Stingrays Eat Food That Is Harder Than Their Own Jaws

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If you’re ever in the Xingu River (a tributary of the Amazon River) in Brazil, look into the fresh flowing water! You might spot one of the endemic spotted rays that call this area home- the Xingu River stingray (Potamotrygon leopoldi). Not only are they unique because they are brilliantly colored and in fresh water, but they are part of an unusual group that eat prey that is tougher than their own jaw. This feeding method is call durophagy, and the fishes that consume these hard-shelled animals have evolved this edge over their competition.

Potamotrygon is a genus of freshwater stingrays in the family Potamotrygonidae native to the rivers of South America. One of these is the bigtooth river stingray (Potamotrygon henlei) which is endemic to the Tocantins-Araguaia River Basin in Brazil. They, like the Xingu River stingray, are black with white/yellow spots.

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“While this is costly, and likely still inefficient in some ways, it has allowed for them to have a competitive edge and reproduce and survive with this additional food source,” said Kelsi Rutledge, a PhD student at the University of California in Los Angeles (USA). “There are tradeoffs for all adaptations. This adaptation is just more rarely seen in these soft skeleton creatures, making it essentially more difficult for them to colonize this niche space. But, it has not prevented durophagy from evolving multiple times in cartilaginous fishes.” In fact, this mode of feeding can be seen in some species of sharks, guitarfish, ratfish (a type of chimaera) and stingrays.

Using medical imaging technology (CT scans) the team of researchers was able to see what the jaws of these stingrays look like. Rutledge and the scientists were interested in figuring out the adaptations and mechanics that allow the Xingu River stingray to crush the snails they are known to love and eat, and how their jaws may change as they age. Rutledge says they were also interested in comparing this ray to other durophagous fish with cartilage skeletons and if they all evolved similar or different adaptations.

The Xingu River stingray (Potamotrygon leopoldi) in their natural habitat and a CT scan of their jaws.

Provided by Kelsi Rutledge for use by Forbes

The team looked at the muscles of these animals, the lever mechanic of the mouth area, tooth development, and mineralization to get their results, now available in the Journal of Morphology. Through manual and virtual dissections, they looked at how the Xingu River stingray muscles compared to other rays, as well as where their muscles inserted on the jaws in relation to their joints. The CT scans also allowed for mineralized tissue to be displayed as brighter in color, and the team used this brightness to see what regions of the jaws were more mineralized.

“The results are definitely different with different rays. The jaws of this ray are actually built more like a durophagous horn shark and less like its closer relatives, [like] an eagle ray, for example,” explained Rutledge. The researchers hypothesized that the strongest area of the Xingu River stingray’s jaws would be under their teeth; however, they instead found that the strongest points were underneath their jaw joints, much like a shark. “Our initial hypothesis was that the jaws would be built similar to the eagle ray but it was not. We also found that the Xingu river stingray had a high mechanical advantage, low 2nd moment of area, very large jaw muscles, reinforcing struts, and increasing mineralization under the teeth through development.” The “2nd moment of area” is actually an engineering metric, and is needed to calculate bending stress. Also known as the “area moment of inertia,” for this research the metric tells the scientists how resistant the jaws are to bending with the crushing force that takes place during eating.

The scientists also found that the jaws of the young pups (baby stingrays) differed from the jaws of an adult Xingu River stingray. The young pups had jaws that were not as well mineralized, had different teeth structure, and the shape of the jaws was most likely to bend with any crushing force. To the researchers, this led them to believe the babies would probably not be able to eat the snails they eat when they are adults. It is unknown what they eat during this life stage, as the stingrays are listed as “Data Deficient” by the International Union for Conservation of Nature (IUCN).

The different shapes of the teeth in the Xingu River stingray (Potamotrygon leopoldi) when they are juveniles and adults.

Provided by Kelsi Rutledge for use by Forbes

Commented Rutledge: “To evolve the jaws needed to crush hard prey, animals are constrained by what they have inherited from their ancestors, and as a result they have to work with what they’ve got. This has resulted in different animals finding different ways to eat tough prey. The Xingu river ray is a cool example of one of these ways. It is the first example of a durophagous stingray that has jaws built like a shark. Learning about the biomechanics of animal processes can also lead to the development of new technologies. Can we use this knowledge to design some sort of crushing machine that uses soft materials?”

Look, so long as you aren’t a snail, you won’t have to worry about stingrays munching on you any time soon. So enjoy this feat of natural engineering at its finest!

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