Shark

Sharks are fascinating fish that have been swimming in the oceans for hundreds of millions of years. They are part of a group called cartilaginous fishes, which means their skeletons are made of flexible cartilage instead of hard bone, like most other fish. Sharks have special skin coverings called dermal denticles that help protect them and improve how smoothly they move through the water.

Sharks come in many different sizes and shapes. Some, like the tiny dwarf lanternshark, are only about as long as your hand, while others, like the huge whale shark, are among the largest animals in the sea. They live in all parts of the world's oceans, from shallow coastal waters to deep waters far below the surface.

Many sharks are top hunters, known as apex predators, playing an important role in keeping the balance of marine life. However, sharks face serious threats from humans, especially from overfishing and the harmful practice of shark finning. Because of these dangers, many shark populations have been decreasing, making conservation very important.

Etymology

Before the 1500s, sailors called sharks "sea dogs," a name you can still see in some species like the dogfish and the porbeagle.

We aren't exactly sure how the word shark came to be. One good guess is that it comes from a Dutch word meaning "villain" or "scoundrel," because sharks are fierce hunters. Another old idea is that the word comes from a Yucatec Maya word for shark, but this theory is no longer believed to be true.

Evolutionary history

The oldest known shark-like fish, called acanthodians or "spiny sharks", appeared about 439 million years ago during the Silurian period. True sharks began appearing later, during the Devonian period. Over time, sharks evolved into many different groups. Today, there are more than 500 species of sharks belonging to thirteen orders.

Sharks are part of a larger group called Chondrichthyes, which also includes rays and skates. They are divided into several groups, such as Carcharhiniformes (which includes blue sharks and hammerhead sharks), Lamniformes (including the great white shark), and Orectolobiformes (the carpet sharks). Other groups include the sawsharks with their special snouts and the angel sharks that look like stingrays.

Anatomy

Sharks have unique bodies that help them survive in the water. Their teeth are special because they grow continuously throughout their lives, replacing old teeth like a conveyor belt. These teeth come in different shapes depending on what the shark eats — some are flat for crushing shellfish, while others are sharp for catching fish.

Sharks also have skeletons made of flexible cartilage instead of hard bone, which makes them lighter. Their fins are supported by soft, elastic rays and help them move smoothly through the water. The skin of a shark is covered with tiny, tooth-like structures called dermal denticles that reduce friction and help them blend into their surroundings with patterns like stripes or spots. Their tails vary in shape, helping them swim efficiently and catch prey.

Physiology

Unlike bony fish, sharks do not have gas-filled swim bladders. Instead, they have a large liver filled with oil, which helps them stay afloat. Some sharks, like the Sand tiger sharks, 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. This is called "ram ventilation." Some sharks must keep moving to breathe, while others can pump water over their gills when resting. Most sharks are cold-blooded, but some, like the great white shark, can keep their bodies warmer than the water around them.

Senses

Sharks have excellent senses that help them find food and navigate the ocean. Their olfactory sense of smell is very strong, allowing them to detect tiny amounts of substances like blood in the water. They can even tell where a smell is coming from by comparing how quickly it reaches each nostril.

Sharks also have good eyes, adapted for seeing in the dim light of the deep sea. Many have a special layer called tapetum lucidum that helps them see better in the dark. Some sharks can change the size of their pupils, similar to humans, and they protect their eyes with a special membrane when hunting. While sight is important, scientists think sharks also rely on other senses to find prey. Their retinas usually give them good night vision, though they may not see colors well.

In addition to smell and sight, sharks can hear sounds from far away and can detect the faint electrical signals that all living things give off. This helps them locate hidden prey, even in sandy ocean floors. Sharks also have a special system called the lateral line, which lets them feel movements and changes in the water around them.

Life history

Shark lifespans differ depending on the species. Most sharks live between 20 to 30 years, but some, like the spiny dogfish, can live more than 100 years. The Greenland shark is thought to be the longest-living vertebrate known, with one specimen estimated to be over 272 years old.

