Sharks are perhaps the most maligned groups of animals on earth. They are seen as the great predators of the sea and a deadly threat to anyone who ventures into the ocean. But this varied (there are hundreds of species) and fascinating group of fish has much more to fear from man than man has to fear from them.
The Tooth of the Matter
When most people think of sharks, they think of teeth. Shark teeth are derived from the same material as their scales. These teeth are constantly produced, shed, and replaced. Sharks can do this because their teeth are not anchored to their jaws or gums, as are those of other vertebrates. Rather, their teeth are connected to a membrane in their mouth that is constantly being pushed forward as new teeth form. Each new tooth is slightly larger than the one before. This system allows the size of the shark's teeth to keep pace with the growth of the rest of the body. A single shark can shed thousands of teeth over its lifetime—this accounts for the many shark teeth found by beach combers throughout the world.
Eggs and Babies
Most shark species do not lay eggs, but actually give birth to live young. There are two different methods by which this occurs. Most sharks are ovoviviparous, which means that the eggs are fertilized, hatch, and develop within the body of the female, though the embryos don't receive any direct nourishment from the mother. However, some are viviparous, which means that females may form a yolk sac placenta that in many ways resembles a mammalian placenta. In this case, newborn sharks even have umbilical cords, just like mammalian babies. While egg-laying fishes may produce thousands of eggs at a time, sharks, like many long-lived animals, reproduce slowly (some species are pregnant for up to two years) and do not give birth to many young (usually between two and four) at a time.
What's for Dinner?
Some species of shark eat tiny zooplankton, while others may eat prey as large as the carcasses of baleen whales. The more quickly moving species—such as the great white, mako, tiger, and hammerhead—eat fish, marine mammals, squid, and even other sharks. The more slowly moving species, such as the angelshark, tend to comb the ocean floor for shellfish. Filter feeding sharks, such as whale sharks and basking sharks, consume large quantities of plankton just as baleen whales do.
Threats to Shark Survival
Despite their reputation as "perfect predators," sharks' very survival is in jeopardy. While these predators are well adapted to their ocean life, they have no way to adapt to the impact of humans. The combination of over-fishing (for meat, fins, and trophies) and habitat degradation in the form of pollution has meant a serious drop in shark populations. Overfishing is a particular problem for sharks because sharks' slow rate of reproduction makes it impossible for populations to rebound from being killed in large numbers. Although some sharks are protected under international and national laws, the difficulty of differentiating one species from another means that people may inadvertently kill species that have protected status.
The World Conservation Union has assessed the conservation status of 546 shark species. Of these, 111 are at significant risk. Twenty species are listed as critically endangered and 25 as endangered. A study published in 2003 in the journal Science concluded that all recorded shark species, with the exception of makos, had declined by more than 50 percent in 8 to 15 years in the Northwest Atlantic. Some species, such as hammerheads and thresher sharks, had declined by as much as 80 percent. This global crisis can only be addressed if we put aside myths about sharks, replacing the fear-filled fiction with a sense of wonder at their adaptations to ocean life and an understanding of the grave peril they face as a result of our actions. The future of these ancient creatures is in our hands.
Updated February 2, 2007
