Scientists have discovered how stingrays can enjoy their hard-shelled meals of snails and mussels despite the fact that their mouths are made of mushy cartilage: The stingray jaws, it turns out, are fortified with a bony material that allows them to crunch open the toughest shells. The find, reported in today's issue of Nature, may point to a bony ancestor for the flimsy fish.
"Stingrays have nice teeth," says Elizabeth Brainerd, an evolutionary biologist at the University of Massachusetts, Amherst, "but no one had thought to ask how the jaw supports them." Brainerd and her colleague Adam Summers were slicing sections of a cownose stingray's jaw to study its mineral content when they noticed that the jaw's surface was covered with tiny formations of calcium minerals called tesserae. More surprisingly, they found tesserae inside the jaw cartilage itself. Previously, scientists had thought that it was impossible for tesserae to grow inside cartilage, because cartilage does not have any blood vessels to ferry the calcium to the innards.
The calcium deposits don't seem to be a response to the ray's feeding habits. Working with colleague Thomas Koob of Shriner's Hospital for Children in Tampa, Florida, the researchers found calcified, dice-shaped blocks in unborn cownose rays. The blocks, concentrated in the mouth region where the stingray chomps on its hard-shelled morsels, also seem to grow as the ray ages. The team stretched and squeezed jaw tissue and found it was 10 times stronger and stiffer than that of jaws in which the researchers had dissolved the calcium. Exactly how the calcium gets into the cartilage is still not clear.
"We didn't know until this paper that cartilaginous fishes could put calcium into the insides of their cartilage," says Brian Hall, a biologist at Dalhousie University in Halifax, Nova Scotia, Canada. Paleontologists who scrutinize fossils of hard tissues "tend to think that cartilage can calcify on the outside," he adds, "but now that we know it can grow inside, they will have to take that into account" to correctly identify which part of the body a fossil came from. The finding is an additional piece of evidence that soft fishes evolved from a bony ancestor, as it retains some way to form hard calcium bonelike structures, Hall adds. He hopes it will shed some light on their evolutionary history.