Shark

Sharks are fascinating fish that have lived in the oceans for hundreds of millions of years. They are special because their bodies are made of cartilage, not bones like most other fish. Sharks have skin covered with tiny tooth-like scales called denticles. These help protect them and make them swim smoothly through the water.

Sharks come in many different sizes. The smallest shark, the dwarf lanternshark, is only about 17 centimeters long. The largest, the whale shark, can grow up to about 12 to almost 19 meters long—making it the biggest fish in the world! Sharks live in all the world's oceans, from shallow coastal waters to deep sea. Some, like the bull Shark, can even travel between saltwater and freshwater.

Many sharks are top predators. This means they are at the top of the food chain. They help keep the balance of ocean ecosystems. However, many shark species are in danger because of overfishing and the harmful practice of shark finning. Protecting sharks is important for keeping our oceans healthy.

Etymology

Until the 1500s, sailors called sharks "sea dogs." You can still see this in names like "dogfish" and the porbeagle.

We do not know exactly how the word shark was first used. One idea is that it comes from a Dutch word meaning "villain" because sharks are strong hunters. Another old idea said it came from a language spoken in Yucatec Maya, but this is not correct anymore.

Evolutionary history

See also: Evolution of fish

Sharks have been around for a very long time. The first shark-like fish, called spiny sharks, lived about 439 million years ago in a time called the Silurian period. True sharks appeared later, during the Devonian period, and have changed into many different kinds over millions of years.

Today, there are more than 500 kinds of sharks. Some well-known sharks are the blue shark, tiger shark, great white shark, and whale shark. Sharks belong to a group of fish called cartilaginous fish, which also includes rays and skates. Scientists study sharks to learn how different kinds are related.

Anatomy

Sharks have special teeth that grow in rows inside their gums and move forward like a conveyor belt when old teeth fall out. These teeth change shape depending on what the shark eats. Some teeth are good for crushing hard shells, while others are sharp for holding fish or cutting larger prey.

Unlike most fish, sharks have skeletons made of flexible cartilage instead of hard bone. This helps them stay light and swim easily. Their fins are supported by soft, flexible rays, and their skin has tiny tooth-like structures called denticles. These denticles help reduce drag in the water and can form patterns like stripes or spots, helping sharks hide. Sharks also use their tails for swimming, and different tail shapes help them chase different kinds of prey.

Physiology

Unlike bony fish, sharks do not have gas-filled swim bladders. Instead, they have a large liver filled with oil that helps them stay afloat. Some sharks, like the sand tiger shark, can store air in their stomachs to help them float.

Sharks breathe by taking in water through their mouths and passing it over their gills. Most sharks can pump water over their gills when resting, but some must keep swimming to breathe. Most sharks match the temperature of their environment, but some, like the great white shark, can keep their bodies warmer than the water around them.

Senses

Sharks have strong senses that help them find food and move through the ocean. One of their best senses is smell. They can find tiny amounts of substances, like a small drop of blood in a lot of water. This helps them find food even when it is far away or hidden.

Sharks also have good eyesight, especially in dim light. They can see in both directions at once and protect their eyes with a special membrane when hunting. In addition to smell and sight, sharks can sense electrical fields made by all living things, which helps them find hidden food. They also have a special system to feel movements and changes in the water around them.

Life history

Sharks live for different lengths of time depending on the species. Most live between 20 and 30 years. The spiny dogfish can live more than 100 years, and whale sharks might also live over 100 years. The Greenland shark may be the longest-lived vertebrate known, with one fish thought to be at least 272 years old.

Sharks have fewer babies than most fish, but their young are more developed when born. They grow slowly, with some species not reaching maturity until they are 13–15 years old. Male sharks have special fins called claspers they use during mating. Scientists have found a few cases where female sharks can have babies without a male through a process called parthenogenesis, but this is rare in the wild.

Sharks have different ways of having their babies. Most sharks give birth to live young that grow inside the mother and are fed from the egg yolk. Some sharks lay eggs that are protected by a tough case, often called a mermaid's purse. Others give birth to live young without using a traditional egg, and these babies are fully formed when they are born.

Behavior

Sharks are often thought of as solitary hunters, but many species live in social groups. Some sharks stay close to the ocean floor and have unique personalities. Even those that usually live alone gather for breeding or when they find plenty of food, traveling great distances.

Sharks can also live in large schools, with groups of over 100 sharks staying together. They may have social hierarchies, where some sharks are dominant over others. When sharks feel threatened, they show this by swimming in exaggerated movements.

Ecology

Most sharks eat meat. Some sharks, like basking sharks, whale sharks, and megamouth sharks, eat tiny plants and animals called plankton. They use their gills to catch these small creatures.

Sharks live in all kinds of ocean waters, from shallow areas to very deep water. Some sharks, like the bull shark, can live in both saltwater and freshwater. They are found almost everywhere in the ocean.

Relationship with humans

Attacks

Only a few sharks can be dangerous to humans. Out of more than 470 types, just four—the great white, oceanic whitetip, tiger, and bull sharks—have been in many serious attacks on people. These sharks are big and strong, but they have been seen close to people without needing special cages.

Sharks usually do not attack humans. They like to eat fish and might sometimes think a person looks like their normal food, like seals. Studies show that humans and sharks can share the water safely.

In captivity

Keeping sharks in aquariums used to be very hard, but now more types can live longer in tanks because of better care and ways to move them. For example, the Monterey Bay Aquarium kept a great white shark for nearly two hundred days before letting it go.

Most sharks are not good for home aquariums. Some smaller types can live in home saltwater tanks, but they need lots of space and care. Public aquariums are better places to see sharks up close.

In culture

In Hawaii

Sharks are important in Hawaiian mythology. Stories tell of people who could change between human and shark forms. Sharks also appear as gods and guardians in Hawaiian culture. One well-known shark god is Kamohoali'i, who is connected to the goddess Pele.

In American Samoa

In American Samoa, there is a famous story about two people who turned into a turtle and a shark. This story is still celebrated today by villagers with special events.

In popular culture

In many Western stories and movies, sharks are shown as scary threats. Famous examples include the Jaws series and films like Deep Blue Sea. But sharks are also used for funny moments in movies like Finding Nemo and cartoons such as Tom and Jerry.

Popular misconceptions

It is a myth that sharks cannot get sick or cancer. Sharks can get illnesses and even cancer. Ideas that shark fins can treat sicknesses like osteoarthritis are not proven by science.

Threats to sharks

Many sharks face big problems because of humans. Fishing for sharks is a major issue, with many sharks caught each year for their meat, fins, and other body parts. This overfishing has caused shark numbers to drop a lot, and some kinds of sharks are now in danger of disappearing.

Sharks are also sometimes killed to protect people, even though this does not make swimming safer. Some places use nets or traps to catch and kill sharks near beaches, but these methods can also harm other sea animals. Losing sharks is bad for the whole ocean because they help keep the marine ecosystem balanced and healthy.

Conservation

Sharks face many threats, and many countries have worked to protect them. In 1991, South Africa became the first country to protect Great White sharks. In 2000, the United States passed a law to stop a harmful practice. Over time, more laws were added to protect sharks around the world.

Many sharks are now at risk because of fishing. Some areas have become shark sanctuaries where sharks are safe from being caught. Scientists are studying sharks to learn how to protect them and keep our oceans healthy.