeth
  characteristic of sharks.
• Each tooth in a shark can be rapidly replaced as
  it becomes worn or damaged. Teeth are arranged
  on a spiral or whorl shaped cartilaginous band in
  which replacement teeth are always developing
  behind the functional tooth.
• Teeth in young sharks may be replaced as often as
  once every 8 days.

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16.6
46
http//www.sharkattackphotos.com/Shark_Miscellaneo
us.htm
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Sand tiger shark (note multiple rows of teeth)
48
Shark Jaws

• A sharks jaws can open in a variety of different
  positions depending on the prey.
• This is because the upper jaw is attached
  flexibly to the chondocranium in two locations
  (front and back) both of which can move. This is
  called a hyostylic jaw suspension.
• (Movement of parts of the head skeleton is called
  cranial kinesis.)

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Shark Jaws

• When the upper jaw is protruded, the
  hyomandibular cartilage which braces the rear of
  the upper jaw (the palatoquadrate) swings to the
  side and anteriorly which increases the distance
  between the right and left jaw articulations and
  the volume of the mouth.

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Shark Jaws

• The increase in volume is possible because the
  upper jaw attachment to the chondocramnium at the
  front is by elastic ligaments and so the upper
  jaw can move.
• The increase in volume powerfully sucks water and
  food into the mouth.

51
Great White Shark http//img.dailymail.co.uk/i/pix
/2007/07_03/19sharkDM_468x591.jpg
52
Shark Jaws

• Protrusion of the upper jaw moves the mouth away
  from the head and allows a bigger bite to be
  taken than would be possible if the upper jaw was
  immobile.

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Biting

• The teeth on the upper jaw (palatoquadrate) have
  evolved to bite chunks from large prey items.
• They are bigger than the teeth on the mandible
  and often curved and serrated, which enables the
  shark to saw off a big chunk of flesh.

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Tiger Shark Teeth
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Biting

• When biting a large prey animal a shark seizes
  the animal sinking its upper and lower teeth into
  it.
• The shark then protrudes its upper jaw which
  pushes its teeth deeper into the wound and
  violently shakes its head from side to side.

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Biting

• The head movements from side to side saw off a
  large chunk of flesh, which results in massive
  bleeding.
• Great Whites kill big prey such as sea lions by
  taking a big bite and then waiting for the victim
  to bleed to death.

57
Prey detection

• Sharks use a series of methods to detect prey
  related to distance.
• Chemoreception is used to detect prey from a
  distance and sharks appear to be able to detect
  odors as dilute a 1 part in 10 billion.

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Prey detection

• Vibrations can also be detected from a distance
  using the lateral line system.
• Once a shark gets relatively close, vision takes
  over.
• Sharks have very good vision at low light
  intensities. There is a high density of rods in
  the retina and a tapetum lucidum just behind the
  retina, which reflects light back through the
  retina.

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Prey detection

• In low light conditions the tapetum lucidum is
  beneficial, but in bright light is not.
• In bright light melanin containing cells expand
  to cover the tapetum lucidum.

60
Prey detection

• If a familiar prey item is located an attack may
  occur quickly.
• If the prey is unfamiliar (e.g. a person) the
  shark may circle to gather more information.
• Such a shark may bump the potential prey with its
  rostrum presumably to gather extra sensory
  information.

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Shark attacks on humans

• 1990s 514 documented unprovoked shark attacks on
  humans. About 13 fatal.
• In a typical year there are 3-4 fatalities
  worldwide.
• In U.S. most shark attacks are in Florida.
• http//www.flmnh.ufl.edu/fish/sharks/statistics/20
  03attacksummary.htm

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Shark attacks on humans

• Great White, Tiger and Bull sharks are the big
  three for shark attacks.
• International shark attack file statistics
  (documented attacks1580-2007)
• White 237 attacks 64 fatalities
• Tiger 88 attacks 28 fatalities
• Bull 77 attacks 23 fatalities

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Bull shark http//www.sharkdiving.us/images/bull/0
7.jpg
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Foraging strategies of sharks

• Various sharks employ different strategies to
  obtain prey.

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Great White Shark

• Great White sharks specialize in feeding on
  colonial seals and sealions, but also take a wide
  variety of other prey including dolphins, other
  sharks, turtles and other fish.
• Around sea lion nursery areas sharks attack the
  mammals as they come and go. They remain deep in
  the water until a victim passes within range
  above and then rocket to the surface like a trout
  after a mayfly often exploding out of the water
  and flinging the prey in the air.

