sharks for Wilga's research on shark 

 feeding and swimming. One day, in- 

 stead 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 in- 

 stead of swallowing them whole. 



In the wild, bamboo sharks have a 

 catholic diet that includes small fish 

 as well as a variety of benthic inver- 

 tebrates such as crab and shrimp. 

 Their teeth seem to reflect their 

 generalist approach to dinner: they 

 are sharp, small, and ideal for grip- 

 ping and then, through vigorous 

 head shaking, tearing prey into 

 chunks. But sharp teeth seem entire- 

 ly at odds with the need to crush 



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 front- 

 most teeth do the biting and 

 chewing; the rows toward the back 

 stay down and out of the way un- 

 til they reach the front of the 

 mouth. At the same time, though, 

 teeth are constantly being re- 

 placed: 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 liga- 

 ment gives shark teeth another un- 

 usual property: they can wiggle a 

 bit to fit around bone or skip over 



tration above]. 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 hard- 

 ness 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 bat- 

 tery into a crushing plate, similar 

 to the palate of true hard-prey spe- 

 cialists such as the horn shark. 



The teeth of the bamboo shark 

 look remarkably similar to those of 

 Elegestolepis, one of the oldest 



Teeth of the white- 

 spotted bamboo shark 

 remain erect when they 

 tear into a fleshy fish 

 (above). When the teeth hit 

 something hard, though (above right), they 

 fold down to form a flat dental plate, suit- 

 able for crushing hard-shelled prey. 



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 experi- 

 ments that showed the mobility of 

 the teeth relative to the jaws, Ram- 

 say got to the root of the problem. 



hard parts of prey. The bamboo 

 shark takes advantage of this mo- 

 bility, enabling its teeth to do dou- 

 ble 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 to- 

 ward the back of the shark's 

 mouth, exposing the front surfaces 

 of the teeth to the prey [see illus- 



V known fossil 



sharks. Per- 

 haps, then, the 

 shark's all-pur- 

 pose strategy is quite old. But it's 

 not universal — great white sharks 

 have no trouble cutting through 

 surfboards with a static set of den- 

 tures, and tiger sharks rip through 

 sea-turtle carapaces like chainsaws 

 through pine. 



Adam Summers (asummers@uci.edu) is 

 an assistant professor of bioengineerittg ami 

 ecology and evolutionary biology at ilic 

 I University of California, Irvine. 



March 2006 NATURAl HIST 



