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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 viviparous.
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.
Developing skate in its egg case
 http://www.youtube.com/watch?v=ZEpOmj
Lb2e8
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.
Sand tiger shark eats siblings in
utero
 http://www.youtube.com/watch?v=ZrqgPjZ
07Ts&feature=related
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 sharks themselves were not fished for.

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 long-line fisheries for tuna,
swordfish, marlin and other prized species
treated sharks as a nuisance by-catch 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.

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 534-631 depending on
who’s classifying them) than there are sharks.

As is the case for sharks, skates and rays have
a cartilaginous skeleton and an enlarged oilfilled liver that reduces their buoyancy. They
also possess the same electro sensors sharks
do (Ampullae of Lorenzini)
Skates and rays
 Skates



and rays have characteristically
dorsoventrally flattened bodies
greatly enlarged pectoral fins, which attach to
the side of the head.
gill slits placed ventrally and eyes dorsally
placed.
http://www.flmnh.ufl.edu/fish/education/questions/rayskatesawfish.jpg
Skates and rays
 There
are also dorsoventrally flattened
sharks e.g. the wobbegong and angel
sharks.
 They
have large wing-like pectoral fins but
these are not attached to the head.
Spotted wobbegong
http://www.biolib.cz/IMG/GAL/122294.jpg
Skates and rays
 Most
skates and rays depend on
undulating their pectoral fins to swim but
some such as the torpedo rays use their
caudal (tail) fin for movement.
Atlantic torpedo ray
http://www.junglewalk.com/shop/Products/Atlantic-Torpedo-Ray-Magnet-4623.htm
Skates and rays

Skates and rays should not be confused with
flatfishes (e.g. sole and halibut), which are bony
fishes.

In flatfish the body is twisted during development
to bring both eyes and gills to the dorsal surface,
but not symmetrically.
Flatfish (flounder a bony fish note the asymmetrical head)
http://biochemicalsoul.com/2009/02/adaptation-of-the-week-flatfish-recapitulation/
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.
Stingray teeth
http://www.flmnh.ufl.edu/fish/Gallery/
Descript/atlanticstingray/atlanticstingray.html
http://misc.thefullwiki.org/Ray_and_Skate
The
mouth is located underneath the body and
can be rapidly protruded to suck up prey.
Differences between skates and
rays

Skates usually have an elongated but thick tail
stalk, which has two dorsal fins and a caudal fin
at the end.
 Generally skates also have a rostrum a pointed
nose-like extension of the braincase.
http://img.21food.com/20110609/product/1
306507703542.jpg
Skates are oviparous.
Skate egg case
http://people.whitman.edu/~yancey/skateEggCase.JPG
Differences between skates and
rays
 Rays
typically have a whip-like tail and the
tail fins are replaced by serrated venomcontaining barbs.
 Rays
are viviparous and most lack a
rostrum
http://www.marinebiodiversity.ca/skatesandrays/Classification%20Overall.htm
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 because the mouth is
usually buried in the sand.
http://media.photobucket.com/image/spirac
les%20in%20rays/nairboarding/sp.jpg
Skates and rays

Skates and rays are usually well camouflaged and sit on
the bottom to hide from predators.

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), like
whale sharks and basking sharks, are planktivores.
Blue spotted ray
Manta Ray
Stingray
A
stingray’s tail possesses a sharp
pointed, barbed spine halfway along the
length of the tail that it uses for defense.
 Many
species of stingray have a venom
gland associated with the spine and the
venom can be fatal to humans. Stingray
spines have been used as spear points by
many cultures.
Stingray barb.
http://elasmodiver.com/Sharkive%20images/Stingray-Barb-004.jpg
http://www.flmnh.ufl.edu/fish/Gallery/Descript
/atlanticstingray/atlanticstingray.html
Electric rays


The up to 40 species of electric rays belong to
the order Torpediniformes.
They can produce an electrical discharge that is
used for hunting and protection.
 The discharge can range from 8 to 220 volts and
be up to 30 amperes (it’s the amperage that’s
dangerous. Amperage is a measure of the
amount of electricity flowing).
Marbled torpedo ray
http://elasmodiver.com/MarbledTorpedoRay.htm
Electric rays

The electric organs are modified from striated
muscle fibers and consist of stacks of flattened
cells supplied with nerves on one side.

Because the cells are stacked in columnar
series (called electroplaques) the small electrical
charges generated by each cell sum together to
produce a large charge that can stun or kill other
fish.
Electric rays

The electric organs are up to one sixth of an electric rays
body weight and are located on the side of the ray just
ahead of the pectoral fins.

In some species the organs are wired to direct a shock
upwards. In others they are pointed downwards.

Electric rays appear to hunt mostly at night and may
slowly move over a fish and then stun it or lurk buried
under sand until a suitable prey fish swims overhead.
Sawfishes

The seven species of sawfishes inhabit tropical
and subtropical coastal bays and estuaries and
prefer shallow silty water.

They are big (1.4 to 7 meters in length).

They possess a distinctive “tooth” edged rostrum
or saw (the teeth are modified tooth-like
structures called denticles). The rostrum is
covered with motion and electro detectors that
allow the sawfish to detect hidden prey.
http://www.elasmoworld.org/sawfish.html
http://www.elasmoworld.org/sawfish.html
Sawfishes
 Usually
sawfish forage on the bottom and
eat crabs, crustaceans and small fish.
They often stir up silt and mud with their
saws to startle prey.
 Sawfish
also slash with the saw to kill or
disable prey and use the saw to defend
themselves also.
Subclass Holocephali: Chimaeras

Chimaeras are a small group (about 33 species)
of deep sea (>80m and usually considerably
deeper) cartilaginous fishes known commonly as
ratfish or ghostfish.

Because they live mainly in deep water they are
not a well known group.

Their lineage diverged from the sharks about
400 mya. Formerly they were more diverse.
Male spotted ratfish
Subclass Holocephali: Chimaeras





Have a large head often with an elongated snout filled with
sensory structures
Have a cover over the gills (operculum) as in bony fishes)
Have a venomous spine located in front of the dorsal fin.
Lack both a spiracle and stomach.
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.
Chimaeras
 Like
other Chondricthyes males use
claspers for mating and eggs are laid in
leathery cases.
 Males
also possess retractable sexual
appendages on the head and in front of
the pelvic fins that apparently are used to
grasp the female during mating.
Chimaeras
 They
appear to mostly feed on sea
urchins, shrimp, and mollusks that they
grind using plate-like grinding teeth.
 Unlike
in sharks the upper jaws are fused
to the skull.