69-757: A sawshark or saw shark is a member of a shark order ( Pristiophoriformes / p r ɪ s t i ˈ ɒ f ɒr ɪ f ɔːr m iː z / ) bearing a unique long, saw-like rostrum (snout or bill) edged with sharp teeth, which they use to slash and disable their prey. There are eight species within the Pristiophoriformes, including the longnose or common sawshark ( Pristiophorus cirratus ), shortnose sawshark ( Pristiophorus nudipinnis ), Japanese sawshark ( Pristiophorus japonicas ), Bahamas sawshark ( Pristiophorus schroederi ), sixgill sawshark ( Pliotrema warreni ), African dwarf sawshark ( Pristiophorus nancyae ), Lana's sawshark ( Pristiophorus lanae ) and
138-426: 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. Tiger sharks have
207-764: A brownish grey color and becomes white along the ventral side. Little else is known about the African Dwarf Sawshark as it is a newly discovered species. The shortnose sawshark ( Pristiophorus nudipinnis ) is similar to the longnose sawshark; however, it has a slightly compressed body and shorter more narrow rostrum. It has 13 teeth in front of its barbels and 6 behind. The shortnose sawshark tends to be uniformly slate grey with no markings on its dorsal side and pale white or cream on its ventral side. Females reach around 124 cm (49 in) long, and males reach around 110 cm (43 in) long. These sharks can live to be up to 9 years old. Like other sawsharks,
276-415: 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 . Shark lifespans vary by species. Most live 20 to 30 years. The spiny dogfish has one of
345-418: 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. The respiratory and circulatory process begins when deoxygenated venous blood travels to
414-499: A few known exceptions, such as the bull shark and the river sharks , which can be found in both seawater and freshwater, and the Ganges shark , which lives only in freshwater. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics . They have numerous sets of replaceable teeth. Several species are apex predators , which are organisms that are at
483-514: A group of elasmobranch fish characterized by a cartilaginous skeleton , five to seven gill slits on the sides of the head , and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii ) and are the sister group to the Batoidea ( rays and kin). Some sources extend the term "shark" as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with
552-423: 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. Other tail adaptations help sharks catch prey more directly, such as
621-416: A long blade-like snout. Although they are similar in appearances, saw sharks are distinct from sawfish . Sawfish are not sharks, but a type of ray . The gill slits of the sawfishes are positioned on the underside like a ray, but the gill slits of the saw shark are positioned on the side like a shark. Sawfish can have a much larger size, lack barbels, and have evenly sized teeth rather than alternating teeth of
690-438: A long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca . 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 . Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between
759-450: A long, thin, and flattened snout. Midway down the snout, nasal barbels protrude on both sides of the snout. Near the barbels, the longnose sawshark possesses a pair of ampullae of Lorenzini. It is unique among the sawshark family by having a longer snout than any of its counter species. The longnose sawshark is not very large with lengths ranging from around 14 inches at birth to 38 inches in males and 44 inches in females. They can also grow to
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#1732781018361828-414: A pair of long barbels about halfway along the snout. They have two dorsal fins , but lack anal fins . Genus Pliotrema has six gill slits , and Pristiophorus the more usual five. The teeth of the saw typically alternate between large and small. Saw sharks reach a length of up to 5 feet and a weight of 18.7 pounds, with females tending to be slightly larger than males. The body of a longnose saw shark
897-428: A prize catch. They dwell in the rage of 37–500 m, preferring to stay in the warmer water. They have between 5 and 7 pups from 7–17 eggs. They have these young in the range of 37–50 m deep to make sure the pups are warm. The longnose sawshark aka the common sawshark ( Pristiophorus cirratus ) is one of 9 species within the family Pristiophoridae. It has unique physical characteristics which include
966-445: 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. 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
1035-617: A shark-like morphology, such as hybodonts . Shark-like chondrichthyans such as Cladoselache and Doliodus first appeared in the Devonian Period (419–359 million years), though some fossilized chondrichthyan-like scales are as old as the Late Ordovician (458–444 million years ago). The earliest confirmed modern sharks (selachimorphs) are known from the Early Jurassic around 200 million years ago , with
1104-405: A similar mechanism for maintaining an elevated body temperature. Larger species, like the whale shark, are able to conserve their body heat through sheer size when they dive to colder depths, and the scalloped hammerhead close its mouth and gills when they dives to depths of around 800 metres, holding its breath till it reach warmer waters again. In contrast to bony fish, with the exception of
1173-555: 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. 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. The hearing sensitivity for most shark species lies between 20 and 1000 Hz. A small opening on each side of their heads (not