66
Great White http//elasmodiver.com/images/Great-Wh
ite-Shark-002.jpg
67
http//imagecache2.allposters.com/images/pic/ NYG/
78027Great-White-Shark-Posters.jpg
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Great White Shark

• Great Whites appear to be relatively intelligent
  and there are reports of them cooperating to
  attack a seal.
• There are also reports that they are very curious
  and they will often raise their head out of the
  water to look something over.

69
Great White Shark

• A lot of attacks on humans by Great Whites are
  likely cases of mistaken identity as a surfer on
  a surfboard looks a lot like a sea lion.
• Frequently people bitten by a Great White are
  released. Humans (and sea otters) lack of
  blubber results in them often being released
  after an initial bite.

70
Shortfin mako

• The shortfin Mako shark specializes in attacking
  fast moving prey such as bluefish, mackerel,
  bonito, swordfish, sailfish as well as dolphins
  and porpoises.
• A study of mako stomach contents of sharks taken
  off of the eastern U.S. found that bluefish made
  up about 77 of the diet by volume.

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Shortfin mako

• Because it hunts such fast prey, makos have to be
  fast and athletic. Its speed has been recorded
  at 50km/h (31 mph), but in bursts it can
  accelerate to 74 km/h (46 mph).
• Makos often leap high out of the water in pursuit
  of prey and there have been several instances of
  hooked makos landing on the decks of fishing
  boats.

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Shortfin mako http//elasmodiver.com/Sharkive20im
ages/ Shortfin20Mako20Shark20053.jpg
73
Thresher Shark

• A Thresher shark is instantly identifiable thanks
  to the enormously elongated upper lobes of its
  tailfin.
• The tail plays a central role in their hunting
  strategy. Either working alone or in groups
  threshers surround groups of pelagic fish and
  stun or disorient them using their tails.

74
http//www.shark-pictures.com/viewpic/thresher-sha
rk-134.html
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Tiger shark

• Tiger sharks are indiscriminate consumers and
  will eat almost anything.
• Their powerful jaws allow them to crack turtles
  shells and clams.
• Stomach contents of captured sharks have included
  seals, sea snakes, birds, fish, squid and even
  old tires.

76
Tiger shark

• Tiger sharks trail only great whites in numbers
  of attacks on people, but because they will eat
  almsot anything they rarely leave after biting a
  human, as great whites often do.

77
Tiger shark http//www.fearbeneath.com/wp-content/
uploads/ 2008/09/tiger-shark-roger-horrocks.jpg
78
Cookiecutter shark

• Cookiecutter are bizarrely specialized predators
  that bite disk-shaped pieces of tissue out of
  much larger animals.
• Cookiecutter sharks attach to their prey with
  their lips and then quickly spin using their
  proportionally enormous teeth to carve out a
  piece of flesh.
• Cookiecutter sharks feed on megamouth, basking
  and whale sharks as well as fish such as tuna and
  marlin as well as dolphins and whales.

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Cookiecutter shark

• Cookiecutters are bioluminescent and appear to
  use this ability to attract victims.
• On the ventral surface cookiecutters glow along
  their whole length except for a dark patch of
  skin under the jaw.
• The bioluminescent areas hide the shark against
  the light of the surface water, but the dark
  patch stands out and acts as a lure for predatory
  fish, which when they attack end up being bitten
  by the shark.

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http//www.shark-pictures.com/viewpic/cookie-cutte
r-shark-teeth-structure-625.html
81
Whale Shark

• Whale sharks are filter feeder that sieve
  plankton, krill and other small prey from the
  water.
• The prey is trapped using 10-cm long gill rakers,
  which are bristle-like structures that sieve the
  water before it passes through the gill slits.
• Whale sharks filter about 1500 gallons (6000
  liters) of water each hour. Basking sharks and
  megamouth sharks also filter feed.

82
http//elasmodiver.com/Sharkive20images/Whale-sha
rk-061.jpg
83
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.

84
Great white shark claspers
85
Reproduction

• During copulation a clasper is inserted into the
  females cloaca and hooked in place by spines at
  the tip.
• Sperm is ejaculated into a groove in the clasper
  and a muscular siphon sac filled with seawater is
  squeezed which washes the sperm down the groove
  into the cloaca from where the sperm swim up the
  females reproductive tract.

86
Reproduction

• The sharks use of internal fertilization is
  coupled with their use of a reproductive strategy
  in which a few young are invested in heavily.
• The energy investment is provided by the female
  who retains and nourishes a small number of
  offspring within her body.

87
Reproduction

• Energy is provided either in the form of egg yolk
  or is delivered to the developing babies via the
  mothers reproductive tract.
• The mode of nutrition depends on whether
  reproduction is oviparous or vivaparous.

