- Sharks and rays have a very varied diet. They are carnivores which means that they eat animals rather than plants and algae. Some sharks commonly eat bony fishes, crustaceans (crabs, lobsters, and other animals with an external skeleton), mollusks(snails, sea slugs, octopus and squids), and different types of worms.
- A shark’s diet is often determined by its habitat. For example, sharks that live out at sea (pelagic sharks) are more likely to eatfish and squid because that is all that is available.
- Sometimes sharks change their diet as they get older. The Great White Shark mainly eats fish when it is young but once it reaches maturity it consumes more marine mammals like seals and sea lions.
- Most sharks prefer live food but they will also consume carrion (dead fish and other animals) that they find on the sea floor.
- Just like filter feeding whales, there are a few sharks that live by filtering plankton from the water. The filter feeding sharks may consume phytoplankton (microscopic plants and algae) while hunting for more nourishing zooplankton (tiny animals and larvae that drifts around on the currents). Ironically, the Whale Shark which is the largest fish in the sea, lives on plankton which is one of the smallest animals. So does the second largest fish; the Basking Shark. Although these sharks have huge mouths, their throats are tiny and they are unable to eat anything larger than a grapefruit. Their teeth which are no longer needed for feeding, have become very small.
- The largest ray (the Manta Ray) is also a plankton feeder. It has a flexible projection on each side of its mouth called cephalic lobes that it uses to funnel plankton towards its mouth.
- Most rays eat small fishes and benthic invertebrates; crabs, snails, and worms etc. that live on or under the sand.
- Sometimes its possible to tell what type of food a shark eats by the shape of its teeth. Sharks that catch fast swimming fishes tend to have very pointed teeth that help them grasp the fish. Sharks that eat hard shelled animals have flattened teeth that form a plate to help them crush the creature’s shell like a nutcracker.
- Tiger Sharks have a reputation for eating anything. They have been found with all sorts of strange things in their stomachs from clothes to license plates. Tiger Sharks have very sharp serrated teeth that are strong enough to bite through the shells of marineturtles.
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Sharks and rays do not have true bones like other fishes. They have cartilage instead which is lighter and much more elastic and allows them to bend in very tight circles.
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Sharks do not have swim bladders. A swim bladder is a gas filled sack inside the body of bony fishes that allows them to stay still without sinking. Sharks compensate by having a very big liver that is filled with oil. Even so, sharks sink unless they keep swimming forward. The exception is the Sandtiger Shark which swallows air to make itself more buoyant.
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A shark’s upper jaw is not fused to its skull like most animals. When a shark bites a large object, it is able to move its upper and lower jaw forward in order to take a bigger bite.
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Unlike other fishes, sharks are able to replace their teeth constantly. New teeth grow from the inner surface of the jaw and rotate forward when the old teeth get worn out or lost during feeding.
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Sharks and rays do not reproduce like other fishes. Most fish release clouds of sperm and eggs into the water column where they mix together. The fertilized eggs then float around until the fish larvae hatch and form schools of tiny fish. Male sharks have two organs called claspers attached to their anal fins. They insert one of these into the female shark’s cloaca (the entrance to the uterus) to transfer sperm (just like in mammals). Some sharks and rays incubate the eggs in their uteruses until the baby sharks are ready to be born. Other sharks and rays (i.e. skates) lay eggs and attach them to the reef.
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Sharks have between 5 and 7 gill slits on each side of their body in front of their pectoral fins. Bony fishes only have one pair. Having many exposed gill slits probably helps transfer more oxygen into their blood faster which allows them to swim very fast when they need to.
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Most shark’s skin is covered in small denticles instead of scales. Denticles are a lot like teeth. They have dentine in the centre and enamel on the surface. This makes shark’s skin very tough and abrasive like sandpaper. The shape and position of some shark’s denticles also helps reduce friction so that they can slip through the water easier.
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Sharks have an extra sense that is able to detect tiny electric fields. They can use this to find food that is buried or to search for animals to eat in the dark or in turbid water.
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Sharks and rays make up the sub-class of fishes called elasmobranches. This sub-class is part of a class of cartilaginous fishes called Chondrichthyes which also includes chimaeras (ratfishes).
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The shark existence of the east coast of the USA decreased in the last 15 years substantially. The number of the hammerhead sharks sank around 89%, those of the thresher sharks around 80%, those of the great white sharks around 79%. The populations of the mako sharks, blue sharks and tiger sharks sank around 40-65%. The existence of the sandbar sharks were reduced in the last 10 years by overfishing by 85-90%.
Canadian researchers announce a decrease of the oceanic whitetip shark by around 99%. They are almost extinguished in certain regions already.
Sharks are not only caught actively for their meat, fins or cartilages. Millions of sharks die as non-usable catch (bycatch) in the nets and longlines of the swimming fish factories.