1242-518: 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. Sharks mature slowly relative to many other fish. For example, lemon sharks reach sexual maturity at around age 13–15. Rostral (anatomical term) Too Many Requests If you report this error to the Wikimedia System Administrators, please include
1311-654: A species of sawshark that inhabits the Philippine coast. It was discovered in 1966 by Dave Ebert, who distinguished it as a new species of sawshark based on its number of rostral teeth. Lana's sawshark was named after Lana Ebert on the occasion of her graduation from the University of Francisco. It has a dark uniform brown color on the dorsal side and a pale white on the ventral side. It is slender bodied, has five gills on each side, and can grow to be around 70 cm. The African dwarf sawshark ( Pristiophorus nancyae )
1380-400: 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
1449-468: A weight of 18.7 pounds. They are known to swim in the waters off the southern coast of Australia's continental shelf. They can also be found in the eastern portion of the Indian Ocean. The longnose sawshark prefers to swim in both the open sea and coastal regions from the surface to a depth of 600m. The longnose Sawshark is known to mainly prey on small crustaceans. It uses its barbels to detect prey on
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#17327810183611518-487: Is a pale yellow to white. This deep water dwelling fish is located off the Northeastern shore of Australia, in depths up to 176–405 m. It averages in size at about 95 cm. Other than its location and appearance little is known of the creature; it is hard to catch due to its ability to travel into the depths of the ocean. The Japanese sawshark ( Pristiophorus japonicus ) is a species of sawshark that lives off
1587-419: Is a small five-gill sawshark that lives off the coast of Mozambique. It was first discovered in 2011 when a specimen was caught off the coast of Mozambique at a depth of 1,600 ft. The African dwarf sawshark has since then been spotted off the coasts of Kenya and Yemen. It can be distinguished from other sawsharks by its location, and by having its barbels closer to its mouth than the end of its rostrum. It has
1656-410: Is covered in tiny placoid scales: modified teeth covered in hard enamel. The body is a yellow-brown color which is sometimes covered in dark spots or blotches. This coloration allows the saw shark to easily blend with the sandy ocean floor. These sharks typically feed on small fish, squid , and crustaceans , depending on species. The function of the sawshark barbels are not well understood, and neither
1725-648: 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. 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
1794-676: Is divided into the superorders Galea (or Galeomorphii ), and Squalea (or Squalomorphii ). The Galeans are the Heterodontiformes , Orectolobiformes , Lamniformes , and Carcharhiniformes . Lamnoids and Carcharhinoids are usually placed in one clade , but recent studies show that Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squaleans are divided into Hexanchiformes and Squalomorpha. The former includes cow shark and frilled shark , though some authors propose that both families be moved to separate orders. The Squalomorpha contains
1863-409: Is high, species of sharks from the family Carcharhinidae have smaller olfactory bulbs. Sharks found in deeper waters also have larger olfactory bulbs. Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril. This is similar to the method mammals use to determine direction of sound. They are more attracted to the chemicals found in
1932-530: Is how they use their rostrum. It is possible they use it in a similar fashion as sawfishes, and hit prey with side-to-side swipes of the saw, crippling them. The saw could also be utilized against other predators in defense. The saw is covered with specialized sensory organs ( ampullae of Lorenzini ) which detect an electric field which is given off by buried prey. Saw sharks life history is still poorly understood. Mating season occurs seasonally in coastal areas. Saw sharks are ovoviviparous meaning eggs hatch inside
2001-455: Is known for its six pairs of gills located on its sides close to the head. They are pale brown in color, with a white underbelly. Along with their color, their size sets them apart from the other types of sawfish: The females are around 136 cm where the males are around 112 cm. Sixgill sawsharks feed on shrimp, squid and bony fish. They are located around the southern portion of South Africa, and Madagascar. Where found, they are considered
2070-581: 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. 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
2139-434: Is observed in the cookiecutter shark . 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
Sawshark - Misplaced Pages Continue
2208-496: Is still evidential in several species termed " dogfish ," or the porbeagle . The etymology of the word shark is uncertain. The most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk , meaning 'villain, scoundrel' ( cf. card shark , loan shark , etc.), which was later applied to the fish due to its predatory behaviour. A now disproven theory
2277-775: Is that it derives from the Yucatec Maya word xook ( pronounced [ʃoːk] ), meaning 'shark'. Evidence for this etymology came from the Oxford English Dictionary , which notes that shark first came into use after Sir John Hawkins ' sailors exhibited one in London in 1569 and posted " sharke " to refer to the large sharks of the Caribbean Sea . However, the Middle English Dictionary records an isolated occurrence of
