- 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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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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Expert findings show sharks and rays are now amongst Europe’s most threatened animals as more are added to the IUCN Red List of Threatened Species
Gland, Switzerland, 20 February 2006 (IUCN) The number of species of sharks and rays on the IUCN Red List of Threatened Species will increase based on the findings of a three-day expert workshop, hosted by the Joint Nature Conservation Committee (JNCC), that examined the conservation status of the species in the Northeast Atlantic and Mediterranean waters.
The workshop confirms the widely-accepted notion that slow-growing sharks and rays are exceptionally vulnerable to over-fishing, and that deep-water species are being depleted at an alarming rate. Some formerly important commercial species are now so rare that they are no longer being sought by fishermen, but their risk of extinction is still rising because of continued incidental capture in fisheries for more abundant species. This situation is exacerbated by the lack of shark fisheries management in European waters.
“Sharks and rays are amongst the most threatened animal groups in the UK today. I welcome the development of a Red List baseline, against which to monitor the hoped-for changes in their status that should arise from increased awareness of their plight,” said Dr Malcolm Vincent, JNCC’s Director of Science.
Nearly 100 species of sharks and rays were evaluated against the IUCN Red List Categories and Criteria. Categories range from Extinct to Least Concern and Data Deficient. Species deemed Vulnerable, Endangered or Critically Endangered are considered threatened with extinction and are added to the global IUCN Red List of Threatened Species. The IUCN Shark Specialist Group, which convened the meeting, will compile these assessments for a regional report that will include recommendations for conservation action.
Proposed additions to the Red List include three species of angel sharks, two species of skates, and several species of deep-water sharks, all of which are considered Critically Endangered in the region, as well as two species of coastal ray, now considered Endangered. The species found to be at lowest risk were generally small and fast-growing coastal species, like cuckoo ray and lesser-spotted catshark, and very deep ocean species that are still beyond the reach of today’s fishing fleets.
Angel sharks, formerly abundant large coastal sharks, were once a common sight in fish markets, but have largely vanished, almost unnoticed, from the European seas that are their world stronghold.
Now officially declared extinct in the North Sea by the International Council for the Exploration of the Sea (fisheries advisers to European countries), the angel shark was nominated in 2001 for strict legal protection in British waters, but we are still waiting for government action on this proposal, said Sarah Fowler, Co-Chair of the Shark Specialist Group. Workshop participants emphasised the urgency of protecting this, and many other imperilled species.
Three species of deep-water sharks, taken as incidental catch in fisheries and increasingly targeted for their meat and rich liver oil, were assessed as threatened. A population decline of 80-95% prompted a Critically Endangered classification for the region’s deep-water gulper shark.
These exceptionally slow-growing sharks are simply not biologically equipped to withstand such intense fishing pressure, said Tom Blasdale, Marine Species Adviser at the JNCC. We welcome recent European Union action to manage deep-water gillnet fisheries, but similar measures are still urgently needed to protect deep-water sharks taken by trawls and longlines.
The shortfin mako shark, a favourite target of commercial and recreational fishermen around the world, was proposed as Vulnerable in the Northeast Atlantic and Critically Endangered in the Mediterranean Sea.
This wide-ranging species is increasingly the target of fisheries and yet lacks any type of protective measures in this region, warned Alen Soldo of the Institute of Oceanography and Fisheries in Croatia. Of particular concern are mako sharks in the Mediterranean, where our findings revealed ongoing fishing pressure well beyond the reproductive capacity of the species.
In contrast to similar workshops held in North America, South Africa, and Australia, the workshop yielded little if any good news, due largely to the lack of shark and ray conservation measures in this region. Protection is granted by just a handful of European countries for the three largest species (basking shark, devil ray, and great white shark). The few European shark and ray quotas in place are routinely set far in excess of actual catches and therefore do not limit fishing pressure. They also cover only part of these stocks. Scientists advice for zero catch of many depleted shark and ray species has been ignored. There are no international limits on shark catch, even as fisheries for wide-ranging shark species (such as mako and blue sharks) expand and evidence of their declines mounts.
