Sharks are trash fish - No Way!
Sharks are a critical part of marine ecosystems, a source for knowledge to help the human condition, and the basis of a valuable fishery.
Sharks have poor vision - Erroneous!
Sharks' eyes, which are equipped to distinguish colors, employ a lens up to seven times as powerful as a human's, and some shark species can detect a light that is as much as ten times dimmer than the dimmest light the average person can see.
All sharks have to swim constantly - Misconceived!
Some sharks can respire by pumping water over their gills through opening and closing their mouths while at rest on the bottom.
The great white shark is a common, abundant species found off most beaches visited by humans - Not!
Great whites are relatively uncommon large predators that prefer cooler waters. In some parts of their range, great whites are close to being endangered.
Whale sharks, the largest species of sharks, are voracious predators - Incorrect!
Whale sharks, which are the largest fish that ever lived, are plankton feeders like the great whales, thus the name.
Most sharks are harmful to people - Untrue!
Of the more then 350 shark species, about 80% are unable to hurt people or rarely encounter people.
A shark is a shark is a shark - Misconstrued!
There is no "typical" shark. The more than 350 species all differ in habitat, lifestyle and body form.
Sharks are hard to kill - Off Base!
Stress of capture weakens a shark, and so some sharks are easily killed in hook-and-line or net fishing.
Shark meat is poisonous to people - Wrong!
Although there have been some reports of people being poisoned by shark meat, the meat from the majority of sharks is edible and delicious when properly handled and prepared.
Sharks have peanut-sized brains and are incapable of learning - Fallacious!
Sharks' relatively large and complex brains are comparable in size to those of supposedly more advanced animals like mammals and birds. Sharks also can be trained.
Most sharks cruise at high speed when they swim - Invalid!
Although some sharks may swim at bursts of over 20 knots (23 miles per hour), most sharks swim very slowly at cruising speeds of less than 5 knots (5.75 miles per hour).
Sharks are not found in freshwater - Forget it!
A specialized osmoregulatory system enables the bull shark to cope with dramatic changes in salinity -- from the freshwaters of some rivers to the highly saline waters of the ocean.
Sharks are not discriminating eaters and scavenge the sea - Wrong!
Most sharks prefer to eat certain types of invertebrates, fish and other animals. Some sharks eat mainly fish. Others eat other sharks or marine mammals. Some sharks are even plankton-eaters.
Sharks prefer human blood - False!
Most sharks don't appear to be especially interested in the blood of mammals as opposed to fish blood.
Sharks eat continuously - Preposterous!
Sharks eat periodically depending upon their metabolism and the availability of food. For example, juvenile lemon sharks eat less than 2% of their body weight per day.
Sharks must roll over on their sides to bite - No!
Sharks attack their prey in whichever way is most convenient, and they can protrude their jaws to bite prey items in front of their snouts.
The origin of sharks is even more obscure than that of most other groups of animals, for they have left very little evidence of their existence. Since their skeletons consist of cartilage rather than bone, the only early shark fossils we have are those of dermal denticles, teeth, spines and the occasional skeleton, preserved under exceptional circumstances.
The oldest shark-like creatures appeared in the fossil record towards the beginning of the Silurian period, about 450 million years ago, but the earliest known fossil teeth of true sharks do not appear until the Early Devonian, about 400 million years ago. Their teeth are small, no more than 4 mm (0.16 in) across, so their owner may have been no more than 30 cm (12 in) long. The skeleton of such a creature, a common ancestor for all the sharks and their relatives, has yet to be found. A candidate for close relative, however, is Antarctilamna, whose fossils have been found in rocks on the Antarctic continent. This fish was just 40 cm (16 in) long, and distinctly shark-like. It had a spine in front of a long dorsal fin, and teeth that had two large splayed cusps, with smaller cusps between them.
