Sharks, the often-feared yet fascinating cartilaginous fish, have been swimming in our oceans for hundreds of millions of years and play a crucial role in the marine ecosystem. From the gigantic whale shark to the tiny dwarf lanternshark, over 500 species exist, each with unique adaptations and characteristics.
1. Ancient Creatures
Sharks are among the oldest animal groups on Earth still existing today. The first shark-like fish appeared over 420 million years ago, even before the dinosaurs. They have survived multiple mass extinctions.
2. Skeleton of Cartilage
Unlike bony fish and land animals, sharks do not have bones. Their skeleton is made entirely of cartilage, the same flexible material that makes up our nose and ears. Cartilage is lighter and more flexible than bone.
3. No Swim Bladder
Most bony fish have a swim bladder, a gas-filled organ that helps regulate buoyancy. Sharks lack this. They partially compensate with a large, oily liver and by constantly swimming to avoid sinking (although some bottom-dwelling sharks can rest).
4. Multiple Rows of Teeth
Sharks don’t just have one row, but multiple rows of teeth lined up behind each other. When a tooth in the front row falls out or is damaged, a tooth from the row behind moves forward to replace it. A shark can go through tens of thousands of teeth in its lifetime.
5. Electroreception: The Sixth Sense
Sharks possess a unique sense called electroreception. They have special pores on their snouts, the Ampullae of Lorenzini, which allow them to detect the weak electrical fields generated by the muscle movements of other animals. This helps them find prey hidden in the sand or in murky water.
6. Teeth on Their Skin
A shark’s skin feels rough like sandpaper (when rubbed from tail to head). This is because the skin is covered with tiny, tooth-like structures called dermal denticles. These reduce drag in the water and provide protection.
7. Huge Diversity in Size
Sharks vary enormously in size. The smallest is the dwarf lanternshark, which only grows to about 20 cm (8 inches) long. The largest is the whale shark, which can exceed 12 meters (40 feet) in length and is the largest fish in the world.
8. Whale Sharks are Filter Feeders
8. Whale Sharks are Filter FeedersDespite their enormous size, whale sharks (and basking sharks) are completely harmless to humans. They are filter feeders that consume plankton, small fish, and krill by filtering huge amounts of water through their gills.
9. Breathing by Swimming
Many pelagic shark species (those living in open water), like the great white shark and mako shark, must swim constantly to force water over their gills and thus obtain oxygen (ram ventilation).
Other species, especially bottom-dwelling sharks like nurse sharks, can actively pump water over their gills and can therefore rest on the seabed.
10. Cold- and Warm-Blooded
Most sharks are cold-blooded (poikilothermic), meaning their body temperature depends on the environment.
Some active species, like the great white shark and makos (family Lamnidae), are partially warm-blooded (regional endothermy). They can maintain the temperature in certain parts of their body (like muscles, eyes, and brain) higher than the surrounding water, making them faster and more efficient in cold water.
11. Different Reproductive Strategies
Sharks exhibit three modes of reproduction:
- Oviparity: Egg-laying (e.g., catsharks, zebra sharks). The eggs are often enclosed in a protective case (‘mermaid’s purse’).
- Ovoviviparity: Eggs develop inside the mother’s body and hatch there, after which the young are born live (e.g., nurse sharks, spiny dogfish). There is no placental connection. Sometimes the strongest embryos eat the other eggs or embryos (oophagy/intrauterine cannibalism).
- Viviparity: Live-bearing. Embryos develop inside the mother and are nourished via a yolk-sac placenta, similar to mammals (e.g., hammerheads, lemon sharks).
12. Long Gestation Periods
Some shark species have extremely long gestation periods. The spiny dogfish is a record holder with a gestation period of nearly two years. The Greenland shark’s might be even longer.
13. Greenland Shark Lives Extremely Long
The Greenland shark, living in the cold, deep waters of the North Atlantic and Arctic Oceans, is one of the longest-living vertebrates on Earth. Research suggests they can live for at least 272 years, and possibly even 400 years or older. They grow extremely slowly.
14. Shark Attacks are Rare
Despite their reputation in movies and media, shark attacks on humans are very rare. Most shark species are harmless to people. The species responsible for most (still rare) incidents are the great white shark, tiger shark, and bull shark.
Often, these are ‘test bites’ out of curiosity or confusion (e.g., mistaking a surfer for a seal). The chance of being struck by lightning or drowning is many times greater than being bitten by a shark.
15. Sharks are Crucial for the Ecosystem
Sharks are apex predators or important mesopredators in most marine ecosystems. They play a crucial role in keeping oceans healthy by:
- Controlling populations of other species.
- Removing sick and weak animals.
- Influencing the behavior of prey animals, which in turn affects lower levels of the food chain (e.g., protecting seagrass beds).
16. Many Species Seriously Threatened
Unfortunately, many shark populations worldwide are severely threatened. The main causes are:
- Overfishing: Both targeted fishing (for meat, fins, oil) and bycatch in other fisheries (like tuna and swordfish fisheries).
- Fins for Shark Fin Soup: Cutting off fins and often discarding the shark back into the water alive (‘shark finning’) is a particularly destructive practice.
- Habitat Loss: Degradation of important habitats like coral reefs and mangroves.
- Pollution: Plastic and chemical pollution.
17. Slow Reproduction Makes Them Vulnerable
Sharks are particularly vulnerable to overfishing because they generally reproduce slowly. They mature late, have long gestation periods, and produce relatively few young. This makes it difficult for populations to recover from heavy fishing pressure.
18. Some Sharks Glow in the Dark
Several deep-sea shark species, such as the dwarf lanternshark and the kitefin shark, are bioluminescent. They can produce their own light, possibly for camouflage (counter-illumination), communication, or luring prey.
19. Migration Over Vast Distances
Many shark species travel enormous distances during their migrations, sometimes thousands of kilometers between feeding grounds and breeding areas. Great white sharks, for example, can migrate between California and Hawaii, or between South Africa and Australia.
20. Sharks in the North Sea
Several shark species also inhabit the North Sea, although they often go unnoticed. Examples include the lesser spotted dogfish, nursehound, spiny dogfish, and the (rarer) starry smooth-hound. The basking shark is also occasionally sighted.
Sharks are impressive and essential inhabitants of our oceans, with a history stretching back much further than humanity’s. Their diverse forms, unique senses, and crucial ecological role deserve respect and protection. Combating overfishing and protecting their habitats are crucial to ensure that these ancient rulers of the sea can continue to exist.
