2Class ChondrichthyesAbout 1000 living species divided into two distinct groupsNeoselachii [also known as elasmobranchs] (sharks, skates and rays) about 950 species.Holocephalii (ratfishes). About 33 species.
3Neoselachii 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.
10SharksSharks 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.
12SharksThe 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.
14SharksThe 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.
15SharksSharks 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.
16SharksSharks 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.
18SharksA 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.
19Characteristics 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.
21Characteristics 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.
23SharksUnlike 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.
25Mako shark skinThe 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).
27Mako shark skinThe 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 Bioinspiration and Biomimetrics; New Scientist 15 Nov 2008, p.16)
28TeethThe 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.
32Shark JawsA 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.)
33Shark JawsWhen 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.
34Shark JawsThe 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.
36Shark JawsProtrusion 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.
37BitingThe 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.
39BitingWhen 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.
40BitingThe 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.
41Prey detectionSharks 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.
42Prey detectionVibrations 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.
43Prey detectionIn 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.
44Prey detectionIf 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.
45Shark 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.
46Shark attacks on humans Great White, Tiger and Bull sharks are the big three for shark attacks.International shark attack file statistics (documented attacks )White 237 attacks 64 fatalitiesTiger 88 attacks 28 fatalitiesBull 77 attacks 23 fatalities
48Foraging strategies of sharks Various sharks employ different strategies to obtain prey.
49Great White SharkGreat 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.
52Great White SharkGreat 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.
53Great White SharkA 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.
54Shortfin makoThe 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.
55Shortfin makoBecause 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.
57Thresher SharkA 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.
59Tiger sharkTiger 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.
60Tiger sharkTiger 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.
62Cookiecutter sharkCookiecutter 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.
63Cookiecutter sharkCookiecutters 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.
65Whale SharkWhale 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.
67ReproductionReproduction 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.
69ReproductionDuring 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.
70ReproductionThe 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.
71ReproductionEnergy 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.
72ReproductionAll 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.
73ReproductionProtuberances 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.
74ReproductionOther sharks are ovoviviparous. The eggs develop within the mothers body and hatch either in her or just after being released from her.
75Embryo of deep sea cat shark. There is a very large yolk sac to Egg case of cat sharkEmbryo of deep sea cat shark.There is a very large yolk sac tosupport the embryo’s growth.
76ReproductionThe 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.
77Placental 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.
78In great white sharks and sand tiger sharks the young feed on extra eggs ovulated by the mother and also on their siblings.
79Life 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.
80Life 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.
81Life history strategy of sharks Survivorship curves can be classified into three general typesType I, Type II, and Type IIIIIIIII501001101,000Percentage of maximum life spanNumber of survivors (log scale)Figure 52.5
82Type I curveType 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).
83Type II curveType 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)
84Type III curveType 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.
85Life 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.
86Fishing and sharksHistorical 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.
87Fishing and sharksIn 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.
90Fishing and sharksDried 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.
92Fishing and sharksAs 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.
93Fishing and sharksThe 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.
95Fishing and sharksData 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.
97Fishing 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.
99Fishing and sharksAlong 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.
100Fishing and sharks Percentage decline in size of individuals caught Oceanic whitetips (-33%)Mako (-50%)Blue (-50%)Dusky (-60%)Silky (-83%)
101Fishing and sharksThis 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.
102Fishing and sharksThe simple truth is that shark populations cannot be intensively harvested sustainably.They are long-lived, slow maturing and slow reproducing.
103Fishing and sharksFor example, female spiny dogfish do not mature until about years of age and produce only 2-14 pups biennially.They can live years, but not with fishing pressure.Other sharks have similar reproductive profiles.
104Fishing and sharksEven with a total ban on fishing, overfished shark populations will take many, many years to recover.
105Skates and raysMore 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.
106Skates and raysSkates 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.
107Skates and raysThe 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.
108Differences 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
110Skates and raysThe 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.
111Skates and raysSkates 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.
114Subclass 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.
116Subclass 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.