The age of sharksBy the start of the Carboniferous period, about 360 million years ago, sharks diversified and proliferated to such an extent that scientists have labelled this the 'golden age of sharks'. In addition to the obviously shark-like species, there were some that can only be described as bizarre. Stethacanthus probably grew up to 3.4 m (11 ft) long and differed from any other shark, living or dead, in having a helmet of small teeth on its head and a curious structure shaped like a triangular shaving brush sticking out of its back, roughly where its anterior dorsal fin should be. This was topped by dozens of teeth, small ones at the front and larger teeth at the back, and was present in both male and female sharks. Such a conspicuous and awkward structure must have had an important function, but what this could have been can only be speculation. It may have played a role in courtship, had something to do with defence, or maybe it enabled Stethacanthus to anchor itself, like a modern remora, to the underside of a larger fish.
Helicoprion. The shark itself is poorly known, but the pattern of its teeth is instantly recognisable. They formed a whorl erupting from the back of a semi-circular 'conveyor belt arrangement', but the teeth did not fall away at the front as in modern sharks. Instead, they were rotated under the apex of the lower jaw, and then back up into a cavity under the jaw where they were stored in a tight spiral. Why these sharks possessed such a bizarre dental arrangement is another mystery.
There were giants, too. Enormous fossil teeth from a shark with a mouth a metre (3 ft) wide and a body 6 m (20 ft) long have been found in Lower Carboniferous deposits in North America. With the giant placoderms gone at the end of the Devonian, these enormous sharks would have been the largest vertebrate predators in the sea.
Modern sharks arrive
The Jurassic and Cretaceous periods were the 'age of reptiles'. In the sea, relatives of the dinosaurs included long-necked plesiosaurs and shark-like ichthyosaurs, but the new breed of sharks--streamlined, fast-swimming and equipped with their remarkable battery of senses--were strong competitors. Some were huge. The cretoxyrhinid sharks, for instance, were active hunters with lengths over 6 m (20 ft) and weights in excess of 1.5 tonnes.
There were also the ptychodonts, a group of sharks known only from their teeth. These were flattened, crushing structures, suitable for tackling hard-bodied creatures, such as ammonites, bivalves and gastropods. Cretaceous deposits also contain the teeth of what look remarkably like those of sand tiger sharks and there were the recognisable teeth of large lamnid sharks, relatives of today's mackerel sharks, from species that must have been over 6 m (20 ft) long. Fossil teeth from ancient porbeagles Lamna, for example, have been unearthed in rocks dating back to the Cretaceous.
That sharks arose at all is something of a miracle, for they began to evolve during a series of devastating episodes in the Earth's history that must have had an impact on sharks and their food. At the end of the Triassic period, another 20 per cent of marine families were hit badly by another mass extinction, and at the end of the Jurassic there was another minor event. Then, at the end of the Cretaceous, about 64 million years ago, about 70 per cent of all living things, including the dinosaurs and their relatives, were wiped out. It was an opportunity, however, for others to flourish, and some of the new sharks survived the catastrophe. There was a distinct loss of diversity in the groups, but the survivors were a tough, well-honed bunch.
These included great filter feeders, such as basking sharks Cetorhinus, straining the water for krill in the manner of modern baleen whales; also, tiny fossil teeth, closely resembling those from the mid-water filter-feeding megamouth Megachasma, have been found in Tertiary rocks. There were fast-swimming hunters, too, including a new group of sharks that would come to dominate the seas up until the present day. These were the carcharonids--the requiem sharks.
With the demise of the giant reptiles, the mammals began to fill some of the vacated niches, and some returned to the sea. The evolving whales, dolphins, seals and sea cows became the main source of food for one group of sharks in particular--the ancient relatives of the great white shark. The earliest fossil teeth resembling those of modern great whites were found in rocks about 65 million years old. Even at this early date, the great white's ancestors were poised in evolutionary terms to exploit the coming superabundance of blubber and meat, and some species took full advantage, including some of the most powerful predators known to have lived in the sea. Today, the last survivors are the great white shark and its close relatives the makos and porbeagles.
Shark behaviors
