If you could travel back in time some 370 million years and snorkel over shallow reefs in Devonian seas, you would, of course, see alien creatures—the antiarchs (armored fishes), perhaps a long-snouted lungfish, and the spiral shells of ammonites. But your most startling encounter would probably be with an all-too-familiar creature: the ancient shark Cladoselache. Its fusiform, or spindle-shaped, body—between three and six feet long—and sharp, multicusped teeth might evoke the same frisson that shark divers so enjoy today.
The survival of the shark, in a form relatively unchanged for hundreds of millions of years, attests to the utility of its body plan. Holding evolution at bay for so long requires flexibility. A shrewd recent observation of shark feeding behavior has led to the realization that some species, perhaps including some very ancient ones, can change their teeth as they change their diet.
Most people's impressions of sharks are dominated by Jaws-type images of school bus-size creatures cutting through the water with their fearsome mouths agape, exposing their jagged teeth before gnashing down on some hapless creature. Most shark predation—the mako's quick lunge at a tuna or the surreptitious sideways snatch of the sand tiger shark—is far less showy.
Some cartilaginous fishes shun fishy prey altogether, preferring creatures that may be harder to process but easier to catch, such as brittle stars, crabs, and sea urchins. The dietary preferences of a species can be inferred by examining its teeth. Triangular teeth, good for cutting and slashing, belong to sharks that eat things larger than their heads; a battery of grasping spikes works well for those wanting to snatch up fish; and a pavement of closely set knobs does the job of crushing hard prey.
The white-spotted bamboo shark (Chiloscyllium plagiosum) is a small, spotted shark native to the Indo-Pacific. It is popular in aquariums because it is usually a willing feeder and lays eggs in captivity. Cheryl A.D. Wilga and her graduate student Jason Ramsay, both marine biologists at the University of Rhode Island in Kingston, maintain a small colony of bamboo sharks for Wilga's research on shark feeding and swimming. One day, instead of feeding the sharks their usual soft diet of squid and fish, Ramsay tossed in a few small New England crabs. To his surprise, the sharks not only eagerly gobbled up the unusual fare but also spat out pieces of shell. Ramsay realized that the sharks had smashed the crabs instead of swallowing them whole.
In the wild, bamboo sharks have a catholic diet that includes small fish as well as a variety of benthic invertebrates such as crab and shrimp. Their teeth seem to reflect their generalist approach to dinner: they are sharp, small, and ideal for gripping and then, through vigorous head shaking, tearing prey into chunks. But sharp teeth seem entirely at odds with the need to crush prey. How could delicate points avoid taking a beating every time the shark chooses a well-armored meal? With a combination of high-speed video, dissections, and experiments that showed the mobility of the teeth relative to the jaws, Ramsay got to the root of the problem.
[media:node/2102 horizontal medium caption left]Shark enthusiasts have long known that sharks have several rows of teeth, all embedded in an elastic dental ligament that, like a conveyor belt, slowly carries the teeth forward from the back of the mouth toward the lip. The frontmost teeth do the biting and chewing; the rows toward the back stay down and out of the way until they reach the front of the mouth. At the same time, though, teeth are constantly being replaced: as the older, front-row teeth break or wear down, they are carried out of the mouth, and the next row of younger teeth moves to the front.
The elasticity of the dental ligament gives shark teeth another unusual property: they can wiggle a bit to fit around bone or skip over hard parts of prey. The bamboo shark takes advantage of this mobility, enabling its teeth to do double duty.
When the bamboo shark’s teeth hit soft flesh, the sharp cusps bite in and grasp the prey. A quick shake of the shark’s head can then rip the prey in half. But when the shark grabs hard-bodied prey, the sharp points can’t make a dent. So instead of letting their edges get dulled on shell, the teeth fold toward the back of the shark’s mouth, exposing the front surfaces of the teeth to the prey [see illustration right]. The flat tooth face is far better suited to the task of smashing open a crab. Imagine your own teeth tilting backward if you bit a walnut shell, but staying upright on contact with a peach.
The beauty of the system is that no special controls are needed to “decide” whether the prey to be processed is hard or soft. The hardness of the prey itself causes the teeth to change from pointy graspers to lumpy crushers. If the prey is hard, the upright front row folds down onto the next row back, turning the entire dental battery into a crushing plate, similar to the palate of true hard-prey specialists such as the horn shark.
The teeth of the bamboo shark look remarkably similar to those of Elegestolepis, one of the oldest known fossil sharks. Perhaps, then, the shark’s all-purpose strategy is quite old. But it’s not universal—great white sharks have no trouble cutting through surfboards with a static set of dentures, and tiger sharks rip through sea-turtle carapaces like chainsaws through pine.