Biologists often say “form follows function,” meaning that, due to the evolutionary process, the morphological features of an organism’s body are fitted to the activities of an organism. A shark’s teeth are no exception to this rule. Over time, evolution has turned a shark’s teeth into the perfect tools for a predator’s manner of feeding.
In fact, if it weren’t for shark’s teeth, we would know very little about prehistoric sharks at all! Since sharks are cartilaginous fish, their teeth are often the only part of their bodies that entered the fossil record. This is because shark teeth are modified placoid scales, which, like our own teeth, consist of a central pulp cavity surrounded by dentine and a hard enamel surface. Shark teeth are used almost exclusively to classify ancient species of sharks. Some prehistoric sharks are known to scientists only by the shape and size of their teeth.
When you consider the number of teeth a shark uses during this lifetime, this information is hardly a surprise. A shark’s teeth are arranged in rows, the number of which varies from species to species. The row nearest the front of the mouth is the “working” row of teeth (though some sharks use up to the first 8 rows of teeth), and they are the largest teeth in a sharks’ mouth. The second row of teeth is smaller than the first row of teeth, the third row of teeth is smaller than the fourth row, and so on. Every time a shark loses a tooth, the tooth in the row behind it moves up to take the lost tooth’s place. This is possible because sharks’ teeth are not embedded in the jaw, but are attached to the skin covering the jaw.
New teeth are continually grown in a groove in the shark’s mouth and the skin acts as a “conveyor belt” to move the teeth forward into new positions. Sharks’ specialized teeth have allowed sharks to develop a very strong jaw. Without the ability to quickly replace teeth, a shark’s jaw could not have developed as powerful of a bite. The number of teeth they routinely lose while catching prey would outweigh the quick-kill benefits of their crushing jaw strength.
The time it takes for a shark use and lose each tooth varies, depending on the species and age. Younger sharks replace their teeth more often. There is also some evidence that sharks’ teeth last significantly longer during colder water temperature in some sharks, a time when they traditionally eat less. While most sharks replace teeth individually, other sharks, like the cookie-cutter shark, replace their entire mouthful of teeth at one time. The number of teeth a shark grows and uses during its lifetime can be enormous – some sharks use up over 30,000 teeth in their lifetime! That’s roughly 937 times the number of teeth the average human has!
While sharks’ teeth do share the common traits described above, they have very different shapes and uses. There are four basic groups of shark diets and because of this there are four basic types of shark teeth.
-Sharks that typically eat fish have long, narrow, needle-like teeth ideal for gripping something as slippery and streamlined as a fish.
-Sharks that are benthic feeders, eating bivalves and crustaceans, have thick, plate-like teeth perfect for crushing the shells of their prey.
-Tiger sharks, great white sharks, and other sharks that primarily eat seals and other mammals have sharp, serrated cutting teeth for tearing off chunks of flesh.
-Finally, we have the gentle giants of the shark family, the basking sharks and whale sharks, that eat krill and other forms of plankton. While they have many teeth, they are tiny and useless, as these sharks feed by filtering water through their gills.