Sharks Classification Kingdom Animalia Phylum Chordata Class Chondrichthyes

Sharks

Classification • Kingdom Animalia – Phylum Chordata • Class Chondrichthyes –Subclass Elasmobranchii » Superorder Selachimorpha • Known as “sea dogs” until the 16 th century – Name “shark” may have come from the Yucatec Maya

Evolution • Earliest sharks evolved 450 million years ago • Today – 440 species – Smallest – dwarf lanternshark (7 in) – Largest – whale shark (largest fish – 40 ft)

Skeleton • No bones – Use cartilage and connective tissue • Strong , but ½ the density of bone – Reduces weight saves energy – Cartilage may be calcified • “bone-like” – No functional rib cage • Must stay in water

Jaw • Covered in a layer of tesserae – Complex rigid surface • Single tile in a mosiac – Tiny hexagonal plates • Crystal blocks of calcium salts – Most sharks have one layer of tesserae • Great white sharks – up to five

Teeth • Embedded into gums rather than the jaw • Arranged into 2 -3 rows of 20 -30 teeth – Continuously replaced • Gums “conveyor belt” them to the front • Can shed 35, 000 teeth in a lifetime • Tooth characteristics determined by diet • Very important to fossil record – all that remains

Tooth Anatomy

Fins • Supported by unsegmented rays composed of proteins

Tail (Caudal) Fins • 6 different designs found in sharks – Shapes evolved due to different habitats and diets – All are heterocercal in design • Top is longer than the bottom

Buoyancy • No swim bladder • Large livers – Up to 30% of body mass – Contain squalene • Oil that is less dense that water • Sharks hunted for this oil –Put in capsules as a health supplement –Used in vaccines –Also available from plant sources

Respiration • Oxygen removed as water passes through the mouth and over the gills – Process known as ram ventilation • Most sharks can pump water over their gills while at rest –Some cannot (obligate ram ventilators) – Gills not covered (as with most fish)

Circulation • Sharks have a two chambered heart

Thermoregulation • Most sharks are poikilothermic (cold blooded) – Body temp is that of the environment • Family Lamnidae sharks can raise their body temp higher than the water temp (includes mako and great white) – Homeothermic – Strip of red muscle in center of body attached to the rete mirabile (miraculous net)

Rete Mirabile • Uses countercurrent flow design

Osmoregulation • Living in salt water • Large production of urea – Shark’s tissues isotonic (in balance with) the sea water • So, few sharks can live in fresh water – Bull sharks can change their kidney funtion to excrete the urea

Senses – Smell • Very keen olfactory sense – Can detect one part per million of blood in sea water – Even greater attraction to chemicals found in guts of organisms • Leads sharks to sewage outfalls

Olfactory Bulbs (in pink)

Senses – Sight • Eyes similar to most vertebrates – Well adapted to the dark marine environment • Use a tapetum lucidum (bright tapestry) –Reflects light back through the retina to increase amount of useable light –Humans do the same (eyeshine in a photo) –Can contract and dilate their pupils

Senses – Sight • Sharks have eyelids, but don’t blink – Water cleanses eyes • Use nictitating membranes – Covering that moves over eyes to protect them during attacks on prey

Senses – Hearing • May be able to hear prey many miles away – Hard to test • Small opening on each side of the head – Leads to the inner ear • Sense vibrations among the water molecules

Senses – Hearing • Lateralis system – System of canals within the lateral line • Runs the length of the body • Pores expose water to the system • Contains sensory cells with hair-like projections • Also helps with balance and movement

Lateralis System

Senses – Electroreception • Sharks can detect electromagnetic fields produced by all living organisms – Sharks have the greatest electrical sensitivity of any organism – Helps them find prey (even under the sand) – Use Ampullae of Lorenzini • Electrorecptor organs –Hundreds to thousands per shark

Pores (on a tiger shark)

Senses – Electroreception • Ocean currents travel through the magnetic field of the earth – Creates electric fields • May allow sharks to navigate the world’s oceans • Allows sharks to detect temperature gradients