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Sharks - Free Sample

Restless Swimmers Conspicuous at first glance are the shark’s gill slits: Contemporary sharks have five; phylogeneti-cally older species have 6 to 7 slits. High sea sharks therefore cannot actively breathe by moving their gill covers like teleostei do. Rather, they must - like marlin and tuna - constantly swim with a slightly open mouth so that water flows through their gills. The only exception here are the sharks that live close to the ground. They can rest on the ground for a while, drawing in water through two holes on their head, just behind the eyes, which are called spiracles. The advantage: The mouth, which is usually in close contact with the seabed, stays shut and neither sand nor sediments find their way into the sensitive gills. Sharks close their gill slits during fast attacks, laying them close to their body to minimize drag. Teeth like Steel Their teeth are very unique. Sharks regenerate their extremely hard, but loosely sitting teeth found in the cartilage jaw "by rows": If a tooth is lost during capture, another will straighten up with a se-ries of additional teeth lying in wait behind a diaphragm. So-called man eater sharks (Carcharhinus types) replace each tooth every 1-2 weeks during the first year of life. Shark teeth almost reach a degree of hardness like steel, as the bite marks on old iron hooks will show. The biting force of the jaws and teeth is enormous. To determine the extent of this force, fiber-glass tubes were filled with fish blood and attached to a steel cable and tension spring scale while being dragged behind a boat. The results shocked the biologists: A 2.5 meter long silvertip shark bit onto and held the savory bait up to a tensile load of 414 kg. Tiger sharks with a length of about 3 meters managed 612 kg while a 4.7 meter long great white shark immediately tore the steel cable from the tension scale with a load-bearing capacity of one ton during the first attack. In addition, examinations of the bite marks revealed that sharks two to three meters in length have a biting strength of three tons per square centimeter. 11


Sharks - Free Sample
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