All About Sharks

Sharks (superorder Selachimorpha) are fish with a full cartilaginous skeleton and a streamlined body (versus fish with boney skeletons). They breathe with the use of five to seven gill slits and have replaceable teeth. They are some of the world's most misunderstood predators, as they very rarely attack humans unless intimidated.

Sharks include species from the hand-sized pygmy shark, a deep sea species of only 22 cm in length, to the whale shark, the largest fish in the ocean, which grows to a length of approximately 41 feet and which, like the great whales, feeds only on plankton through filter feeding.

The sharks on exhibit at the Indianapolis Zoo are dogsharks, a relatively small species.  The bull shark is the best known of several species to swim in both salt and fresh water (it is found in Lake Nicaragua, in Central America) and in deltas.
Photo by Kerrie Best

Physical characteristics

Skeleton

The skeleton of a shark is very different from that of bony fishes such as cod; it is made from cartilage, which is very light and flexible, although the cartilage in older sharks can sometimes be partly calcified, making it harder and more bone-like. The shark's jaw  is thought to have evolved from the first gill arch. It is not attached to the cranium and has extra mineral deposits to give it greater strength.

Respiration

Like other fishes, sharks extract oxygen from seawater as it passes over their gills. Some sharks have a modified slit called a spiracle located just behind the eye, which is used in respiration. Due to their size and the nature of their metabolism, many sharks have a higher demand for oxygen than bony fishes and cannot rely on ambient water currents to provide enough oxygenated water. If these sharks were to stop swimming, the water circulation would drop below the level necessary for respiration and the animal would suffocate. Some sharks, such as the nurse shark, can pump water over their gills as they rest.

Buoyancy

Unlike bony fishes, sharks do not have gas-filled swim bladders, but instead rely on a large liver filled with oil. The liver may constitute up to 25% of their body mass and its effectiveness is limited, so sharks employ dynamic lift to maintain depth and sink when they stop swimming. Some sharks, if inverted, enter a natural state of tonic immobility where they do not move - researchers use this condition for handling sharks safely.

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, while in other species they could last several months. The lower teeth are primarily used for holding prey, while the upper ones are used for cutting into it. The teeth range from thin, needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish.

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. Different tail shapes have evolved in sharks adapted for different environments.

Speed

In general, sharks swim ("cruise") at an average speed of 5 mph, but when feeding or attacking, the average shark can reach speeds upward of 12 mph. The shortfin mako may range upwards of 31 mph and is considered to be the fastest shark and one of the fastest fish. The great white shark is also capable of considerable bursts of speed. These exceptions may be due to the "warm-blooded"  nature of these sharks' physiology.

Skin

Unlike bony fishes, sharks have a complex skin made of flexible collagenous fibers arranged around their bodies.  This works as an outer skeleton, providing attachments for their swimming muscles and thus saving energy. The skin also helps them reduce turbulence when swimming.

Evolution

The fossil record of sharks extends back over 450 million years - before land vertebrates existed and before many plants had colonized 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.

Instead of bones, sharks have cartilagenous skeletons, with a bonelike layer broken up into thousands of isolated apatite prisms. When a shark dies, the decomposing skeleton breaks up and the apatite prisms scatter. Complete shark skeletons are only preserved when rapid burial in bottom sediments occurs.  Mostly only the fossilized teeth of sharks are found, although often in large numbers.  As the teeth consist of mineral apatite (calcium phosphate), they are easily fossilized.

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. At this point in the Earth's history these rocks made up the soft sediment of the bottom of a large, shallow ocean that stretched across much of North America. Cladoselache was only about 1 m long with stiff triangular fins and slender jaws. Its teeth had several pointed cusps, which would have been worn down by use. The discovery of whole fish found tail first in their stomachs suggest that they were fast swimmers with great agility.

From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. One of these, the xenacanths, 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. Fossil mackerel shark teeth occurred in the Lower Cretaceous. The oldest white shark teeth date from 60 to 65 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution there are at least two lineages: one with coarsely serrated teeth that probably gave rise to the modern great white shark, and another with finely serrated teeth and a tendency to attain gigantic proportions. This group includes the extinct megalodon.

It is believed that the immense size of predatory sharks such as the great white may have arisen from the extinction of the dinosaurs and the diversification of mammals. It is known that at the same time these sharks were evolving some early mammalian groups evolved into aquatic forms. Certainly, wherever the teeth of large sharks have been found, there has also been an abundance of marine mammal bones, including seals, porpoises and whales. These bones frequently show signs of shark attack. There are theories that suggest that large sharks evolved to better take advantage of larger prey.

Classification

There are more than 360 described species of sharks, which are related to rays and skates.

Reproduction

The sex of a shark can be easily determined. The males have modified pelvic fins that have become a pair of claspers. The name is somewhat misleading as they are not used to hold on to the female, but fulfill the role of the mammalian penis.

Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the female. In less flexible species the two sharks swim parallel to each other while the male inserts a clasper into the female's oviduct. Females in many of the larger species have bite marks that appear to be a result of a male grasping them to maintain position during mating. The bite marks may also come from courtship behavior: the male may bite the female to show his interest. In some species, females have evolved thicker skin to withstand these bites.

