Expert: Jana Connell RVT, CVT - 12/13/2009

Question
My child asked me, "Why do sharks shed their teeth?"  and I didn't have an answer for her.  Would you possibly know why they do this?

Answer
Well we aren't supposed to answer homework questions so I will give you this bit of info from wikipedia:

Sharks, as well as other Chondrichthyes, have a polyphydont  dentition, in which old teeth are shed continually throughout the fish's lifetime, and new ones are rotated into place on a conveyor belt-like structure.[1]  Shark teeth, through the same mechanism as the scales on sharks' skin[citation needed], develop along the inner surface of the jaw cartilage, and are attached to the dental membrane. When the tooth forms in the gum tissue, the crown cap develops first, followed by the root.[1]  The resulting teeth are composed of a dentine root and core, covered by a layer of enamel. The root and the enamel of the blade may sometimes be separated by a thin band of enamel, known as the bourlette or chevron. Depending on the species of shark, the blade of the tooth may or may not be serrated, and may exhibit coarse or fine, irregular or regular serrations. Again depending on the species of shark, cusplets may be present on one or both sides of the blade.
Anatomy of a shark's tooth

The teeth of sharks are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the shark's life. When they lose a working tooth it will be replaced by the next tooth behind it. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. Typically a shark has two to three working rows of teeth with 20 to 30 teeth in each row, although a whale shark has about 300 teeth in each row.[2] The replacement rate has not been measured in most sharks but normally the teeth seem to be replaced every two weeks. The lemon shark replaces its teeth every 8–10 days, and the great white shark replaces its teeth about every 100 days for young sharks and about every 230 days for old sharks.[3] Most sharks shed individual teeth, but the cookiecutter shark sheds the whole lower jaw at once.

The shape of a shark's tooth depends on its diet; those that feed on mollusks and crustaceans have dense flattened teeth for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are greatly reduced and non-functional.[4]

It never says WHY it just says how. I don't think they know but it could be because of the nature of their pray and the fact that sharks are made of all cartilage and no bone, so their teeth really have no way to stay in. They grow up to four rows at a time.