Sharks roamed the oceans before dinosaurs walked the land. They
even survived the Permian mass extinction 252 million years ago,
when over 90% of all species died off. Today, however, these top
predators are disappearing from our oceans. They are in trouble
on the Great Barrier Reef, have almost been eliminated from the
Atlantic Ocean, and are a regular on the endangered species
list. This magnificent animal, which has been around for over
400 million years, is falling victim to overfishing. Sharks are
being taken from the oceans before they can replenish their
populations. Many people believe these creatures hold the secret
to long life. Reports of sharks with abnormalities are so rare
that it's commonly thought they do not get cancer. Sharks have
cartilage instead of bones, and lack bone marrow where humans
produce immune cells. Sharks produce their immune cells in the
spleen, thymus, esophagus and in unique tissues associated with
the gonads. Since these immune cells circulate in the
bloodstream, they offe