Scientists from government agencies, universities, and private institutions participated in the workshop including authors of published papers on shark and skate population status and experts who develop advice on shark quotas for European and international fisheries of the Northeast Atlantic. Experts from England, Scotland, Ireland, Italy, Spain, Portugal, Croatia, Russia, Sweden, Canada, and the USA took part.
The IUCN Red List of Threatened Species is the world’s most authoritative guide to the status of biological diversity. The workshop was the eighth in a global series to assess all of the world’s shark and ray species and develop regional conservation priorities. Resulting Red List proposals are preliminary until accepted by the global Shark Specialist Group network.
Filed under Save Sharks · Tagged with abundant species, action, addition, advice, Adviser, age, air, Alen, angel, angel sharks, animal, animal groups, assessment, Atlantic, Australia, author, awareness, baseline, Basking, basking shark, Biological, blue, blue sharks, British, capacity, capture, catch, Categories, catshark, cent, Chair, Class, classification, Co, coast, commercial species, committee, Concern, conservation, conservation action, Continue, contrast, Council, Criteria, Critically, Croatia, cuckoo, Data, day, decline, deep ocean, Deficient, Development, devil, Director, diver, dr malcolm, Dr Malcolm Vincent, ear, Endangered, England, EST, Europe, European, european waters, exception, excess, expert, expert workshop, Exploration, Extinct, extinction, favourite, Feb, February, fin, finding, fish, fisheries, fisheries management, fishermen, fishing, fishing fleet, fishing fleets, fleet, form, gel, gene, general, gill, gillnet, Gland, gland switzerland, Government, Great, great white shark, group, gulper, hand, handful, hold, host, Important, Institute, International, Italy, IUCN, iucn red list, iucn shark specialist group, JNCC, Join, Joint, joint nature conservation committee, lack, land, Least, leg, line, List, liver, liver oil, longline, longlines, mako, mako sharks, management, Marine, market, meat, Mediterranean, Mediterranean Sea, mediterranean waters, meeting, Nature, need, Network, news, North, North America, North Sea, Northeast Atlantic, notion, number, ocean, Oceanography, official, oil, Ongoing, paper, part, place, plight, population, Portugal, pressure, product, proposal, Proposed, protect, protection, range, rat, rate, ratio, ray, reach, recreation, Red, region, report, rise, risk, round, row, Sarah Fowler, Science, Scotland, sea, serv, shark, shark fish, shark fisheries, shark specialist group, shark species, Sharks, shop, shortfin, side, sight, situation, skate, skates, skin, Soldo, South Africa, Spain, Specialist, Species, species of sharks, stand, status, stronghold, Switzerland, TALE FOR SHARKS, target, Tens, threat, Threatened, today, Tom Blasdale, type, UK, Union, urgency, US, USA, use, Vulnerable, water, water sharks, water species, White, white shark, Wide, With, work, workshop, world
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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There is something unique about sharks’ teeth! A shark without teeth could not survive; it would starve. Therefore, unlike many other animals like dogs and canines, sharks continuously get new teeth to replace those that fall out. A shark’s mouth generally contains five or more rows of teeth, one behind the other. All rows, with the exception of the first, lay flat in the animal’s mouth. The next row rises up to replace any teeth that have fallen out or were broken. Sharks always make new teeth and have always spare rows of teeth. Sharks’ teeth are adapted to what they eat. Unlike humans, sharks do not chew. They are not omnivores, but carnivores. They use their teeth to grasp prey and, if needed, tear the prey into smaller chunks they can swallow. Most shark teeth are very sharp. Sharks’ jaws are powerful and the sharp teeth are capable of cutting through bone and even thin steel chains.
Shark teeth vary from being ferocious-looking curved spikes to flat triangular points, to points that are so small that they are not used for anything at all. The larger sharks, like the great white and the tiger shark, have triangular teeth with jagged edges. This helps to keep hold of large fish and animals so as to tear chunks of meat from their bodies or slice through a turtle’s shell. A sand tiger’s teeth, on the other hand, are long and narrow which make them look frightening, but in fact this type of shark is not very aggressive. The shape of its teeth is ideal for grabbing hold of slippery prey, like fish and squid. However, the whale shark, one of the biggest sharks on earth, has very small teeth. Whale sharks don’t use their teeth for biting because they simply filter their food.
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