SHARK, The King of The Deep Blue Sea.

SHARK

Physical characteristics

Skeleton

The skeleton of a shark is very different from that of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made from rubbery cartilage, a tissue lighter and more flexible than bone.
The general rule is that there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have been found to be covered with both two and three layers, and even more, depending on the body size. The jaws of a large white shark even had five layers.

Respiration

Like other fish, sharks extract oxygen from seawater as it passes over their gills. Shark gill slits are not covered like other fish, but are in a row behind its head. A modified slit called a spiracle is located just behind the eye; the spiracle assists the water intake during respiration and even plays a major role in bottom dwelling sharks, but is also reduced or missing in active pelagic sharks. While moving, water passes through the mouth of the shark and over the gills — this process is known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small subset of shark species that spend their life constantly swimming, a behaviour common in pelagic sharks, have lost the ability to pump water through their gills. These species are obligate ram ventilators and would presumably asphyxiate if unable to stay in motion. (Obligate ram ventilation is also true of some pelagic bony fish species.)

Osmoregulation

In contrast to bony fish, the blood and other tissue of sharks and Chondrichthyes in general is isotonic to their marine environments because of the high concentration of urea and trimethylamine oxide, allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in fresh water, and they are therefore confined to a marine environment.

Teeth

The teeth of carnivorous sharks are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the shark's life; some sharks can lose 30,000 teeth in a lifetime. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some sharks rows of teeth are replaced every 8–10 days,

Tails

The tails (caudal fins) of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape.

The tiger shark's tail has a large upper lobe which delivers the maximum amount of power for slow cruising or sudden bursts of speed.

Some tail adaptations have purposes other than providing thrust. The cookiecutter shark has a tail with broad lower and upper lobes of similar shape which are luminescent and may help to lure prey towards the shark.

Dermal denticles

Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibres and arranged as a helical network surrounding their body.Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.

Body temperature

A few of the larger species, such as the shortfin mako, Isurus oxyrinchus, and the great white, are mildly homeothermic - able to maintain their body temperature above the surrounding water temperature. This is possible because of the presence of the suprahepatic rete, a counter current exchange mechanism that reduces the loss of body heat.

Lifespan

Maximum shark ages vary by species. Most sharks live for 20 to 30 years, while the spiny dogfish lives a record lifespan of more than 100 years. Whale sharks (Rhincodon typus) have been hypothesized to also live over 100 years.

Evolution

The fossil record of sharks extends back over 450 million years - before land vertebrates existed and before many plants had colonised the continents. The first sharks looked very different from modern sharks. The majority of the modern sharks can be traced back to around 100 million years ago. Mostly only the fossilized teeth of sharks are found, although often in large numbers. In some cases pieces of the internal skeleton or even complete fossilized sharks have been discovered.

Instead of bones, sharks have cartilagenous skeletons, with a bone-like layer broken up into thousands of isolated apatite prisms. When a shark dies, the decomposing skeleton breaks up and the apatite prisms scatter.

Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago, which has been found within the Paleozoic strata of Ohio, Kentucky and Tennessee.

From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. One of these, the acanthuses, was almost exclusive to freshwater environments. By the time this group became extinct (about 220 million years ago) they had achieved worldwide distribution. The other group, the hybodonts, appeared about 320 million years ago and was mostly found in the oceans, but also in freshwater.
Modern sharks began to appear about 100 million years ago.

Classification

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks.
There are more than 360 described species of sharks split across are eight orders of sharks.

Asexual reproduction

In December 2001, a pup was born from a female hammerhead shark who had not been in contact with a male shark for over three years. This has led scientists to believe that sharks can reproduce without the mating process.
After three years of research, this assumption was confirmed on May 23, 2007, after determining the shark born had no paternal DNA, ruling out any sperm-storage theory as previous thought. It is unknown as to the extent of this behaviour in the wild, and how many species of shark are capable of parthenogenesis. This observation in sharks made mammals the only remaining major vertebrate group in which the phenomenon of asexual reproduction has not been observed.

Shark senses

Sense of smell

Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater.

Sense of sight

Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to the marine environment with the help of a tissue called tapetum lucidum.

Sense of hearing

Sharks also have a sharp sense of hearing and can hear prey many miles away.

Electroreception

The Ampullae of Lorenzini are the electroreceptor organs of the shark, and they vary in number from a couple of hundred to thousands in an individual. Sharks use the Ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. This helps sharks find its prey (mostly the hammer head). The shark has the greatest electricity sensitivity known in all animals.

Lateral line

This system is found in most fish, including sharks. It is used to detect motion or vibrations in the water. The shark uses this to detect the movements of other organisms, especially wounded fish. The shark can sense frequencies in the range of 25 to 50 Hz.

Behaviour

Studies on the behaviour of sharks have only recently been carried out leading to little information on the subject, although this is changing. The classic view of the shark is that of a solitary hunter, ranging the oceans in search of food; however, this is only true for a few species, with most living far more sedentary, benthic lives.

Shark intelligence

Despite the common myth that sharks are instinct-driven "eating machines", recent studies have indicated that many species possess powerful problem-solving skills, social complexity and curiosity. The brain-mass-to-body-mass ratios of sharks are similar to those of mammals and other higher vertebrate species.

Shark sleep

Some say a shark never sleeps. It is unclear how sharks sleep. Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, it does not use its nares, but rather its spiracles.

Habitat

A December 10, 2006 report by the Census of Marine Life group reveals that 70% of the world's oceans are shark-free. They have discovered that although many sharks live up to depths as low as 1,500 metres (5,000 ft), they fail to colonize deeper, putting them more easily within reach of fisheries and thus endangered status.

Shark attacks

Sharks rarely attack humans unless provoked. In 2006 the International Shark Attack File (ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities per year between 2001 and 2006 from unprovoked shark attacks is 4.3.
Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 360 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger, and bull sharks.

Shark fishery

An estimate states that, every year, 26 to 73 million (median value is at 38 million) sharks are killed by people in commercial and recreational fishing. In the past, sharks were killed simply for the sport of landing a good fighting fish (such as the shortfin mako sharks). Shark skin is covered with dermal denticles, which are similar to tiny teeth, and was used for purposes similar to sandpaper. Other sharks are hunted for food (Atlantic thresher, shortfin mako and others), and some species for other products.
Sharks are a common seafood in many places around the world, including Japan and Australia.

Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fish that are harvested. This has caused concern among biologists regarding the increase in effort applied to catching sharks over time, and many species are considered to be threatened.
Some organizations, such as the Shark Trust, campaign to limit shark fishing. According to Seafood Watch, sharks are currently on the list of fish that American consumers, who are sustainability minded, should avoid.

Sharks in cultural tradition

In ancient Greece, it was forbidden to eat shark flesh at women's festivals.
A popular myth is that sharks are immune to disease and cancer; however, this is untrue. There are both diseases and parasites that affect sharks. The evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few, if any, scientific or statistical studies that have shown sharks to have heightened immunity to disease.