Underwater Predators: Sharks

What do you think of first when you think of sharks? Fearsome, big teeth, of course. Sharks, however, have many other interesting features that make them stand out from other denizens of the sea.

The main difference from other fishes is that their skeleton is made from cartilage rather than bone. This cartilage makes sharks very flexible, allowing them to twist 360 degrees and whirl around and bite an unsuspecting diver or fisherman.

Sharks don’t have an air bladder, and if they stop swimming they will sink. To overcome this disadvantage, they have very large, oil-filled livers giving them some buoyancy. An advantage of not having a swim bladder is that it gives sharks great vertical mobility allowing them to rapidly move upward in the water column without the development of bends. In addition, their pectoral fins act as glide-planes and provide great lift as the shark swims.

Shark meat has an unpleasant taste due to the presence of high concentrations of the waste product urea in the tissue. Sharks store urea to maintain an osmotic balance with seawater so as not to have a water loss problem.

Shark reproduction is very different from that of most bony fishes, having a very low output from their internal fertilization and production of large young.

Sharks also have very low growth rates, a problem that is related to the problem of overfishing. The most economically important sharks are the sandbar, bull, and lemon which do not mature until about 12 to 18 years of age. Slow growth is the norm; for example, a tagged immature male sandbar shark was recaptured 15 years later and had only grown about 19 inches and was still immature.

Sharks can see color, as indicated by the presence of cone cells in their retinas. Similar to cats, they have a light-reflecting layer to enhance their night vision. This is important to divers to realize that swimming and diving in shark infested waters at night is more dangerous.

The reason that chumming works so well in attracting sharks is their acute sense of smell. This could be a warning not to dive with even the smallest cut or abrasion.

An interesting sense that sharks possess is one called electroreception. There is a system of jelly-filled pores around the head and mouth called “ampullae of Lorenzini” that can detect small electric fields of less than 0.01 microvolt. This has been used to develop a small shark repelling apparatus for divers to wear that seems to be effective in warding off sharks.

Why Do Sharks Attack Humans?

Sharks do not attack humans for the sole purpose of hunger. In fact, sharks do not know what the feeling of hunger is, and in fact, can go for many months without eating. This is not to say that sharks do not attack with the intention of seeking prey. Many attacks on divers and surfers especially can be attained to searching for food. To a shark, a surfer on a surfboard slightly resembles that of a seal or sea lion, or a diver in a black wetsuit can look like other prey.

Sharks also attack humans because they have been provoked or agitated by the person. Many spear-fishers have been attacked by reef sharks because when they spear fish, the blood from the fish and it’s vibrations can sometimes result in a feeding frenzy by many sharks. Bright colours can also be counted for attacks. As many people have believed in the past, sharks do in fact can see colours, and do indeed have very good eyesight. Avoid wearing the colours of orange and yellow, as this can aggravate the shark, and possibly lead to attacks.

Sharks are in fact attracted by splashing and vibrations in the water, and it can sometimes be attributed to attacks. Most scientists have not been able to predict why and where sharks attacks.

The following is a list of preventative measures you, as a swimmer or diver can do to prevent the possibility of shark attacks:

  • Don’t tease or entice sharks!
  • If you cut or injure yourself… get out! Do not stay in the water with blood around you. Sharks can smell blood from over a mile away.
  • Don’t swim in waters that have been deemed dangerous. Avoid swimming in murky waters.
  • If you feel something brush up against you…. get out of the water to check to make sure that you have not been bitten. Many shark attack victims have noted the lack of pain from being bitten, doctors and scientists have not been able to conclude why this occurs.. so if you have been brushed against by something, get out and investigate. Finally, if you don’t feel right in the water. Then get out! Nothing can be said for “gut feeling.”
  • Watch other fish and turtles in the area–if they start acting erratic–be alert that a shark might be in the area.

Are All Attacks Fatal?

Most shark attacks are not fatal, however, there are a percentage of attacks that are fatal. There are only 4 sharks who consistently attack people: The Great White, The Tiger, The Bull, and The Oceanic White Tip. There are, however, other large sharks that have attacked humans, and can potentially dangerous.

When most sharks attack, the first bite is usually a “tester.” Like most people, when sampling food, they bite once, revel in the taste, and then decide whether or not to continue… with most sharks, sampling occurs as well. The trouble is, with the sampling of a Great White or other larger predatory sharks, the first bite is so massive or severe that many people die from their injuries, and do not actually die from being consumed. A lot of fatalities can be attributed to people bleeding to death or dying from shock.

There are different modes of shark attacks and investigations that sharks go through when they come across humans. The following list shows what a shark can do when it comes across a human.

  • Indifference (rare)
  • Approach with swift visual inspection from a distance without follow-up
  • Approach with surveillance circling – without follow up or follow-up, contact and attack
  • Approach with brush-past, without follow-up (wounding possible)
  • Charge with collision (upwards trajectory generally)
  • Charge with single or double investigative bite without tearing
  • Charge with biting and removal of flesh (death in 45% of cases)
  • Multiple feeding-frenzy charge (death in 100% of cases)


Ernest S. Campbell, M.D., FACS

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