88
Reproduction

• All skates and some sharks are oviparous and lay
  eggs soon after fertilization. The eggs hatch
  later.
• Most oviparous sharks produce large eggs with big
  yolks and a proteinaceous case is secreted around
  the fertilized egg.

89
Reproduction

• Protuberances on the case entangle in vegetation
  or the substrate and hold it in place.
  Development takes 6-10 months within the case.
• Movements of the embryo bring in oxygen and flush
  out wastes.

90
Reproduction

• Other sharks are ovoviviparous. The eggs develop
  within the mothers body and hatch either in her
  or just after being released from her.

91
Egg case of cat shark
Embryo of deep sea cat shark. There is a very
large yolk sac to support the embryos growth.
92
Reproduction

• The remaining species of shark are viviparous and
  the offspring are nourished by a placenta,
  unfertilized eggs or smaller siblings.
• These forms of food supply are collectively
  referred to as matrotrophy.

93
Placental feeding of young

• Some sharks develop long stringy extensions of
  the oviduct. These secrete a milky substance
  into the mouths and gill openings of the young.
• The commonest form of viviparity in sharks uses a
  yolk sac placenta which allows the developing
  baby to obtain nutrition from its mothers blood
  stream.

94

• In great white sharks and sand tiger sharks the
  young feed on extra eggs ovulated by the mother
  and also on their siblings.

95
Life history strategy of sharks

• Sharks because they invest heavily in individual
  offspring produce relatively few young.
• This reproductive strategy is similar to that of
  humans and elephants.

96
Life history strategy of sharks

• Humans, elephants and sharks all have a high
  expectation of survival and they have what is
  called a type I survivorship curve.

97
Life history strategy of sharks

• Survivorship curves can be classified into three
  general types
• Type I, Type II, and Type III

Figure 52.5
98
Type I curve

• Type I curve typical of animals that produce few
  young but care for them well (e.g. humans,
  elephants).
• Death rate low until late in life where rate
  increases sharply as a result of old age (wear
  and tear, accumulation of cellular damage,
  cancer).

99
Type II curve

• Type II curve has fairly steady death rate
  throughout life (e.g. rodents).
• Death is usually a result of chance processes
  over which the organism has little control (e.g.
  predation)

100
Type III curve

• Type III curve typical of species that produce
  large numbers of young which receive little or no
  care (e.g. Oyster).
• Survival of young is dependent on luck. Larvae
  released into sea have only a small chance of
  settling on a suitable substrate. Once settled
  however, prospects of survival are much better
  and a long life is possible.

101
Life history strategy of sharks

• Because sharks are slow breeders their
  populations are very vulnerable to an increase in
  adult mortality and/or a reduction in survival of
  offspring.
• In recent years fishing has drastically increased
  adult mortality and caused many shark populations
  to decline sharply.

102
Fishing and sharks

• Historical records by early explorers, merchants
  and others often mention the number and large
  size of the sharks that trailed their ships.
• Sharks were competitors for the schools of
  herring, mackerel, capelin and other commercial
  that humans hunted, but not fished for
  themselves.
• In the early 20th century the seas of the world
  still teemed with sharks, but that has changed
  dramatically.

103
Fishing and sharks

• In the 1950s longline fisheries for tuna,
  swordfish, marlin and other prized species
  treated sharks as a nuisance bycatch and many
  were cut free.
• Today the growing wealth of Asian countries where
  shark fins are a delicacy has made them a
  valuable catch.

104
http//advocacy.britannica.com/blog/advocacy/wp-co
ntent/uploads/shark-fin.jpg
105
Shark Fins http//www.lessfeelsbetter.net/upload/1
215781554SHARKFI8crop.jpg
106
Fishing and sharks

• Dried sharkfin can cost more than 500 a kilo and
  sharkfin soup up to 90 a bowl.
• Longliners can set lines as much as 100km long
  that contain 30,000 baited hooks and the catches.
• In 1997 Hawaiian longliners caught more than
  100,000 sharks and tossed almost 99 of the body
  mass back. Why? They just kept the fins.

107
http//mythix.com/images/ projects/
Shark_Finning_ -_dead_shark_in_ocean.jpg
http//www.scubadiving.com/upload/images/Travel/20
070326_sharkfinning_head.jpg
108
Fishing and sharks

• As worldwide fish stocks have declined and often
  collapsed less desirable species including sharks
  have been targeted by commercial fishing fleets.
• As cod stocks collapsed, species such as spiny
  dogfish (marketed as rock cod) began to be
  served as a replacement in fish and chips.

