By Dr. Becker
It's a topic that, for some people, is the stuff made of nightmares, especially after watching the 1970s cult classic, "Jaws." But for scientists and anyone who finds sharks fascinating, several new and little-known facts are helping experts in the field (or more correctly, the ocean) understand these powerfully equipped creatures of the deep.
Today I Found Out highlights a number of features sharks possess, not only for their own survival but to perpetrate the species.1 One of those facts is the incredible ability they have to grow unlimited rows of teeth throughout their lifetimes.
Yes, finding a shark's tooth on the beach doesn't necessarily mean there's at least one shark out there with fewer teeth to bite you with; they can grow more! What's more, scientists report that a single Selachimorpha (its scientific name) can grow as many as 50,000 teeth if it lives to be a grizzled old shark. You're not alone in wondering how they're able to do that.
But first of all, it's important to touch on one of the most frequently asked questions about the sea creatures that strike the most fear into the hearts of men: Just how deadly are they? Well, according to Florida Museum of Natural History's Department of Ichthyology,2 only about a dozen people are killed by sharks per year, which doesn't hold a candle to the reverse circumstances: approximately 20 million to 30 million sharks per year die at the hands of humans.
How Sharks Develop New Teeth
Perhaps you already knew that sharks have multiple rows of teeth, but it varies depending on the shark variety — five rows for some and 15 rows for others. A bull shark, for instance, could take care of corn on the cob pretty quickly with the unbelievable 50 rows of teeth — seven different kinds — totaling about 350 teeth at any given time.
In fact, the moment one of those teeth starts getting loose, one right behind it starts pushing forward, so that the minute the front one is lost, there's an almost immediate replacement. Simultaneously, another replacement tooth will begin forming right behind the second one, and so on, like "a conveyor belt of death."3 The whole process may take one day to a couple of weeks.
The largest teeth are always in front, with progressively smaller sets or series of teeth set behind it, sort of like a "choir" of teeth. Those in back are the ones most recently formed, and it continues as long as the shark is alive. Today I Found Out explains:
"These front teeth are referred to as the shark's 'working teeth' and are the ones primarily used for biting, tearing flesh and all of the other things a shark is thinking about doing when it's swimming around below your legs as you dangle them in the ocean …
... This isn't to say that the second, third or even the teeth located towards the back of a shark's mouth aren't used when a shark is devouring something like your lower half, it's just that the front series of teeth are typically subjected to the most stress when this occurs."4
One reason sharks are so prone to tooth loss is that their teeth are not as deeply rooted as human teeth are, but they're many times more powerful. It's those front ones they lose most often, which are replaced every few weeks, but they naturally "shed" teeth on a regular basis as their teeth are used to do what they do.
A few interesting factoids on shark teeth are that older sharks don't regrow their missing teeth as quickly as younger ones do, and in some species, cold water seems to help them retain their teeth longer. Fishermen who've caught sharks could most likely tell you their teeth were in near-perfect shape because they lose them fast enough to prevent them from ever getting broken or dulled down.
Shark Tooth 'Flex' and Other Facts
National Graphic quoted R. Aidan Martin, director of ReefQuest Centre for Shark Research in Vancouver, Canada, in explaining other ways besides biting that sharks use their teeth, which, for a great white, is similar to the way humans use their hands. In live sharks, each tooth has 10 to 15 degrees of flex, so when it opens its mouth, the tooth bed is pulled back, which Martin equated with the way a cat's whiskers splay out.
"Great whites are curious and investigative animals. That's what most people don't realize. When great whites bite something unfamiliar to them, whether a person or a crab pot, they're looking for tactile evidence about what it is. Combine that with the flexibility of each tooth, and you realize a great white can use its jaws like a pair of forceps. They're very adept at grabbing things that snag their curiosity."5
In the throes of curiosity, shark have been known to nibble not just on fish, sea turtles, cetaceans such as dolphins, manta rays and squid, but also on surf boards, paddle boards, buoys and brightly hued kayaks. Sometimes they bite into things and spit them out, apparently because they don't like whatever it is.
Shark teeth, both old and new, are collected by people all over the world, and it's not that difficult, because experts say there are literally many trillions of sharks' teeth lining the ocean floor. Because they're made of dentin, which is a hard, calcified tissue (the same material as elephant tusks) surrounded by a harder layer of enamel, the teeth don't generally decompose as other bones do, so they're readily available for scientific study.
Sharp Senses Sharks Possess and Other Oddities
The featured article refers to the "frighteningly efficient mechanisms" sharks use in their quest for food. Besides their teeth, they also have streamlined bodies, which help them travel underwater at amazing speeds.
Shark skin has an odd roughness, somewhat like sandpaper, because it's made of dermal denticles, also known as placoid scales that are actually similar to teeth. They're even enamel covered, making them vitro-dentine. Unfortunately, it's this skin that drives people to kill sharks so they can make leather products as well as shagreen, a type of sandpaper. The scales help sharks swim more efficiently. Further:
"The fact that sharks are completely covered in tooth-like structures may seem alarming, but not only are they resourceful when it comes to swimming … they also form a barrier of protection. Interestingly enough, however, while the dermal (denticles) are arranged in a pattern on the shark, they do not grow as the shark grows. Instead, the shark just sprouts more placoid scales as necessary."6
SharkSider notes how adept these animals are at detecting electrical fields and minute changes in water pressure. Their heads are covered with a sensory network called the ampullae of Lorenzini, (which look like tiny holes if you got close enough to see them) but they're actually cells that help sharks "sense" if, say, his prey is hiding in the sand. Neuromasts are used to detect any movement. Magnetic fields, water temperature and pressure are also picked up by sharks, abilities that come in handy for undersea hunting forays.
Sharks have great hearing, as their inner ear can pick up sound as well as acceleration and gravity. Add to that incredibly sharp vision and a keen sense of smell (lemon sharks can even detect tuna oil at a concentration of just one part per 25 million, SharkSider adds) and it gives you a better understanding of what this animal's prey are up against.