There are several hundred species of sharks, ranging in size from less than ten inches to over 50 feet. These amazing animals have a fierce reputation, but fascinating biology. Here we’ll explore ten things that define sharks.


1. Sharks are cartilaginous fish.
The term “cartilaginous fish” means that the structure of the animal’s body is formed of cartilage, instead of bone. Unlike the fins of bony fishes, the fins of cartilaginous fishes cannot change shape or fold alongside their body. Even though sharks don’t have a bony skeleton like many other fish, they are still categorized with other vertebrates in the Phylum Chordata, Subphylum Vertebrata, and Class Elasmobranchii. This class is made up of about 1,000 species of sharks, skates and rays.

2. There are over 400 species of sharks.3. Sharks have rows of teeth.
The teeth of sharks don’t have roots, so they usually fall out after about a week. However, sharks have replacements arranged in rows and a new one can move in within one day to take the old one’s place. Sharks have five to 15 rows of teeth in each jaw, with most having five rows.

4. Sharks do not have scales.
A shark has tough skin that is covered by dermal denticles, which are small plates covered with enamel, similar to that found on our teeth.

5. Sharks have a lateral line system, which detects movements in the water.
Sharks have a lateral line system along their sides, which detects water movements. This helps the shark find prey and navigate around other objects at night or when water visibility is poor. The lateral line system is made up of a network of fluid-filled canals beneath the shark’s skin. Pressure waves in the ocean water around the shark vibrate this liquid. This, in turn is transmitted to jelly in the system, which transmits to the shark’s nerve endings and the message is relayed to the brain.

6. Sharks sleep differently than we do.
Sharks need to keep water moving over their gills to receive necessary oxygen. Not all sharks need to move constantly, though. Some sharks have spiracles, a small opening behind their eyes, that force water across the shark’s gills so the shark can be still when it rests. Other sharks do need to swim constantly to keep water moving over their gills and their bodies, and have active and restful periods rather than undergoing deep sleep like we do. They seem to be “sleep swimming,” having parts of their brain less active while they remain swimming.

7. Some sharks lay eggs, others give birth to live young.
Some shark species are oviparous, meaning they lay eggs. Others are viviparous and give birth to live young. Within these live-bearing species, some have a placenta like human babies do, and others do not. In those cases, the shark embryos get their nutrition from a yolk sac or unfertilized egg capsules filled with yolk. In the sandtiger shark, things are pretty competitive. The two largest embryos consume the other embryos of the litter! All sharks reproduce using internal fertilization, though, with the male shark using his "claspers" to grasp the female and then he releases sperm, which fertilize the female’s oocytes. The fertilized ova are packaged in an egg case and then eggs are laid or the egg develops in the uterus.

8. Sharks are long-lived species.
While nobody seems to know the true answer, it is estimated that the whale shark, the largest shark species, can live up to 100-150 years, and many of the smaller sharks can live at least 20-30 year.

9. Sharks are not vicious man-eaters.
Bad publicity around a few shark species has doomed sharks in general to the misconception that they are vicious man-eaters. In fact, only 10 out of all the shark species are considered dangerous to humans. All sharks should be treated with respect, though, as they are predators, often with sharp teeth that could inflict wounds.

10. Humans are a threat to sharks.
Humans are a greater threat to sharks than sharks are to us. Many shark species are threatened by fishing or bycatch, amounting to the death of millions of sharks each year. Compare that to shark attack statistics - while a shark attack is a horrifying thing, there are about 10 fatalities worldwide each year due to sharks. Since they are long-lived species and only have a few young at once, sharks are vulnerable to overfishing. One threat is the wasteful practice of shark-finning, a cruel practice in which the shark’s fins are cut off while the rest of the shark is thrown back in the sea. The U.S. is considering the Shark Conservation Act of 2009, which would eliminate the practice of shark finning in the U.S.

Thanks to About.com

Kathy Dowsett
www.kirkscubagear.com