In the year 1991 the longline fishery brought in 8.3 million sharks world-wide. It is assumed that about half of it were blue sharks. More than 87% of these 8.3 million sharks were thrown away!
In the USA, along the Atlantic Coast and the Gulf of Mexico, annually approx. 2.5 million sharks are fished by sportfishing (game fishing). From that approx. 20-40% (500’000 – 1’000’000 sharks) got killed.
The population of all sorts of sharks is worldwide extremely threatened. In total 82 shark and ray species are registered on the so called “Red List” of the World Conservation Union.
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The dark markings indicate gene expression in the electrosensory organs in the head of an shark.
Gainsville, Florida (Feb 6 2006 18:53 EST) Sharks are known for their almost uncanny ability to detect electrical signals while hunting and navigating.
Now researchers have traced the origin of those electrosensory powers to the same type of embryonic cells that gives rise to many head and facial features in humans.
The discovery, reported by University of Florida scientists in the current edition of Evolution & Development, identifies neural crest cells, which are common in vertebrate development, as a source of sharks’ electrical ESP.
It also fortifies the idea that before our early ancestors emerged from the sea, they too had the ability to detect electric fields.
“Sharks have a network of electrosensory cells that allows them to hunt by detecting electrical signals generated by prey,” said Martin Cohn, a developmental biologist with the departments of zoology and anatomy and cell biology, and the UF Genetics Institute. “That doesn’t mean they can only detect electric fish. They can sense electricity generated by a muscle twitch, even if it’s the weak signal of a flounder buried under sand.”
Likewise, sharks are widely thought to use the Earth’s magnetic field for navigation, enabling them to swim in precise paths across large expanses of featureless ocean, Cohn said.
“If you think of this in the big picture of evolution of sensory systems, such as olfaction, hearing, vision and touch, this shows sharks took a pre-existing genetic program and used it to build yet another type of sensory system,” Cohn said.
UF and University of Louisiana researchers analyzed electroreceptor development in the embryos of the lesser spotted catshark, an animal that is largely motionless during the day and hunts at night, mainly in the seagrass beds of the eastern Atlantic Ocean.
Using molecular tests, scientists found two independent genetic markers of neural crest cells in the animal’s electricity-sensing organs. Analysis shows these cells migrate from the brain and travel into the developing shark’s head, creating the framework for the electrosensory system – a previously unknown function of a much-studied group of cells, according to Renata Freitas, a doctoral candidate in UF’s zoology department and first author of the paper.
The process mirrors the development of the lateral line that allows fish to mechanically sense their environment, and organs of the inner ear that enable people to keep their balance. But scientists suspect as human ancestors emerged from the sea, they discarded their lateral lines as well as their ability to sense electrical fields.
“Our fishy ancestors had the anatomy for it,” said James Albert, a former UF biologist who is now at the University of Louisiana. “You can imagine how valuable this system would be if you were aquatic, because water is so conductive. But it doesn’t work on land – air doesn’t conduct electricity as well. When it happens, it’s called a lightning bolt and you don’t need special receptors to sense it.”
All primitive animals with backbones could sense electricity, according to Michael Coates, an associate professor of organismal biology and anatomy at the University of Chicago. Mammals, reptiles and birds lost the sense over time, as did most fish alive today.
But in sharks and a few other species, such as sturgeons and lampreys, electrosensory capability endured.
“Most fish you see today have large eyes,” Coates said. “But sharks are predators that do not particularly rely on vision. If you see a hammerhead shark searching for flatfish, it moves its head back and forth, almost as if it were using a metal detector. Knowing that the electrosensory system may have developed with involvement of neural crest cells is valuable for people trying to reconstruct vertebrate evolution. It gives us further indication of how all of the various sensory systems come on line.”
But the idea that the neural crest truly is the source of the electrosensory system will raise eyebrows, scientists say.
“It’s a very interesting paper for two reasons,” said Glenn Northcutt, a distinguished professor of neuroscience at the University of California, San Diego, and a leading expert in vertebrate neurobiology. “For the first time, someone has shown which molecules may be responsible for guiding the development of the receptors of the lateral line system. I think this will hold true and is a very important finding. But I’m skeptical about the claim the neural crest gives rise to electroreceptors. It still requires a definitive experiment, where the developing neural crest cells are marked with dye, the embryo develops and the dye clearly shows up in the electroreceptors.”
Dye tests are a classical way of mapping cell movements during development, and have been used to explore the origins of limbs and brain cells. In the current research, scientists used genetic markers to trace neural crest cells.