109
Fishing and sharks

• The Norwegian fishing fleet targeted sharks of
  the genus Lamna (porbeagles or salmon sharks) for
  intensive fishing to sell as steaks as a
  substitute for swordfish.
• Initial harvests were as much as 8060 tons in a
  year from the northeast Atlantic. Within seven
  years the catch collapsed to 207 tons and hasnt
  been over 100 tons since the 1970s.

110
Porbeagle http//dsc.discovery.com/sharks/shark-t
ypes/porbeagle-shark.jpg
111
Fishing and sharks

• Data from analyses of catch records worldwide
  show similar massive declines worldwide (see
  Callum Roberts The Unnatural History of the
  Sea for sources).
• More than 90 of sharks have been taken from
  massive areas of the worlds oceans.

112
Sharks caught on longlines. http//onfinite.com/li
braries/1353690/324.jpg
113
Fishing and sharks

• Some species populations have been devastated.
• Once the oceanic whitetip was probably the
  commonest large animal in the world. Today its
  numbers have declined 150-fold in the Gulf of
  Mexico and probably by the same amount elsewhere.

114
Oceanic whitetip (Red Sea) http//www.flmnh.ufl.ed
u/fish/gallery/
115
Fishing and sharks

• Along with declines in numbers another pattern
  common to other fisheries has emerged, the sizes
  of the animals caught has fallen.
• Between the 1950s and 1990s the size of
  individuals caught fell in a variety of species.

116
Fishing and sharks

• Percentage decline in size of individuals caught
• Oceanic whitetips (-33)
• Mako (-50)
• Blue (-50)
• Dusky (-60)
• Silky (-83)

117
Fishing and sharks

• This decline is because fishing often
  preferentially removes older animals and even if
  it doesnt fishing pressure is so intense that
  animals dont live long enough to grow large.

118
Fishing and sharks

• The simple truth is that shark populations cannot
  be intensively harvested sustainably.
• They are long-lived, slow maturing and slow
  reproducing.

119
Fishing and sharks

• For example, female spiny dogfish do not mature
  until about 10-12 years of age and produce only
  2-14 pups biennially.
• They can live 40-50 years, but not with fishing
  pressure.
• Other sharks have similar reproductive profiles.

120
Fishing and sharks

• Even with a total ban on fishing, overfished
  shark populations will take many, many years to
  recover.

121
Skates and rays

• More than half of all elasmobranchs are skates
  and rays.
• More species (about 534) than there are sharks.
• They have characteristically dorsoventrally
  flattened bodies and greatly enlarged pectoral
  fins, which they swim with using a wavelike
  motion.

122
Skates and rays

• Skates and rays should not be confused with
  flatfishes (e.g. sole and halibut), which are
  bony fishes.
• Skates and rays have gill slits placed ventrally
  and eyes dorsally placed.
• In flatfish the body is twisted during
  development to bring both eyes and gills to the
  dorsal surface, but not symmetrically.

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Skates and rays

• The group is specialized for bottom dwelling and
  feeding on hard foods (e.g. molluscs and
  crustaceans) that have to be ground up.
• Teeth are flat crowned plates that form an
  arrangement like paving stones.
• The mouth is located underneath the body and can
  be rapidly protruded to suck up prey.

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Differences between skates and rays

• Skates have an elongated but thick tail stalk,
  which has two dorsal fins and a caudal fin at the
  end.
• Skates are oviparous.
• Generally skates also have a rostrum a pointed
  nose-like extension of the braincase.
• Rays typically have a whip-like tail and the fins
  are replaced by serrated venom-containing barbs.
• Rays are viviparous and most lack a rostrum

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http//www.flmnh.ufl.edu/fish/education/questions/
rayskatesawfish.jpg
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Skates and rays

• The spiracles (openings behind the eye) 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.

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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).
• Most species are bottom feeders that eat
  invertebrates. However, the largest species
  (e.g. manta rays) as in sharks are planktivores.

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Blue spotted ray
Manta Ray
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Skate egg case http//people.whitman.edu/yancey/s
kateEggCase.JPG
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Subclass Holocephali Chimaeras

• Chimaeras are a small group (about 33 species) of
  deep sea (gt80m) cartilaginous fishes known
  commonly as ratfish or ghostfish.
• Because they live mainly in deep water they are
  not a well known group.

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Male spotted ratfish
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Subclass Holocephali Chimaeras

• They have a large head, plate-like grinding
  teeth, a cover over the gills and lack both a
  spiracle and stomach.
• They appear to mostly feed on sea urchins,
  shrimp, and mollusks.
• The tail is thin and tapers to a point (hence the
  name ratfish) and not much use in swimming.
  Instead chimaeras depend on flapping their
  pectoral fins for much of their movement.