Most sharks arepredators: they hunt and eatfish, marinemammals, and other sea creatures. However, thewhale shark - the largest fish in the world - eatskrill, like truewhales.
It is widely believed that sharks are "silent killers". However, a recent study shows that sharks emit a low growl from their throats which resonates through theirscales.
Just a few million years ago, a giant shark calledMegalodon swam in the seas. It was 18meters long, twice as long as the closely-related great white shark, and it ate whales.Megalodon died out 1.6 million years ago.
Much of what we understand aboutprehistoric sharks comes from the study of theirfossils. While sharks have skeletons made of softcartilage that can fall apart before fossilizing, theirteeth are harder and easily fossilized. Prehistoric sharks, like their modern descendants, would grow and shed many thousands of teeth over their lifetime. For this reason, shark teeth are one of the most common fossils.
Sharks come in many different shapes and sizes, but most are long and thin (also called streamlined), with very strongjaws.
Sharks haveskin covered in millions of tiny teeth-likescales that point to thetail. If you rub along a shark towards the tail, it feels smooth, but if you rub the other way, it is rough.
Sharks' teeth are constantly replaced throughout their lives. Sharks eat soviolently they often break a few teeth, so new teeth grow continuously in a groove just inside themouth and move forward from inside the mouth on "conveyor belts" formed by the skin which they are attached to. In its lifetime, a shark can lose and regrow as many as 30,000 teeth.
Even with all those teeth, though, sharks cannotchew. So they bite theirprey andjerk it around so they can pull off a chunk to swallow. The chunks of food that a shark swallows end up in itsstomach, where they aredigested. This is a slow process, however, so a meal might take several days to digest. This is why a shark does not eat every day.
Sharks have different-shaped teeth, depending on what they eat. For instance, some sharks have sharp, pointy teeth, while bottom-dwelling sharks havecone-shaped teeth for crushingshells. Because there are so many different kinds of sharks, and because each kind has its own kind of special teeth, many people enjoy collecting shark teeth.
Shark teeth collectors can guess how large a shark was by measuring the shark tooth. First, they measure the length of the tooth in inches. Every inch of tooth equals 10feet of shark length: so if a shark tooth is 2 inches long, the tooth came from a shark that was around 20 ft long. SomeMegalodon teeth are 6 inches long - suggesting that this shark was 60 feet long.
Sharks come in various shapes and sizes, so theirprey differs depending on the type of shark and where they live. Sharks typically eat up to 3% of their body weight every two to three days. Sharks' meals are spaced apart because they often have trouble chewing despite their large teeth; this meansdigestion can take anywhere from 68 to 82 hours.
While most sharks arecarnivores, a recent study shows that there are exceptions. The bonnethead shark isomnivorous, feeding on small animals andsea grass. It has been discovered[who?] that wild whale sharks may be omnivorous as well, though this may be due to scarcity in their usual diet. If this is the case, we may see more sharks broaden their diet to include plant life as food scarcity worsens.
Sharks find their prey by using their sense of smell and vibrations as well as the movement of their prey. Thelateral line is anorgan found in the skin, which sharks use to sense the vibrations of their prey. Sharks have different hunting strategies to catch their prey depending on size andspecies. Larger sharks hunt down their meal and often swallow them whole or eat them in large chunks. Some will twist their prey into thesand or stun them with their tail, while others will hunt for small animals such ascrustacean.
Prey for different shark species varies greatly. Hammerhead sharks eat a lot of squid andoctopus. Dolphins and small whales are often prey for large sharks like the great white or tiger sharks, while whale sharks mainly subsist on animals likeplankton and krill.
Sharks use theirfins forswimming, stabilizing, steering, and lift. Each fin is used in a different manner.
Parts of a shark (showing which fins are typically removed for sale)
There are one or two fins present along thedorsal midline called the first and seconddorsal fin. These fins help the shark from constantly rolling around. They may or may not havespines. When spines are present, they are used for defense, and may also have skin glands with them that produce an irritating substance.
Thepectoral fins are behind the head and extend outwards. These fins are used for steering during swimming and help to provide the shark with lift.
Thepelvic fins are behind the pectoral fins, near thecloaca, and are also stabilizers.
Not all sharks haveanal fins, but if they do have them, they are found between the pelvic and caudal fins.
The tail region itself consists of the caudal peduncle and the caudal fin. The caudal peduncle sometimes has notches known as "precaudal pits", which are found just ahead of the caudal fin. The peduncle may also be horizontally flattened into lateral keels. The caudal fin has both an upper lobe, and a lower lobe, which can be of different sizes; the shape depends on the shark species.
The primary use of the caudal fin is to provide a "push" while the shark swims. The upper lobe of the caudal fin produces most of the push, and usually forces the shark downwards. The pectoral fins and the shape of the body (like an airfoil) work together to counter this force.
