Transcript Document

Thresher Shark
http://dsc.discovery.com/sharks/shark-types/thresher-shark.jpg
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.
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.

Figure 24.07
Diversity of sharks
Figure 24.co
Hammerhead
Shark
Hammerhead sharks
Two skates
Great White Shark
Whale shark
Figure 24.12
Two species of ray
Spotted Ratfish
http://www.elasmodiver.com/BCMarinelife/images/Spotted-ratfish.jpg
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.
Basking Shark
http://oursurprisingworld.com/wp-content/uploads/
2008/02/disgusting_fishes_7-basking-shark.jpg
Whale shark
http://animals.nationalgeographic.com/
staticfiles/NGS/
Shared/StaticFiles/animals/images/
primary/whale-shark-with-fish.jpg
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.
http://www.flmnh.ufl.edu/fish/Gallery/descript
/Megamouth/cookie.JPG
Cookie-cutter shark
http://vivaldi.zool.gu.se/Fiskfysiologi_2001/Course_material/
Introduction_fish_evolution/Images/Cookie_cutters.GIF
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.
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.
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
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.
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.
Dorsal intercalary plate
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.
Dorsal intercalary plate
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.
Figure 24.18
16.15
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).
Shortfin mako Shark
http://elasmodiver.com/images/Shortfin-Mako-022.jpg
Mako shark skin

The 200 micrometer long scales when held at 90
degrees to the shark’s 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)
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.
Figure 24.08
16.6
http://www.sharkattackphotos.com/Shark_Miscellaneous.htm
Figure 24.09
Sand tiger shark (note multiple rows of teeth)
Shark Jaws

A shark’s 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.)
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.
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.
Great White Shark
http://img.dailymail.co.uk/i/pix/2007/07_03/19sharkDM_468x591.jpg
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.
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.
Tiger Shark Teeth
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.
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.
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.
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.
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.
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.
Shark attacks on humans

1990’s 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
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
Bull shark
http://www.sharkdiving.us/images/bull/07.jpg
Foraging strategies of sharks
 Various
sharks employ different strategies
to obtain prey.
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.
Great White
http://elasmodiver.com/images/Great-White-Shark-002.jpg
http://imagecache2.allposters.com/images/pic/
NYG/78027~Great-White-Shark-Posters.jpg
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.

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.
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.
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.
Shortfin mako
http://elasmodiver.com/Sharkive%20images/
Shortfin%20Mako%20Shark%20053.jpg
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.
http://www.shark-pictures.com/viewpic/thresher-shark-134.html
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.
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.
Tiger shark
http://www.fearbeneath.com/wp-content/uploads/
2008/09/tiger-shark-roger-horrocks.jpg
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.
Cookiecutter shark

Cookiecutters are bioluminescent and appear to
use this ability to attract victims.
 On the ventral surface cookiecutter’s 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.
http://www.shark-pictures.com/viewpic/cookie-cutter-shark-teeth-structure-625.html
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.
http://elasmodiver.com/Sharkive%20images/Whale-shark-061.jpg
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

During copulation a clasper is inserted into the
female’s 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 female’s reproductive tract.
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.
Reproduction
 Energy
is provided either in the form of
egg yolk or is delivered to the developing
babies via the mother’s reproductive tract.
 The
mode of nutrition depends on whether
reproduction is oviparous or vivaparous.
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.
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.
Reproduction
 Other
sharks are ovoviviparous. 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 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.
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.
 In
great white sharks and sand tiger
sharks the young feed on extra eggs
ovulated by the mother and also on their
siblings.
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.
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.
Life history strategy of sharks

Survivorship curves can be classified into
three general types

Type I, Type II, and Type III
Figure 52.5
Number of survivors (log
scale)
1,000
I
100
II
10
III
1
0
100
50
Percentage of maximum life span
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).
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)
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.
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.
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.
Fishing and sharks
 In
the 1950’s 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.
http://advocacy.britannica.com/blog/advocacy/wp-content/uploads/shark-fin.jpg
Shark Fins
http://www.lessfeelsbetter.net/upload/1215781554SHARKFI8crop.jpg
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.
http://mythix.com/images/
projects/ Shark_Finning_
-_dead_shark_in_ocean.jpg
http://www.scubadiving.com/upload/images/Travel/20070326_sharkfinning_head.jpg
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.
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 hasn’t
been over 100 tons since the 1970’s.
Porbeagle
http://dsc.discovery.com/sharks/shark-types/porbeagle-shark.jpg
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 world’s oceans.
Sharks caught on longlines.
http://onfinite.com/libraries/1353690/324.jpg
Fishing and sharks
 Some
species populations have been
devastated.
 Once
the oceanic whitetip was probably
the commonest large animal in the world.
Today it’s numbers have declined 150-fold
in the Gulf of Mexico and probably by the
same amount elsewhere.
Oceanic whitetip (Red Sea)
http://www.flmnh.ufl.edu/fish/gallery/
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 1950’s and 1990’s the size of
individuals caught fell in a variety of
species.
Fishing and sharks
 Percentage
decline in size of individuals
caught





Oceanic whitetips (-33%)
Mako (-50%)
Blue (-50%)
Dusky (-60%)
Silky (-83%)
Fishing and sharks
 This
decline is because fishing often
preferentially removes older animals and
even if it doesn’t fishing pressure is so
intense that animals don’t live long enough
to grow large.
Fishing and sharks
 The
simple truth is that shark populations
cannot be intensively harvested
sustainably.
 They
are long-lived, slow maturing and
slow reproducing.
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.
Fishing and sharks
 Even
with a total ban on fishing,
overfished shark populations will take
many, many years to recover.
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.
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.
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.
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
http://www.flmnh.ufl.edu/fish/education/questions/rayskatesawfish.jpg
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.
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.
Blue spotted ray
Manta Ray
Skate egg case
http://people.whitman.edu/~yancey/skateEggCase.JPG
Subclass Holocephali: Chimaeras

Chimaeras are a small group (about 33 species)
of deep sea (>80m) cartilaginous fishes known
commonly as ratfish or ghostfish.

Because they live mainly in deep water they are
not a well known group.
Male spotted ratfish
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.