- 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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he rapid decline of great sharks in the world’s oceans is disrupting the marine ecosystem by allowing more lowly fish to thrive, scientists warn today.
Overfishing of the ancient predators has lead to a sudden uprising of species they prey on, causing an abundance of skates, rays and smaller sharks, which are steadily devastating populations of shellfish, including scallops, oysters and clams, the researchers claim.
The findings suggest that the demise of the great sharks, whose primitive ancestors cruised the seas long before the rise and fall of the dinosaurs, may have unforeseen knock-on effects on marine life lower down the food chain.
Records from fisheries and research vessels dating from the 1970s to 2005 have revealed a dramatic nosedive in great shark populations. Tiger sharks and scalloped hammerheads may have declined more than 97% since the mid-1980s, while numbers of smooth hammerheads and bull sharks are believed to have fallen by 99% off the east coast of the US.
Writing in the journal Science, a team of marine biologists led by Ran Myers at Dalhousie University in Halifax, Nova Scotia, analyse fish research surveys over the past 16 to 35 years. The records show that while the abundance of 11 great shark species fell dramatically over the past 35 years, 12 of the 14 fish species they prey on had increased sharply.
In the waters along the US Atlantic coast, numbers of cownose rays, a staple of the great shark diet that can grow to four feet across, jumped 8% a year to an estimated population of around 40 million.
The explosion of the cownose population coincides with an almost complete collapse of scallops in the waters, leaving only those protected behind marine fences for local fisheries to take.
Without sharks to keep their numbers in check, researchers fear the migrating rays will drive down shellfish populations as they swim through, to the point where they are unable to recover .
Julia Baum, a co-author of the paper, said: “With fewer sharks around, the species they prey upon, like cownose rays, have increased in numbers, and in turn, hordes of cownose rays dining on scallops, have wiped the scallops out.”
Sharks are targeted by fisheries for their fins and meat, but are also taken as by-catch by fleets fishing for tuna and swordfish. As many as 73 million sharks are killed each year around the globe for the finning trade.
Ellen Pikitch, executive director of the Pew Institute for Ocean Science in Miami, said: “This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops. These unforeseen and devastating impacts underscore the need to take a more holistic, ecosystem-based approach to fisheries management.”
Charles Peterson, a researcher on the paper and marine biologist at the University of North Carolina in Chapel Hill, said the study highlighted the importance of maintaining populations of the ocean’s top predators. “Despite the vastness of the oceans, its organisms are interconnected, meaning that changes at one level have implications several steps removed. Through our work, the ocean is not so unfathomable, and we know better now why sharks matter,” he added.
In British waters, historic overfishing has seen the common ray decline to the point that surveys in the western channel have failed to spot any since the 1930s. More recently, numbers of blue and porbeagle sharks are believed to have fallen. The porbeagles are believed to be taken by Danish and French fleets, while Spanish long-line vessels take blue sharks migrating into British waters.
Last year, a team lead by David Sims of the Marine Biological Association in Plymouth tagged six blue sharks off the coast of Portugal to investigate their fate. Two were landed by fisheries within three months. “The ones that get here may be the survivors,” he said.
Dr Sims said the lack of hard data makes it extremely difficult to produce reliable assessments of fish populations, adding that many predators have such varied diets that cascade effects through ecosystems are complex and often difficult to pinpoint.
“There’s no doubt the fisheries are having an impact on the big shark populations, but what we really don’t know is what the ecosystem effects of that will be. There could be other factors involved that haven’t been measured,” he added.
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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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