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

Figure 24.01
16.1

Figure 24.02
16.2

 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.

 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 (Devonian
-- 416-360 mya). although there are older fossils of scales.

 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 threepronged teeth. As in modern sharks the teeth were arranged on a ligamentous band in a whorl-shaped arrangement.

 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.

Cladoselache picture
Cladoselache http://www.dinosoria.com/poissons/cladoselache_03.jpg
Cladoselache lacked the rostrum (snout) of modern sharks

 By the Jurassic (200-146 mya) 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.

Megalodon
 Megalodon ( Carcharodon megalodon ) is another extinct shark species that is the largest predatory shark known.
 Megalodon occurred from 28-1.5 mya and at approximately 16m long (and with a mass estimated at 47 metrics tonnes) it resembled a massive great white shark and was the top ocean predator of its era.

“Megalodon (gray and red) with the whale shark (violet), great white shark
(green), and a human (blue) for scale. Note: The maximum size attained by C. megalodon is indicated by the 20 m scale.” From Wikipedia.
http://upload.wikimedia.org/wikipedia/commons/thumb/0/07/Megalodon_scale
1.png/220px-Megalodon_scale1.png

http://upload.wikimedia.org/wikipedia/com mons/thumb/7/71/Megalodon_tooth_ruler.j
pg/220px-Megalodon_tooth_ruler.jpg
http://upload.wikimedia.org/wikipedia/com mons/thumb/7/7a/Megalodon_shark_jaws_
museum_of_natural_history_068.jpg/300px
Megalodon_shark_jaws_museum_of_natur al_history_068.jpg

http://upload.wikimedia.org/wikipedia/com mons/thumb/1/1f/Carcharodon_megalodon
.jpg/220px-Carcharodon_megalodon.jpg

Megalodon
 Megalodon preyed on anything it wanted including pinnipeds, sea turtles, and frequently whales.
 Fossil whale skeletons contemporaneous with Megalodon show bite damage clearly inflicted by Megalodon .

Megalodon
 Megalodon when attacking large prey is believed to have focused its attack on the rib cage area crushing the delicate organs within.
 It also is believed to have bitten off appendages immobilizing the prey.

Megalodon
 Megalodon’s bite force has been estimated at up to 10 times that of a great white shark and up to five times that of a
Tyrannosaurus rex .
 The large serrated teeth were deep rooted and Megalodon likely behaved like a great white, which shakes its head after biting, thus sawing through its prey.

Megalodon
 The formation of the Isthmus of Panama three mya resulted in large changes in ocean currents and reduced worldwide ocean temperatures.
 Megalodon preferred warmer sea temperatures and the loss of warm water habitat coupled with a substantial decline in the diversity of whales
(also as a result of changing ocean temperatures) is most likely the cause of the species extinction.

Thresher Shark http://dsc.discovery.com/sharks/shark-types/thresher-shark.jpg

 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

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 long and up to 6 m wide.

Figure 24.07
Diversity of sharks

Figure 24.co
Hammerhead
Shark

Hammerhead sharks
Great White Shark
Two skates
Whale shark

Figure 24.12
Skate (left) and ray (right)

Spotted Ratfish http://www.elasmodiver.com/BCMarinelife/images/Spotted-ratfish.jpg

 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 and the megamouth shark, but most of the others are predators of fish, marine mammals, crustaceans and whatever else they can catch.

Whale shark http://animals.nationalgeographic.com/ staticfiles/NGS/
Shared/StaticFiles/animals/images/ primary/whale-shark-with-fish.jpg
Basking Shark http://oursurprisingworld.com/wp-content/uploads/
2008/02/disgusting_fishes_7-basking-shark.jpg

 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 various species of dogfish, the megamouth, and cookie-cutter sharks.

Lesser spotted dogfish http://www.malcolmnobbs.com/favourite/medium/lesser_spotted_dogfish2.jpg

Megamouth shark http://upload.wikimedia.org/wikipedia/com mons/d/dd/Megamouth_shark_japan.jpg
Cookie cutter sharks http://vivaldi.zool.gu.se/Fiskfysiologi_2001/Course_material/
Introduction_fish_evolution/Images/Cookie_cutters.GIF

Spiny dogfish http://elasmodiver.com/Sharkive%20images/Spiny%20dogfish%20059.jpg

 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 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.

 The elasmobranch heterocercal tail contains numerous radial skeletal elements which make it flexible and the vertebral column extends into the dorsal lobe.
 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 shark’s tendency to sink, while the large flat pectoral fins also provide lift to keep the head up.

Figure 24.08
16.6

 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.

 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.

 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

 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

 Barnacles are a major source of drag on boats and on many marine mammals e.g. whales. However, shark skin doesn’t allow barnacles to grip and grow.
 http://www.youtube.com/watch?v=1hihcIP
G7xo&feature=relmfu

 The shortfin mako shark is capable of swimming in brief bursts at speeds approaching 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

 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)

 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 not embedded in the jaw but 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)

 The type of teeth a shark possesses is dictated by the prey it eats.
 Sharks that feed on crustaceans, mollusks and similar hard shelled prey have dense arrays of flattened teeth designed for crushing.
 Fish feeders have long pointed needle-like teeth for gripping.

Porbeagle shark’s (a fish eater) teeth http://www.elasmodiver.com/Sharkive%20images/PorbeagleSharkTeeth001.jpg

 Sharks that feed on large prey such as mammals have pointed lower teeth and triangular serrated upper teeth for cutting.
 Plankton feeders such as basking sharks have small non functional teeth.

Blue shark upper (top) and lower jaws (bottom) http://www.marinebiodiversity.ca
/shark/english/teeth.htm

 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 .)

 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.

 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

 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.

 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

 When biting a large prey animal a shark seizes the animal and sinks 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.

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

 http://www.youtube.com/watch?v=oIy_eYj
20_0

 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.

 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.

 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.

 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.

 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

 Great White, Tiger and Bull sharks are the big three for shark attacks.
 International shark attack file statistics (documented attacks from1580-2007)

White 237 attacks 64 fatalities


Tiger 88 attacks 28 fatalities
Bull 77 attacks 23 fatalities
 This total of shark deaths in 400 years is approximately equal to the number of people who die annually in Britain from falling out of bed.

Bull shark http://www.sharkdiving.us/images/bull/07.jpg

 Various sharks employ different strategies to obtain prey.

 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. They 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

 http://www.youtube.com/watch?v=nt2ayKadD0&feature=related

 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.


 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.

 The shortfin Mako shark specializes in attacking fast moving prey such as tuna, 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.

 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.

 The musculature in a mako shark that drives its tail is located closer to the central axis of the body than in most other fish. This reduces the amount of energy used in side to side motion and enhances thrust. (Tuna have a similar muscle arrangement).

I http://www.youtube.com/watch?v=dcO1UHVNkE

Shortfin mako http://elasmodiver.com/Sharkive%20images/
Shortfin%20Mako%20Shark%20053.jpg

 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

 http://www.youtube.com/watch?v=BQrrW9
VRMFQ
 5:40-8:20

 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 sharks trail only great whites in numbers of attacks on people, but because they will eat almost 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

 Tiger sharks and turtles
 http://www.youtube.com/watch?v=Q_D51
Ui_XMI
 Tiger shark aggression towards video camera
 http://www.youtube.com/watch?v=5oiWFb
DEe6I

 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.



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

 http://www.youtube.com/watch?v=syISlRJ
Taw4

 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

 http://www.youtube.com/watch?v=uQrBw
N39LJI