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e 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 neuromast a cell similar to hair cells present in the vertebrate ear that interact with the surrounding aquatic environment this helps sharks distinguish between the currents around them obstacles off on their periphery and struggling prey out of visual view the shark can sense frequencies in the range of 25 to 50 hz 71 life history the claspers of male spotted wobbegong shark egg the spiral egg case of a port jackson shark shark lifespans vary by species most live 20 to 30 years the spiny dogfish has one of the longest lifespans at more than 100 years 72 whale sharks rhincodon typus may also live over 100 years 73 earlier estimates suggested the greenland shark somniosus microcephalus could reach about 200 years but a recent study found that a 5 02 metre long 16 5 ft specimen was 392 120 years old i e at least 272 years old making it the longest lived vertebrate known 74 75 reproduction unlike most bony fish sharks are k selected reproducers meaning that they produce a small number of well developed young as opposed to a large number of poorly developed young fecundity in sharks ranges from 2 to over 100 young per reproductive cycle 76 sharks mature slowly relative to many other fish for example lemon sharks reach sexual maturity at around age 13 15 77 sexual sharks practice internal fertilization 78 the posterior part of a male shark s pelvic fins are modified into a pair of intromittent organs called claspers analogous to a mammalian penis of which one is used to deliver sperm into the female 79 mating has rarely been observed in sharks 80 the smaller catsharks often mate with the male curling around the female in less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the female s oviduct females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during mating the bite marks may also come from courtship behavior the male may bite the female to show his interest in some species females have evolved thicker skin to withstand these bites 79 asexual there have been a number of documented cases in which a female shark who has not been in contact with a male has conceived a pup on her own through parthenogenesis 81 82 the details of this process are not well understood but genetic fingerprinting showed that the pups had no paternal genetic contribution ruling out sperm storage the extent of this behavior in the wild is unknown mammals are now the only major vertebrate group in which asexual reproduction has not been observed scientists say that asexual reproduction in the wild is rare and probably a last ditch effort to reproduce when a mate is not present asexual reproduction diminishes genetic diversity which helps build defenses against threats to the species species that rely solely on it risk extinction asexual reproduction may have contributed to the blue shark s decline off the irish coast 83 brooding sharks display three ways to bear their young varying by species oviparity viviparity and ovoviviparity 84 78 ovoviviparity most sharks are ovoviviparous meaning that the eggs hatch in the oviduct within the mother s body and that the egg s yolk and fluids secreted by glands in the walls of the oviduct nourishes the embryos the young continue to be nourished by the remnants of the yolk and the oviduct s fluids as in viviparity the young are born alive and fully functional lamniforme sharks practice oophagy where the first embryos to hatch eat the remaining eggs taking this a step further sand tiger shark pups cannibalistically consume neighboring embryos the survival strategy for ovoviviparous species is to brood the young to a comparatively large size before birth the whale shark is now classified as ovoviviparous rather than oviparous because extrauterine eggs are now thought to have been aborted most ovoviviparous sharks give birth in sheltered areas including bays river mouths and shallow reefs they choose such areas for protection from predators mainly other sharks and the abundance of food dogfish have the longest known gestation period of any shark at 18 to 24 months basking sharks and frilled sharks appear to have even longer gestation periods but accurate data are lacking 84 oviparity some species are oviparous laying their fertilized eggs in the water in most oviparous shark species an egg case with the consistency of leather protects the developing embryo s these cases may be corkscrewed into crevices for protection the egg case is commonly called a mermaid s purse oviparous sharks include the horn shark catshark port jackson shark and swellshark 84 85 uterus placenta and fetus of a silvertip shark viviparity viviparity is the gestation of young without the use of a traditional egg and results in live birth 86 viviparity in sharks can be placental or aplacental 86 young are born fully formed and self sufficient 86 hammerheads the requiem sharks such as the bull and blue sharks and smoothhounds are viviparous 76 84 behavior the classic view describes a solitary hunter ranging the oceans in search of food however this applies to only a few species most live far more social sedentary benthic lives and appear likely to have their own distinct personalities 87 even solitary sharks meet for breeding or at rich hunting grounds which may lead them to cover thousands of miles in a year 88 shark migration patterns may be even more complex than in birds with many sharks covering entire ocean basins sharks can be highly social remaining in large schools sometimes more than 100 scalloped hammerheads congregate around seamounts and islands e g in the gulf of california 43 cross species social hierarchies exist for example oceanic whitetip sharks dominate silky sharks of comparable size during feeding 76 when approached too closely some sharks perform a threat display this usually consists of exaggerated swimming movements and can vary in intensity according to the threat level 89 speed in general sharks swim cruise at an average speed of 8 kilometres per hour 5 0 mph but when feeding or attacking the average shark can reach speeds upwards of 19 kilometres per hour 12 mph the shortfin mako shark the fastest shark and one of the fastest fish can burst at speeds up to 50 kilometres per hour 31 mph 90 the great white shark is also capable of speed bursts these exceptions may be due to the warm blooded or homeothermic nature of these sharks physiology sharks can travel 70 to 80 km in a day 91 intelligence sharks possess brain to body mass ratios that are similar to mammals and birds 92 and have exhibited apparent curiosity and behavior resembling play in the wild 93 94 there is evidence that juvenile lemon sharks can use observational learning in their investigation of novel objects in their environment 95 sleep all...