AMHERST, Mass. - University of Massachusetts biologists have led a team that has determined how some stingrays, despite having skeletons made of cartilage, are able to crush hard-shelled prey such as mollusks and snails. This is the first time that scientists have found, within the cartilage skeleton of a modern animal, rigid support structures made of minerals; such structures had previously been found only in fossils of ancient sharks. Details will be published in the Oct. 1 issue of the journal Nature. UMass graduate student Adam Summers is the main author, along with assistant professor Elizabeth Brainerd, and Thomas Koob of Shriners Hospital for Children in Tampa, Fla.
Stingrays are related to sharks, and members of one group of stingrays feed on hard-shelled prey, according to Summers. In the study, he focused on the cownose ray, an Atlantic coast fish that is known for gathering in huge schools very near shore.
"The relationship between form and function is an underpinning of biology," said Summers, noting many animals are well-suited to their environments, a trait which extends to the materials that comprise animals'' bodies. While bone and cartilage both offer structure, bone provides more strength. This raises the question of how the cownose manages to eat hard-shelled food using its soft, cartilaginous jaws. "If you were going to make a nutcracker, chances are you wouldn''t make it out of cartilage," Summers said.
Stingrays accomplish the task in two ways. First, there is a thin layer of mineralization around the cartilage of their jaws, making the jaw rigid and strong, rather than soft and pliable. The mineralization is actually multiple layers of tiny blocks called tesserae, Summers said, laid down in a tile-like way. Secondly, stingrays develop a network of hollow, supportive struts within their jaws. The struts, which are also comprised of tesserae, are concentrated in the areas where toothplates crush prey, providing strength where it is most needed, Summers said
The finding also shows that cartilage in a modern animal can provide strength in a way similar to bone, by laying down a thick outer layer and developing inner struts, Summers said. The finding appears to provide scientists with an evolutionary freeze-frame, according to Summers. The manta, a related stingray which grows to be 30 feet across with its wing-like fins extended, eats plankton, but has jaws with struts. Scientists believe that the manta''s ancestors ate hard-shelled prey, and that the struts are left over from a diet its descendants gave up millions of years ago.