2346-687: The Devonian . Anachronistidae , the oldest probable representatives of Neoselachii, the group containing modern sharks (Selachimorpha) and rays (Batoidea) to the exclusion of most extinct elasmobranch groups, date to the Carboniferous . Selachiimorpha and Batoidea are suggested by some to have diverged during the Triassic . Fossils of the earliest true sharks may have appeared during the Permian , based on remains of " synechodontiforms " found in
2415-737: The Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes , and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the Batoidea . There are more than 500 species of sharks split across thirteen orders , including several orders of sharks that have gone extinct: Shark teeth are embedded in the gums rather than directly affixed to
2484-463: The bull shark , which has developed a way to change its kidney function to excrete large amounts of urea. When a shark dies, the urea is broken down to ammonia by bacteria, causing the dead body to gradually smell strongly of ammonia. 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
2553-511: The clade Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes . The Elasmobranchii also include rays and skates ; the Chondrichthyes also include Chimaeras . It was thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks, but current molecular studies support monophyly of both groups of sharks and batoids. The clade Selachimorpha
2622-441: The coelacanth , 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% ) 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
2691-461: 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. 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
2760-456: 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. 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. The main component of the system is the neuromast,
2829-414: 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
Sawshark - Misplaced Pages Continue
2898-466: The southern sawshark ( Pristiophorus nudipinnis ) is found in south-eastern Australian waters. Much of its distribution overlaps with that of the Common sawshark, however, it seems to occur less frequently. This species is similar in size as the common sawshark, but has a broader rostrum (saw) and a more even brown coloration. It also grows to be heavier than the common sawshark. Since the color pattern of
2967-1065: The Early Permian of Russia, but if remains of "synechodontiformes" from the Permian and Triassic are true sharks, they only had low diversity. Modern shark orders first appeared during the Early Jurassic, and during the Jurassic true sharks underwent great diversification. Selachimorphs largely replaced the hybodonts , which had previously been a dominant group of shark-like fish during the Triassic and Early Jurassic. Batoidea [REDACTED] Heterodontiformes [REDACTED] Orectolobiformes [REDACTED] Carcharhiniformes [REDACTED] Lamniformes [REDACTED] Hexanchiformes [REDACTED] Squatiniformes [REDACTED] Pristiophoriformes [REDACTED] Squaliformes [REDACTED] Sharks belong to
3036-517: The Short Nose lives a benthic lifestyle and feeds on benthic invertebrates. It uses its barbels to detect life on the ocean floor which it then paralyzes with its rostrum. The species is ovoviviparous and tends to give birth to a litter of 7–14 pups biannually. It inhabits ocean floors off the coast of Australia. The Bahamas sawsharks ( Pristiophorus schroeder ) have very little information on them. Studies are being done daily to learn more about
3105-417: The anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. 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. In environments with low visibility, shark species generally have larger olfactory bulbs. In reefs, where visibility
3174-568: The basking shark are small and non-functional. 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. Because sharks do not have rib cages, they can easily be crushed under their own weight on land. The jaws of sharks, like those of rays and skates, are not attached to
3243-416: 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. 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
3312-457: The coast of Japan, Korea, and Northern China. It swims at a depth of 500 m. It has around 15–26 large rostral teeth in front of the barbels, which are equal distance from the gills to the snout, and about 9–17 teeth behind the barbels. Like all sawsharks, the Japanese sawshark is ovoviviparous, and feeds on crustaceans and bottom dwelling organisms. Lana's sawshark ( Pristiophorus lanae ) is
3381-405: The common sawshark may be more or less defined, the easiest way to separate this species from the common sawshark is the location of the barbels, which are closer to the mouth than the co-occurring common sawshark. Unlike the common sawshark, the southern sawshark likely feeds mainly on fishes. The tropical sawshark ( Pristiophorus delicatus ) a pale brown with a yellow hue, and an underbelly that
3450-480: The deep sea dweller. They are located near Cuba, Florida, and the Bahamas (hence their name) where they dwell in the depths of 400–1000 m. As far as their appearance they can be identified by their snouts with teeth which appear as a saw, as well as their length, they are averaged at 80 cm in length. Saw sharks and sawfish are cartilaginous fish possessing large saws. These are the only two fish that have
