Shark Facts—Bite Off More Than You Can Chew
Worldwide
- About 400 described species of sharks are known; however, scientists describe new species all the time.
- Worldwide there is an average of 50 to 70 shark attacks every year. The number of attacks has increased over the decades as a result of increased human populations and the use of the oceans for recreational activity.
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Shortfin mako shark. Credit: Walter Heim
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United States
- NOAA Fisheries manages more than 40 species of sharks in U.S. waters.
- In the United States, federal law prohibits shark finning, the process of removing shark fins at sea and discarding the rest of the shark.The practice has been prohibited by federal law since 2000 and shark conservation was further strengthened in 2010 when Congress passed the Shark Conservation Act. The Shark Conservation Act requires that all sharks in the United States, except for smooth dogfish and sharks harvested in state waters, be brought to shore with their fins naturally attached.
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Blue shark. Credit: NOAA |
Size and Age - The largest shark, and also the largest fish in the ocean, is the whale shark (Rhincodon typus). This massive plankton-feeder reaches lengths more than 60 feet. The smallest shark is a deepwater dogfish shark (Etmopterus perryi). Found in in the Caribbean Sea, this species matures at less than 8 inches.
- Fossil records indicate that ancestors of modern sharks swam the seas more than 400 million years ago—making sharks older than dinosaurs!
- While longevity data are not available for many sharks, maximum ages do vary by species. Some sharks like the smooth dogfish (Mustelus canis) might only live 16 years, while others such as the porbeagle shark, (Lamna nasus) may live as long as 46 years. Whale sharks (Rhincodon typus), the largest fish in the world, might live longer than 100 years.
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Whale shark. Credit: NOAA
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Physiology - Sharks lack true bone. Instead, they have cartilaginous skeletons. Cartilage is a type of connective tissue strong enough to give support but softer than true bone.
- Shark skin feels exactly like sandpaper because it is made up of tiny teeth-like structures called placoid scales, also known as dermal denticles. These scales point towards the tail and help reduce friction from surrounding water when the shark swims. Because of this, if someone rubbed the skin from the head towards the tail, it would feel very smooth. In the opposite direction, shark skin feels very rough. As the shark grows, the placoid scales do not increase in size, but rather the shark grows more scales.
- Sharks have several adaptations that can aid their ability to be neutrally buoyant. Their cartilaginous skeletons are much lighter than true bone, and they have large livers full of low-density oils, which provide some buoyancy.
- Sharks lack a swim bladder, but some species of shark, like the sand tiger (Carcharias taurus), can actually gulp air into their stomach for additional buoyancy.
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Sand tiger shark. Credit: NOAA
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Reproduction - Sharks exhibit a great diversity in reproductive modes. There are egg-laying (oviparous) species and live-bearing (viviparous) species. Oviparous species lay eggs that develop and hatch outside the mother's body with no parental care after the eggs are laid. The embryos are nourished by a yolk-sac inside the egg capsule.
- Viviparous species can be separated into two categories: placental (having a placenta, or true connection between maternal and embryonic tissue), or aplacental (lacking a placenta). Among the aplacental species, there are those whose embryos rely primarily on a yolk-sac for nutrition during gestation and those that consume yolk-filled, unfertilized egg capsules (oophagy). There is even one species, the sandtiger (Carcharias taurus), in which the two largest embryos that were fertilized first, consume the other embryos of the litter (adelphophagy). There is no parental care after birth among viviparous sharks.
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Nurse shark. Credit: NOAA
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