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Sharks have a great deal to fear from humans. Compared to the 10-15 people killed by sharks each year; over 100 million sharks perish at the hands of humans annually and many populations may face extinction. Sharks are killed for many reasons, including fear; food, sport, and ‘machismo’, but the great majority perish due to simple greed. Many shark products have commercial value, including: the flesh; the skin for high-quality leather; teeth and jaws for ornaments; liver oil for cosmetics, medicines, vitamin A, and skin-care products; and cartilage for false cancer ‘cures’. However; the product that drives the market are the fins. After drying, collagen fibers are extracted from them, cleaned, and processed to make ‘shark fin soup’. In spite of the fact that these fibers have little flavor or nutritional value, the soup is considered a delicacy, and may sell in the Orient for more than $100 ( £65) a bowl.
Over the years, shark fisheries have come and gone. In the early part of the century, sponge fishermen in Florida killed sharks to boil them down for their oil. The oil was then thrown on the ocean to smooth the surface of the water and make it easier to see the sponges from the boat. That ended when a plague killed off the sponges. In the 19405 to .19505 a number of shark fisheries sprang up to supply the market for vitamin A. That ended with the discovery of a method for its synthetic production. However; most shark fisheries, such as the one for dogfish sharks to supply the ‘fish and chips’ market in the UK, have ended only when the number of sharks dropped too low for the fishery to be sustained.
The explosive growth of the Chinese economy and rapid expansion of trade with the outside world during the 1985 and 1995 created an unprecedented situation. Suddenly there was an insatiable demand for shark fins of almost any size or type. Improvements in shipbuilding and navigational electronics meant that shark fishing boats could now go anywhere in the world, moving from one place to another as local shark populations were destroyed. The fins are now so much more valuable than the rest of the shark that the carcass is often discarded after the fins are removed, to save storage space on the boat. Often the fins are sliced off when the shark is still alive and the mutilated shark is dumped back into the water to die a slow and agonizing death.
Why should we be concerned about this situation? After all, wouldn’t the ocean be much safer without sharks? The answer is no. The chance of being attacked by a shark is already less than the chance of being struck by lightning. The real dangers for people in the water are drowning, exposure, and being struck by a boat. In the USA, for example, drowning incidents outnumber shark attacks by 1,000 to I. Without sharks, the whole experience of being in a natural ocean wilderness would be immeasurably reduced. It would be like being on the Serengeti with no lions or cheetahs. In losing the opportunity to view these magnificent and superbly-adapted predators in the wild, we are also losing part of our spiritual connection with nature.
But something else would be changed as well -the whole ecology of the ocean. Predators control the populations of their prey species in a beneficial way. They eliminate diseased and genetically defective individuals, and they stabilize population fluctuations. On land, when we have removed the natural predators of deer; for example, their populations have exploded until they overgrazed their food supply and died of starvation and disease. In the ocean we are not sure what all the consequences of removing the apex predators from the food pyramid might be. We do have one example, though. A shark fishery in Tasmania collapsed after two years of over fishing. Shortly afterwards, the fishery for spiny lobsters also collapsed and fishermen observed a lot of octopus in the area. Octopuses are both major predators of spiny lobster and an important food item for sharks. It seems that once the numbers of octopus were no longer controlled by the sharks, they became too numerous and decimated the lobsters. Economically, for those other than shark fishermen, it doesn’t make sense to allow sharks to be fished out, not only because of the possible damage to sustainable fisheries, but also because of the loss of earnings from divers coming to see sharks. Worldwide, shark-watching has become a multi-million dollar business.
Why do shark populations collapse so quickly when people begin fishing them? The answer lies in the life history of these animals. In many aspects, sharks are more similar to mammals such as whales, dolphins, or ourselves, than to other fish. Whereas most fish reach maturity in only a few years and produce thousands or millions of eggs per year; sharks take many years to reach maturity. Some species may not begin to reproduce until they are over 15 years old. Some species produce as few as two pups biannually, averaging only one offspring per year: So when a population is over fished, it may take many years to recover; or it may never recover: Some scientists believe that sharks should never be fished at all, that their biology is too fragile to withstand any exploitation. Perhaps sharks should have the total protection given to marine mammals in many countries. Unfortunately, sharks do not have big ‘fan clubs’ as dolphins do.
Although both are large predators with slow reproductive rates, sharks are handicapped, from a.: public relations perspective, by the fact that their mouths appear to be frowning, and that they must open their mouths to pass water over their gills, exposing their teeth. Dolphins, on the other hand, always appear to be smiling, because of the shape of their mouths. Since they breathe through the blowholes on top of their heads, they do not have to open their mouths and expose their formidable teeth in order to get oxygen. But even the dolphin’s smile may not protect it from the greed inspired by the high prices being offered by international buyers of shark fins. In a number of countries, fishermen are slaughtering dolphins to chop up for shark bait.
We should create save our sharks and dolphins lanyards to create awareness
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