Unlike many other fish, sharks have a slow reproduction rate. They produce fewer, well-developed young instead of many small ones. For example, lemon sharks don’t reach maturity until about age 13–15. Sharks have unique ways of reproducing, including internal fertilization where the male uses special fins called claspers to deliver sperm to the female. Some female sharks can even reproduce without a male through a process called parthenogenesis, though this is rare in the wild.

Sharks have different methods of bearing their young. Most are ovoviviparous, meaning the eggs hatch inside the mother and the young are born alive. Some sharks are oviparous, laying eggs protected by a tough case often called a mermaid's purse. Others are viviparous, giving birth to live young without an egg, like hammerheads and bull sharks.

Behavior

Sharks are often thought of as lonely hunters, but many live in social groups and have unique personalities. Even those that usually live alone gather for breeding or when they find good places to hunt, sometimes traveling thousands of miles a year. Their travel patterns can be very complex, covering large areas of the ocean.

Sharks can also live together in large groups. For example, over 100 scalloped hammerheads might come together around underwater mountains or islands, such as in the Gulf of California. Some sharks even have social rankings, where one type of shark might lead another when eating.

When sharks feel threatened, they show this by swimming in big, clear movements to warn others away.

Ecology

Most sharks are carnivorous, meaning they eat meat. Some, like basking sharks, whale sharks, and megamouth sharks, eat tiny plants and animals by filtering water through special structures in their gills. Other sharks hunt different kinds of food. For example, angel sharks hide on the ocean floor and suck in prey, while great white sharks bite large pieces from their food.

Sharks live in all the world's seas, from shallow waters to very deep ocean. Some, like the bull shark, can also live in rivers and other freshwater places. They are found almost everywhere in the ocean, except in the very deepest parts.

Relationship with humans

Attacks

Sharks are generally not a threat to humans. Out of over 470 species, only a few have been involved in attacks. The great white, oceanic whitetip, tiger, and bull sharks are known for sometimes attacking people, but these are rare events.

Sharks usually prefer eating fish and may mistake humans for their normal prey. Researchers have found that sharks and humans can coexist in the water without incidents.

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 transport methods. The Monterey Bay Aquarium was the first to successfully keep a great white shark for a period of time. While many sharks are not suitable for home aquariums, some smaller species can live in large home saltwater tanks.

In culture

In Hawaii

Sharks have an important place in Hawaiian mythology. Stories tell of shark gods and people who could change between shark and human forms. One famous shark god is Kamohoali'i, a brother of the goddess Pele. He helped protect people and was very respected.

In American Samoa

In American Samoa, there is a special place called Turtle and Shark where a legend says two people turned into a turtle and a shark. People there still perform rituals to honor this story.

In popular culture

In many Western stories and movies, sharks are shown as scary threats, like in the Jaws films. However, in some cartoons and funny movies, sharks are used for comedy, like in Finding Nemo.

Popular misconceptions

It is a myth that sharks cannot get sick or cancer. Sharks can suffer from diseases and cancer, just like other animals. There is no proof that shark products can treat human illnesses.

Threats to sharks

Sharks face many dangers from human activities. One big problem is fishing. Many sharks are caught for their meat and fins every year. In some places, like Japan and Australia, shark is a popular food. Sadly, some people catch sharks just to take their fins for soup and then throw the rest of the shark back into the water, where it usually cannot survive. This practice is called shark finning.

Sharks also face threats from special fishing programs meant to keep people safe. For example, in Queensland and New South Wales in Australia, sharks have been caught and killed using special nets and traps. These programs have also harmed other animals like dolphins and turtles. Scientists say these programs are not good for the ocean because sharks help keep the marine ecosystem 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 the harmful practice of shark finning, which was later strengthened in 2010.

Many sharks are now at risk of disappearing forever. In 2009, scientists found that one-third of oceanic shark species were in danger because of fishing and finning. Efforts continue worldwide to help protect sharks and their important role in the ocean ecosystem.