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in scientific studies 29 they are sometimes given their own order echinorhiniformes 28 anatomy main article shark anatomy general anatomical features of sharks teeth main article shark tooth the teeth of tiger sharks are oblique and serrated to saw through flesh shark teeth are embedded in the gums rather than directly affixed to the jaw and are constantly replaced throughout life multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward like a conveyor belt some sharks lose 30 000 or more teeth in their lifetime the rate of tooth replacement varies from once every 8 to 10 days to several months in most species teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row which is observed in the cookiecutter shark 30 tooth shape depends on the shark s diet those that feed on mollusks and crustaceans have dense and flattened teeth used for crushing those that feed on fish have needle like teeth for gripping and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting the teeth of plankton feeders such as the basking shark are small and non functional 31 skeleton shark skeletons are very different from those of bony fish and terrestrial vertebrates sharks and other cartilaginous fish skates and rays have skeletons made of cartilage and connective tissue cartilage is flexible and durable yet is about half the normal density of bone this reduces the skeleton s weight saving energy 32 because sharks do not have rib cages they can easily be crushed under their own weight on land 33 jaw the jaws of sharks like those of rays and skates are not attached to the cranium the jaw s surface in comparison to the shark s vertebrae and gill arches needs extra support due to its heavy exposure to physical stress and its need for strength it has a layer of tiny hexagonal plates called tesserae which are crystal blocks of calcium salts arranged as a mosaic 34 this gives these areas much of the same strength found in the bony tissue found in other animals generally sharks have only one layer of tesserae but the jaws of large specimens such as the bull shark tiger shark and the great white shark have two to three layers or more depending on body size the jaws of a large great white shark may have up to five layers 32 in the rostrum snout the cartilage can be spongy and flexible to absorb the power of impacts 35 fins fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia filaments of elastic protein resembling the horny keratin in hair and feathers 36 most sharks have eight fins sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail first direction 33 dermal denticles further information fish scale placoid scales the dermal denticles of a lemon shark viewed through a scanning electron microscope unlike bony fish sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body this works as an outer skeleton providing attachment for their swimming muscles and thus saving energy 37 their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming 38 some species of shark have pigmented denticles that form complex patterns like spots e g zebra shark and stripes e g tiger shark these markings are important for camouflage and help sharks blend in with their environment as well as making them difficult for prey to detect 39 for some species dermal patterning returns to healed denticles even after they have been removed by injury 40 tails tails provide thrust making speed and acceleration dependent on tail shape caudal fin shapes vary considerably between shark species due to their evolution in separate environments sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion this is because the shark s vertebral column extends into that dorsal portion providing a greater surface area for muscle attachment this allows more efficient locomotion among these negatively buoyant cartilaginous fish by contrast most bony fish possess a homocercal caudal fin 41 tiger sharks have a large upper lobe which allows for slow cruising and sudden bursts of speed the tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet whereas the porbeagle shark which hunts schooling fish such as mackerel and herring has a large lower lobe to help it keep pace with its fast swimming prey 42 other tail adaptations help sharks catch prey more directly such as the thresher shark s usage of its powerful elongated upper lobe to stun fish and squid physiology buoyancy unlike bony fish sharks do not have gas filled swim bladders for buoyancy instead sharks rely on a large liver filled with oil that contains squalene and their cartilage which is about half the normal density of bone 37 their liver constitutes up to 30 of their total body mass 43 the liver s effectiveness is limited so sharks employ dynamic lift to maintain depth while swimming sand tiger sharks store air in their stomachs using it as a form of swim bladder bottom dwelling sharks like the nurse shark have negative buoyancy allowing them to rest on the ocean floor some sharks if inverted or stroked on the nose enter a natural state of tonic immobility researchers use this condition to handle sharks safely 44 respiration like other fish sharks extract oxygen from seawater as it passes over their gills unlike other fish shark gill slits are not covered but lie in a row behind the head a modified slit called a spiracle lies just behind the eye which assists the shark with taking in water during respiration and plays a major role in bottom dwelling sharks spiracles are reduced or missing in active pelagic sharks 31 while the shark is moving water passes through the mouth and over the gills in a process known as ram ventilation while at rest most sharks pump water over their gills to ensure a constant supply of oxygenated water a small number of species have lost the ability to pump water through their gills and must swim without rest these species are obligate ram ventilators and would presumably asphyxiate if unable to move obligate ram ventilation is also true of some pelagic bony fish species 45 46 the respiratory and circulatory process begins when deoxygenated venous blood travels to the shark s two chambered heart here the shark pumps blood to its gills via the ventral aorta where it branches into afferent branchial arteries gas exchange takes place in the gills and the reoxygenated blood flows into the efferent branchial arteries which come together to form the dorsal aorta the blood flows from the dorsal aorta throughout the body the deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses from there venous blood re enters the heart ventricle and the cycle repeats 47 thermoregulation most sharks are cold blooded or more precisely poikilothermic meaning that their internal body temperature matches that of their ambient environment members of the family lamnidae such as the shortfin mako shark and the great white shark are homeothermic and maintain a higher body temperature than the surrounding water in these sharks a strip of aerobic red muscle located near the center of the body generates the heat which the body retains via a countercurrent exchange mechanism by a system of blood