3519-1168: The horny keratin in hair and feathers. 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. 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. Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming. 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. For some species, dermal patterning returns to healed denticles even after they have been removed by injury. 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
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#17327810183613588-418: The inner ear has been lost. 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. 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
3657-402: 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. 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
3726-451: 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 in comparison to 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
3795-453: 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. In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts. Fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling
3864-543: The longest lifespans at more than 100 years. Whale sharks ( Rhincodon typus ) may also live over 100 years. 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. Unlike most bony fish , sharks are K-selected reproducers, meaning that they produce
3933-735: The mother. They have litters of 3–22 pups every 2 years. After 12 months of pregnancy, the pups are born at 30 cm long. While in the mother, pups' rostral teeth are angled backwards to avoid harming the mother. The life expectancy of sawsharks is still poorly understood, but they are thought to live to 10 years or more. Among the different species of sawshark, all are listed on the IUCN Red List of 2017 as either data deficient or of least concern Saw sharks do not see much human interaction because of their deep habitats. There are currently ten known species of sawsharks across two genera in this family: The sixgill sawshark ( Pliotrema warreni )
4002-470: The ocean floor which it then hits with its snout to immobilize it. Like all other sawsharks, the common sawshark has a long snout with rows of small teeth and barbels on either side. It has five gill slits on either side of its head and between 19 and 25 teeth on each side. Sawsharks appear to be one of the types of elasmobranch that are difficult or impossible to age using most commonly-used approaches that rely on vertebral banding. The shortnose sawshark aka
4071-553: The oldest known member being Agaleus , though records of true sharks may extend back as far as the Permian . Sharks range in size from the small dwarf lanternshark ( Etmopterus perryi ), a deep sea species that is only 17 centimetres (6.7 in) in length, to the whale shark ( Rhincodon typus ), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. They are found in all seas and are common to depths up to 2,000 metres (6,600 ft). They generally do not live in freshwater, although there are
4140-462: The saw shark. Clear difference is that a sawfish has no barbels and a saw shark has a prominent pair halfway along the saw. The saw shark uses these like other bottom fish, as a kind of antennae, feeling the way along the ocean bottom until it finds some prey of interest. Both the saw shark and the sawfish utilize the electroreceptors on the saw, ampullae of Lorenzini , to detect the electric field given off by buried prey. Shark Sharks are
4209-403: 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
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#17327810183614278-413: 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
4347-414: The stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth. 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
4416-423: 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
4485-403: The thresher shark's usage of its powerful, elongated upper lobe to stun fish and squid. 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. Their liver constitutes up to 30% of their total body mass. The liver's effectiveness
4554-456: The top of their food chain . Select examples include the bull shark , tiger shark , great white shark , mako sharks , thresher sharks , and hammerhead sharks . Sharks are caught by humans for shark meat or shark fin soup . Many shark populations are threatened by human activities. Since 1970, shark populations have been reduced by 71%, mostly from overfishing . Until the 16th century, sharks were known to mariners as "sea dogs". This
4623-472: The tropical sawshark ( Pristiophorus delicatus ). Sawsharks are found in many areas around the world, most commonly in waters from the Indian Ocean to the southern Pacific Ocean . They are normally found at depths around 40–100 m, but can be found much lower in tropical regions. The Bahamas sawshark was discovered in deeper waters (640 m to 915 m) of the northwestern Caribbean . Sawsharks have
4692-548: The word shark (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology. The oldest total-group chondrichthyans, known as acanthodians or "spiny sharks", appeared during the Early Silurian , around 439 million years ago. The oldest confirmed members of Elasmobranchii sensu lato (the group containing all cartilaginous fish more closely related to modern sharks and rays than to chimaeras ) appeared during
4761-576: 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. 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. Unwanted items may never get past
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