Sharks

Classification Kingdom Animalia Phylum Chordata Class Chondrichthyes Subclass Elasmobranchii Superorder Selachimorpha Known as “sea dogs” until the 16th 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