Sharks

whale-shark

Sharks, fun facts

Sharks have inhabited our oceans for more than 400 million years. They have survived the 5 great extinctions that have taken place on Earth, including the one that wiped out the dinosaurs. Since then and until today, these fish have evolved in surprising ways.

They are extraordinary animals with an anatomy that does nothing but improve over time. Sharks are master hunters with incredible precision. They are found at the top of the food chain and are essential for maintaining the balance of marine ecosystems.

Habitat

Sharks live in all the oceans around the world. Their distribution depends on factors such as depth, temperature and the availability of food.

They can adapt to a wide range of habitats. Sharks live in shallow waters, on the bottom of the ocean, or in the open sea. However, they tend to prefer tropical and temperate zones.

Some species of sharks live in freshwater lakes and rivers. There are even some, like the bull shark, that can swim between salt and fresh water.

Sharks come in all shapes and sizes

Some measure only a few centimetres, like the lantern or the pygmy shark, while others, like the whale shark, the largest fish that exists today, can be up to 12 meters long.

The megalodon shark lived 20 million years ago and has been the largest shark that ever existed. It had an average of between 13 and 15 meters long.

How do sharks sleep

sharks
White tip reef shark resting on the sandy bottom at Blue Lagoon, Padangbai

There are different species of shark, and depending on the way they rest, we can distinguish two types. 

The first are those that need to move continuously to obtain oxygen and breathe. Only then does oxygen enter their gills.

These sharks rest while swimming. This can be done because they do not really sleep, but instead leave part of their brain inactive, resting.

Sharks control their movements with the spinal cord and not with the brain. Because of this, they can deactivate different parts of the brain and continue swimming, alternating periods of full consciousness with semi-unconsciousness. An example of this type of shark that rests while still swimming is the white shark.

The second type of sharks is those that can stop swimming to sleep. They can do this thanks to the fact that they have structures called spiracles, through which they can obtain oxygen without the need to constantly swim.

Thanks to this mechanism, these sharks rest immobile on the seabed, so their daily energy use can be significantly reduced compared to those of the first group.

However, none of the different types of sharks has a swim bladder, so they cannot float if they are still. For this reason, sharks that have spiracles when they want to rest have to go down to the seabed or look for caves to be able to stop swimming.

Sharks do not have eyelids and for this reason, they cannot close their eyes. They sleep with their eyes open.

Their skeleton is made of cartilage instead of bones

This makes them very light and allows them to move very fast.

The fastest shark is the mako. It can swim up to 74 km per hour.

Shark’s skin is rough, tough, and thick

blacktip-sharks
Black tip reef shark, Drop Off, Tulamben

It is composed of dermal denticles, which are made from the same material as teeth.

The denticles limit friction with the water, making the shark swim easily, quickly and silently.

Sharks have jaws with multiple rows of teeth that continually renew

Teeth are rootless and break easily. When this happens, the new tooth can appear as fast as in one day.

A shark can lose up to 1,000 total teeth per year.

Although their mouths are full of teeth, sharks do not chew their food but use them to tear food apart.

Diet

Sharks are mostly predators, so their diet is generally carnivorous. They eat fish, marine mammals, molluscs, or even other sharks. It all depends on the size and habitat of the shark. Some are even scavengers.

However, other species of sharks, such as the whale and the basking shark are filter feeders. They ingest phytoplankton, algae, and krill, as well as larvae, small crustaceans, squid, or even small fish.

Some sharks can go without food for long periods and apply energy-saving measures such as moving very slowly. This happens with the Arctic boreal shark that can live more than 400 years, this technique has made it the longest living vertebrate in the world.

They have a great sense of smell

white-tip-reef-sharks
White tip reef shark, Gili Mimpang, Candidasa

They can smell a single drop of blood from at least 100 meters away, depending on the nostril through which they perceive the smell, they go to the left or right.

They can sense electrical impulses

All sharks have an organ in their snouts, ampullae of Lorenzini, that allows them to sense electrical fields in the water emitted by other fish and marine life. In addition, the vast majority of sharks have a similar lateral line that detects movement in the water.

They have excellent sight

They can see very well even in the dark, allowing them to hunt at night.

Despite its excellent vision, the ocean’s largest predator cannot distinguish colours.

Hammerhead sharks have 360-degree vision

Sharks have no vocal cords

wobbegong-sharks
Wobbegong shark, Gili Tepekong, Candidasa

Therefore, they make no vocal sounds whatsoever. Instead, they communicate through body language.

Mating and reproduction

Sharks grow slowly and take until they reach sexual maturity, and when they reproduce, they produce only a few young.

Mating is a bit wild. The male holds the female with its teeth, biting her fin, often causing large wounds that will mark the female for a lifetime.

Sharks exhibit great diversity in their reproductive modes. There are oviparous (egg-laying) species and viviparous (live-bearing) species.

Gestation time is as varied as the types of sharks in the oceans. It can last between 3 to 4 months or up to 2 years.

From the moment of birth, the young do not receive the protection of their mother as in other creatures of the animal kingdom, but they learn by themselves to survive.

Dangers

There are more than 500 known shark species, and many are in danger of extinction or in decline due to various factors such as overfishing.

Sharks are not dangerous to humans

Of the nearly 400 species, only 12 are considered dangerous to humans and only three are responsible for most attacks on humans: the white shark, the tiger and the bull. Instead, humans kill at least 100 million sharks a year and have put many shark species on highly threatened species lists.

Humans are not part of the diet of sharks. Most of the attacks happen by mistake since the shark mistakes a surfer for a seal or some other type of marine mammal or as a way of “proving” what this curious creature that has been found is about.

Be a responsible diver

Whenever you dive with sharks, remember:

Stay calm and don’t make any sudden movements.

Never stand in the way of the shark.

As you move, don’t turn your back on the shark.

Where we can dive in Bali with sharks

The best place to see sharks in Bali is undoubtedly the islands of Candidasa. Whitetip reef sharks live on these volcanic formed islets. During the season we can also see wobbegong or carpet shark. And if you are lucky enough during the plankton season, you can occasionally see the largest fish that lives in the oceans, the whale shark.

You may also like to read about mola mola

Back to marine life in Bali

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