Sharks normally produce around a dozen pups (blue sharks have been recorded as producing 135 and some species produce as few as two). These pups are either protected by egg cases or born live. No shark species are known to provide post-natal parental protection for their young, but females have a hormone that is released into their blood during the pupping season that apparently keeps them from feeding on their young.

Smell

Sharks have keen olfactory senses, with some species able to detect as little as one part per million of blood in seawater. They are attracted to the chemicals found in the guts of many species, and as a result often linger near or in sewage outfalls.  Sharks generally rely on their superior sense of smell to find prey, but at closer range they also use the lateral lines running along their sides to sense movement in the water, and also employ special sensory pores on their heads to detect electrical fields created by prey and the ambient electric fields of the ocean.

Sight

Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, although their eyesight is well adapted to the marine environment with the help of a tissue called tapetum lucidum. This tissue is behind the retina and reflects light back to the retina, thereby increasing visibility in the dark waters. The effectiveness of the tissue varies, with some sharks having stronger nocturnal adaptations. Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some have nictitating membranes. This membrane covers the eyes during predation, and when the shark is being attacked. However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey.

Hearing

Sharks also have a sharp sense of hearing and can hear prey many miles away. A small opening on each side of their heads (not to be confused with the spiracle) leads directly into the inner ear through a thin channel.

The shark has the greatest electricity sensitivity known in all animals. This sense is used to find prey hidden in sand by detecting the electric fields inadvertently produced by all fish. It is this sense that sometimes confuses a shark into attacking a boat: when the metal interacts with salt water, the electrochemical potentials generated by the rusting metal are similar to the weak fields of prey, or in some cases, much stronger than the prey's electrical fields: strong enough to attract sharks from miles away. The oceanic currents moving in the magnetic field of the Earth also generate electric fields that can be used by the sharks for orientation and navigation.

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.

Behavior

The classic view of the shark is of a solitary hunter, ranging the oceans in search of food; this is only true for a few species, with most living far more sedentary lives. Even solitary sharks meet for breeding or on rich hunting grounds, which may lead them to cover thousands of miles in a year. Migration patterns in sharks may be even more complex than in birds, with many sharks covering entire ocean basins.

Some sharks can be highly social, remaining in large schools, sometimes up to over 100 individuals for scalloped hammerheads congregating around seamounts and islands e.g. in the Sea of Cortez. Cross-species social hierarchies exist with oceanic whitetip sharks dominating silky sharks of comparable size when feeding.

When approached too closely some sharks will perform a threat display to warn off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat.

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.

Sharks have even been known to engage in playful activities (a trait also observed in cetaceans and primates). Porbeagle sharks have been seen repeatedly rolling in kelp and have even been observed chasing an individual trailing a piece behind them.

Sleep

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, they do not use their nares, but rather their spiracles. If a shark tried to use their nares while resting on the ocean floor, they would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish's spinal cord, rather than its brain, coordinates swimming, so it is possible for a spiny dogfish to continue to swim while sleeping. It is also possible that a shark can sleep with only parts of its brain in a manner similar to dolphins.

Shark Attacks

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, tiger, oceanic whitetip and bull sharks. These sharks, being large, powerful predators, may sometimes attack and kill people, but all of these sharks have been filmed in open water, without the use of a protective cage.

The perception of sharks as dangerous animals has been popularized by publicity given to a few isolated unprovoked attacks, such as the Jersey Shore Shark Attacks of 1916, and through popular fictional works about shark attacks, such as the Jaws film series. The author of Jaws, Peter Benchley, had in his later years attempted to dispel the image of sharks as man-eating monsters.

Habitat & Conservation

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 m, they fail to colonize deeper, putting them more easily within reach of fisheries and thus endangered status.

The majority of shark fisheries around the globe have little monitoring or management. With the rise in demand of shark products there is a greater pressure on fisheries. Stocks decline and collapse because sharks are long-lived apex predators with comparatively small populations, which makes it difficult for them breed rapidly enough to maintain population levels. Major declines in shark stocks have been recorded in recent years - some species have been depleted by over 90% over the past 20-30 years with a population decline of 70% not being unusual. Many governments and the UN have acknowledged the need for shark fisheries management, but due to the low economic value of shark fisheries, the small volumes of products produced and the poor public image of sharks, little progress has been made.

Many other threats to sharks include habitat alteration, damage and loss from coastal developments, pollution and the impact of fisheries on the seabed and prey species.

Shark Fishery

Every year, somewhere between 26 to 73 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. In the Australian State of Victoria, shark is the most commonly used fish in fish and chips.  Sharks are often killed for shark fin soup: the finning process involves capture of a live shark, the removal of the fin with a hot metal blade, and the release of the live animal back into the water. Sharks are also killed for their meat. The meat of dogfishes, smoothhounds, catsharks, skates and rays is in high demand by European consumers. The situation in Canada and the United States is similar: the blue shark is sought as a sport fish while the porbeagle, mako and spiny dogfish are part of the commercial fishery. There have been cases where hundreds of de-finned sharks were swept up on local beaches without any way to convey themselves back into the sea.  Conservationists have campaigned for changes in the law to make finning illegal in the U.S.

Shark cartilage has been advocated as effective against cancer and for treatment of osteoarthritis. However, a trial by Mayo Clinic found no effect in advanced cancer patients.

Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fishes 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.

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.