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.
Great White Shark Hammerhead sharks Whale shark Two skates
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.
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.
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.
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.
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.
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.
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/2003attacksummary.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