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, which 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, while 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, and some, like the bull shark, can even travel between saltwater and freshwater.

Many sharks are top predators, meaning they are at the top of the food chain and 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," which you can still see in names like "dogfish" and the porbeagle.

We aren't exactly sure how the word shark came to be. The most likely idea is that it comes from a Dutch word meaning "villain" because sharks are fierce hunters. Another old idea, which isn’t true anymore, thought the word came from a language spoken in Yucatec Maya, but this doesn’t seem right.

Evolutionary history

See also: Evolution of fish

Sharks have a long history on Earth. The earliest shark-like fish, called spiny sharks, appeared about 439 million years ago during a time called the Silurian period. True sharks began to appear later, during the Devonian period, and have evolved into many different types over millions of years.

Today, there are more than 500 kinds of sharks grouped into thirteen orders. Some well-known types include the blue shark, tiger shark, great white shark, and whale shark. Sharks are part of a larger group of fish called cartilaginous fish, which also includes rays and skates. Scientists study their family tree to learn how different shark types are related to each other.

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—crushing teeth for hard shells, sharp teeth for holding fish, or cutting teeth for larger prey.

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

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 very strong senses that help them find food and navigate the ocean. One of their best senses is smell. They can detect tiny amounts of substances, like a tiny drop of blood in a lot of water. This helps them find prey even when it's far away or hidden.

Sharks also have good eyesight, especially in dim light, thanks to a special layer in their eyes that helps them see better in the dark. They can also 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 produced by all living things, which helps them locate hidden prey. They also have a special system to feel movements and changes in the water around them.

Life history

Shark lifespans vary by species, with most living 20 to 30 years. The spiny dogfish can live more than 100 years, and whale sharks may also live over 100 years. Recent studies show that the Greenland shark could be the longest-lived vertebrate known, with one specimen estimated to be at least 272 years old.

Unlike most fish, sharks produce fewer but more developed young. They mature slowly, with some species reaching sexual maturity around age 13–15. Sharks have special fins called claspers that males use to deliver sperm during mating. Scientists have also observed a few cases where female sharks can have babies without a male through a process called parthenogenesis, though this is rare in the wild.

Sharks have different ways of having their babies. Most give birth to live young that develop inside the mother and are nourished by the remains of the egg yolk. Some sharks lay eggs 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 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. Shark migrations can be very complex, sometimes crossing entire oceans.

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 are carnivorous, meaning they eat meat. Some sharks, like basking sharks, whale sharks, and megamouth sharks, have special ways of eating tiny plants and animals called plankton. They use their gills to trap 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 even live in both saltwater and freshwater. They are found almost everywhere in the ocean, except in very deep water below 3,000 meters.

Relationship with humans

Attacks

Only a few sharks are dangerous to humans. Out of more than 470 species, just four—the great white, oceanic whitetip, tiger, and bull sharks—have been involved in many serious attacks on people. These sharks are large and powerful, but they have also been filmed without the need for protective cages.

Sharks do not usually attack humans. They often prefer the blood of fish and may sometimes mistake a person for one of their normal prey, like seals. Research shows that humans and sharks can coexist in the water safely.

In captivity

Keeping sharks in aquariums used to be very difficult, but now more species can live longer in captivity thanks to better care and transportation methods. For example, the Monterey Bay Aquarium kept a great white shark for nearly two hundred days before releasing it.

Most sharks are not suitable for home aquariums. Some smaller species can live in home saltwater tanks, but they need a lot 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 legend about two people who turned into a turtle and a shark. This story is still celebrated today by villagers performing rituals.

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. However, 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 are immune to disease or cancer. Sharks can get sick and even develop cancer. Claims that shark fins can treat illnesses like osteoarthritis are not supported by science.

Threats to sharks

Many sharks face serious threats from human activities. Fishing for sharks is a big problem, with millions of sharks caught each year for their meat, fins, and other body parts. Overfishing has caused shark populations to drop sharply, and some species are now in danger of disappearing forever.

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 often harm other sea animals too. Losing sharks hurts 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 shark finning, a harmful practice. Over time, more laws were added to protect sharks around the world.

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