vessels called the rete mirabile miraculous net the common thresher and bigeye thresher sharks have a similar mechanism for maintaining an elevated body temperature 48 larger species like the whale shark are able to conserve their body heat through sheer size when they dive to colder depths the scalloped hammerhead closes its mouth and gills when diving to depths of around 800 metres holding its breath until it reaches warmer waters again 49 osmoregulation in contrast to bony fish with the exception of the coelacanth 50 the blood and other tissue of sharks and chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea up to 2 5 51 and trimethylamine n oxide tmao allowing them to be in osmotic balance with the seawater this adaptation prevents most sharks from surviving in freshwater and they are therefore confined to marine environments a few exceptions exist such as the bull shark which has developed a way to change its kidney function to excrete large amounts of urea 43 when a shark dies the urea is broken down to ammonia by bacteria causing the dead body to gradually smell strongly of ammonia 52 53 research in 1930 by homer w smith showed that sharks urine does not contain sufficient sodium to avoid hypernatremia and it was postulated that there must be an additional mechanism for salt secretion in 1960 it was discovered at the mount desert island biological laboratory in salsbury cove maine that sharks have a type of salt gland located at the end of the intestine known as the rectal gland whose function is the secretion of chlorides 54 digestion digestion can take a long time the food moves from the mouth to a j shaped stomach where it is stored and initial digestion occurs 55 unwanted items may never get past the stomach and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth 56 one of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines this short length is achieved by the spiral valve with multiple turns within a single short section instead of a long tube like intestine the valve provides a long surface area requiring food to circulate inside the short gut until fully digested when remaining waste products pass into the cloaca 55 fluorescence a few sharks appear fluorescent under blue light such as the swell shark and the chain catshark where the fluorophore derives from a metabolite of kynurenic acid 57 senses smell the shape of the hammerhead shark s head may enhance olfaction by spacing the nostrils further apart sharks have keen olfactory senses located in the short duct which is not fused unlike bony fish between the anterior and posterior nasal openings with some species able to detect as little as one part per million of blood in seawater 58 the size of the olfactory bulb varies across different shark species with size dependent on how much a given species relies on smell or vision to find their prey 59 in environments with low visibility shark species generally have larger olfactory bulbs 59 in reefs where visibility is high species of sharks from the family carcharhinidae have smaller olfactory bulbs 59 sharks found in deeper waters also have larger olfactory bulbs 60 sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril 61 this is similar to the method mammals use to determine direction of sound they are more attracted to the chemicals found in the intestines of many species and as a result often linger near or in sewage outfalls some species such as nurse sharks have external barbels that greatly increase their ability to sense prey sight eye of a bigeyed sixgill shark hexanchus nakamurai shark eyes are similar to the eyes of other vertebrates including similar lenses corneas and retinas though their eyesight is well adapted to the marine environment with the help of a tissue called tapetum lucidum this tissue is behind the retina and reflects light back to it thereby increasing visibility in the dark waters the effectiveness of the tissue varies with some sharks having stronger nocturnal adaptations many sharks can contract and dilate their pupils like humans something no teleost fish can do sharks have eyelids but they do not blink because the surrounding water cleans their eyes to protect their eyes some species have nictitating membranes this membrane covers the eyes while hunting and when the shark is being attacked however some species including the great white shark carcharodon carcharias do not have this membrane but instead roll their eyes backwards to protect them when striking prey the importance of sight in shark hunting behavior is debated some believe that electro and chemoreception are more significant while others point to the nictating membrane as evidence that sight is important since presumably the shark would not protect its eyes were they unimportant the use of sight probably varies with species and water conditions the shark s field of vision can swap between monocular and stereoscopic at any time 62 a micro spectrophotometry study of 17 species of sharks found 10 had only rod photoreceptors and no cone cells in their retinas giving them good night vision while making them colorblind the remaining seven species had in addition to rods a single type of cone photoreceptor sensitive to green and seeing only in shades of grey and green are believed to be effectively colorblind the study indicates that an object s contrast against the background rather than colour may be more important for object detection 63 64 65 hearing although it is hard to test the hearing of sharks they may have a sharp sense of hearing and can possibly hear prey from many miles away 66 the hearing sensitivity for most shark species lies between 20 and 1000 hz 67 a small opening on each side of their heads not the spiracle leads directly into the inner ear through a thin channel the lateral line shows a similar arrangement and is open to the environment via a series of openings called lateral line pores this is a reminder of the common origin of these two vibration and sound detecting organs that are grouped together as the acoustico lateralis system in bony fish and tetrapods the external opening into the inner ear has been lost electroreception main article electroreception electromagnetic field receptors ampullae of lorenzini and motion detecting canals in the head of a shark the ampullae of lorenzini are the electroreceptor organs they number in the hundreds to thousands sharks use the ampullae of lorenzini to detect the electromagnetic fields that all living things produce 68 this helps sharks particularly the hammerhead shark find prey the shark has the greatest electrical sensitivity of any animal sharks find prey hidden in sand by detecting the electric fields they produce ocean currents moving in the magnetic field of the earth also generate electric fields that sharks can use for orientation and possibly navigation 69 lateral line main article lateral line this system is found in most fish including sharks it is a tactile sensory system which allows the organism to detect water speed and pressure changes near by 70 the main component of the system is the...
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