The strong, non-lunate caudal fin in mostbenthic shark species allows the shark to swim close to the seabed (such as thenurse shark). However, the fastest swimming sharks (such as themako sharks) tend to have lunate-shaped (crescent-shaped) caudal fins.[3]
Sharks can determine the direction of a certain scent based on the timing of scent detection in eachnostril.[5] This is similar to the methodmammals use to determine direction of sound. Some species, such asnurse sharks, have externalbarbels that greatly increase their ability to sense prey.
Sharks are attracted to thechemicals found in theintestines of many species. For this reason, they often linger near or insewage outfalls.
Sharkeyes are similar to the eyes of othervertebrates; they have similarlenses,corneas andretinas. Theireyesight is welladapted to themarine environment. They can contract and dilate theirpupils, like humans, something noteleost fish can do. A tissue behind theretina reflects light back, thereby increasing sight in darker waters.
Electromagnetic field receptors (Ampullae of Lorenzini) and motion detecting canals in the head of a shark
Sharks have tiny holes all over their snouts, especially between theeyes and the tip of thesnout. Inside these holes arenerve receptors called theampullae of Lorenzini.[6]p23 They can senseelectricity in the water.
Animals in the water give off electricity: every time an animal'sheart beats or moves, tiny currents ofelectricity are created. These tinyelectric currents make signals that travel through water. Sharks can sense these currents and may use this sense when they catch their prey, even more than they use theirsight.
It is hard to test sharks' hearing. However, they may have a sharp sense of hearing and can possibly hear prey many miles away.[7] A small opening on each side of their heads (not thespiracle) leads directly into theinner ear through a thin channel.
Thelateral line detects changes in waterpressure. It is open to the environment by a line ofpores. This and the sound-detecting organs are grouped together as the 'acoustico-lateralis system', because they have a common origin. In bony fish andtetrapods, the external opening into the inner ear has been lost.
This system is found in other fish as well. It detects motion or vibrations in water. The shark can sensefrequencies in the range of 25 to 50Hz.[8]
Baby sharks (called pups) are born with a full set of teeth, and are capable of taking care of themselves. Once born, they quickly swim away from their mothers, who sometimes feed on the pups. Litters vary from one or two pups (as withgreat white sharks) to one hundred pups (blue sharks andwhale sharks).[10]
Most sharks areovoviviparous, meaning the eggs hatch inside the female's body. The babies develop within the mother, but there is noplacenta to nourish the pups. Instead the young feed on the egg'syolk. The pups eat anyunfertilized eggs and sometimes each other. Very few pups in a litter survive until birth due to this form ofsiblingcannibalism.
Some sharks areviviparous, meaning that the females give live birth: the eggs hatch inside the female's body, and the babies are fed by aplacenta. The placenta helps transfernutrients andoxygen from the mother'sbloodstream, and transfers waste products from the baby to the mother for elimination.
New sharks are still being found. Dave Ebert found ten newspecies in aTaiwanmarket. Over the past threedecades he has named 24 new species. They include sharks,rays,sawfish andghost sharks– these cartilaginous fish are all related.[11]
Some sharks are notendangered, but some are hunted for food (likeshark fin soup) or sportfishing.[12] In 2013 five species of shark, along with two species ofmanta ray, received international protection as part of the Convention on International Trade inEndangered Species.[13]
It is thought[who?] that 100 million sharks are killed by commercial and recreational fishing[when?].[14][15] Sharks are a commonseafood in many places, includingJapan andAustralia.
In the Australian state ofVictoria, shark is the most commonly used fish infish and chips, in which fillets are battered anddeep-fried or crumbed and grilled. In fish and chip shops, shark is called "flake".
InIndia, small sharks or baby sharks (calledsora inTamil andTelugulanguages) are sold in local markets. Since the flesh is not matured (not adult), cooking the flesh breaks it into powder. The powder is thenfried in oil andspices (called sora puttu/sora poratu). The soft bones can be easily chewed. They are considered a delicacy in coastalTamil Nadu.
↑"Shark Fins".marinebiodiversity.ca. Archived fromthe original on 5 December 2013. Retrieved30 July 2013.
↑Martin, R. Aidan."Smell and taste". ReefQuest Centre for Shark Research. Retrieved2009-08-21.
↑Jayne M. Gardiner, Jelle Atema 2010. The function of bilateral odor arrival time differences in olfactory orientation of sharks. Current Biology20 (13),<rc-c2d-number> 1187-1191 </rc-c2d-number>Archived 2012-03-08 at theWayback Machine
↑Long J.A. 1995.The rise of fishes: 500 million years of evolution. Johns Hopkins, Baltimore. Chapter 5, p100: Class Placodermi
