Stingrays solve problems with eating troublesome prey in surprising ways

Kaitlyn Lowder

Ph.D. student Matt Kolmann studies freshwater and marine rays to understand how jaw morphology relates to diet

Imagine trying to crush a snail shell with the cartilage in the tip of your nose. Stingrays don’t have bones, just cartilage covered with smallcalcified plates in places. Despite lacking the truly bony jaws of many vertebrates, some stingrays not only feed on hard or tough prey but specialize on them. Matt Kolmann studies both freshwater and marine rays as part of his Ph.D. work at the University of Toronto and found that they solved this tough problem in a variety of surprising ways.
Ph.D. student Matt Kolmann during an expedition he led to the Demerara Estuary, Guyana.
Kolmann’s pursuit of these impressive stingrays took him to the murky Rio Ucayali in Peru, home to many predators with impressive bites, including jaguars and alligator-like caiman. Getting in the water was “a little unnerving,” he laughs. There, he found Potamotrygon motoro, the ocellate river stingray, which feeds almost exclusively on insect larvae in some parts of its range. Although this may not sound like much of an accomplishment for the stingray, the outside of insects is covered by a supportive and protective shell called an exoskeleton. As such, the exoskeleton is tough—chitin, a fibrous sugar, runs throughout it. Even though it was known that insects comprise part of P. motoro’s diet, Kolmann realized that no one understood how these boneless animals are able to break up tough prey. “I think the coolest studies I’ve seen in science have been things where everyone sort of knows it happens, but no one really thinks of it being different or interesting.” After bringing P. motoro back to the lab, Kolmann began to explore how the stingrays handle prey. Cameras situated underneath clear tanks captured the feeding action. Although this might seem like the easiest part of Kolmann’s work, it was still challenging. “There are some days where [the stingrays] just don’t want to do anything. They just stare at you.”
A P. motoro inspects her tank during a feeding trial. Photo credit: Matt Kolmann
Once he cajoled the rays into cooperating, he saw just how they are able to eat insects. All sharks and rays can protrude their jaws, and video analysis shows that P. motoro uses this technique when grabbing insect larvae. Notably, it shoves out its mouth on an angle, which many other rays that eat different prey are unable to do. Then, the sides of the jaws squeeze together, perhaps moving the upper and lower teeth in such a way that shears the insects apart. Kolmann says the insects look like they’re getting broken apart at the joints, likely allowing for digestive juices to later enter.

This video, shot from beneath the ray, shows P. motoro feeding on an insect larva in the lab. The ray shoves out its jaw in quick, almost imperceptible movements to grab the insect, then spits it out to bite yet again. Video credit: Matt Kolmann

To get a clearer idea of what’s going on internally during feeding, Kolmann and his colleagues Dr. Mason Dean (Max Planck Institute of Colloids and Interfaces) and Dr. James Weaver (Harvard University) enlisted the help of an unusual tool–expanding foam normally reserved for DIY home insulation. Filling up the mouth of a dead ray with this foam protrudes the jaw. Subsequent CT scans revealed that the teeth, which usually lie flat, change orientation during this feeding movement. “The teeth of sharks and rays attach to a ligament, and when that’s flexed or pulled taut, they sort of spring up. So [these rays] go from having these boring, flattened teeth to having these cuspidate teeth” during feeding, Kolmann says. The orientation of teeth and jaw movement may be the key to successful insect-eating.
A CT-scan of P. motoro’s body. Its jaws are near the top and filled with small tiles of teeth. Photo credit: Matt Kolmann
This project has become part of a larger question woven throughout Kolmann’s work. If form relates to function, how does the diversity of jaws relate to different prey types? While the freshwater stingray P. motoro has the ability to munch on insects, marine stingrays (such as eagle, bat, and cownose rays in the family Myliobatidae) tackle other seemingly difficult prey items, namely crustaceans, snails, and mussels. While all of these may be considered “hard” prey, they are not all “hard” in the rigid vocabulary of materials science. The outer structures of the prey respond to forces in dissimilar manners. Some, like mussel and snail shells, are stiffer and resist deformation, but cracks may form more easily. Crustacean exoskeletons, in contrast, are tougher, more bendable, and can absorb more energy before cracking. As a result, rays have to approach these different kinds of prey in different ways. “Treating those two [prey types] as the same, from an ecology standpoint, is backwards,” Kolmann says.
These four rays in the family Myliobatidae (including eagle, bat, and cownose rays) have distinctly different jaw shapes and eat different kinds of “hard” prey. From Kolmann et al 2015. Journal of Experimental Biology 218: 3941-3949.
By linking CT scans of jaws with diet, Kolmann and his colleagues found that rays that primarily eat tough prey have different jaw morphologies than those that eat primarily stiff prey. Does jaw shape enable rays to specialize on prey with particular material properties? Using the CT scans of eagle, bat, and cownose rays, Kolmann again took an unusual approach. He and Dr. Adam Summers of University of Washington’s Friday Harbor Labs milled aluminum into simplified, metallic, “James Bond-y stingray jaws” to compare performance without factoring in differences like tooth material. And the unlucky prey that is smashed in the aluminum jaws? Live varnish clams, mussels, dogwinkle snails, and things one might not see in the stomach of even the worst garbage-slurping shark–ceramic tubes and 3D-printed plaster dogwinkle shells. Prey were loaded between these jaws and force was applied until they catastrophically broke. After many rounds of smashing, Kolmann learned that all the jaws performed the same, regardless of their shape. This is surprising, since they also found that dogwinkles (a snail) are 1.5-3x stronger than mussels or varnish clams. In this instance, quite large differences in jaw shape do not enable rays to specialize on different kinds of “hard” prey. Changes in form do not result in changes in function.
Crushing a 3D-printed plaster dogwinkle shell between two milled aluminum jaws. From Kolmann et al 2015. Journal of Experimental Biology 218: 3941-3949.
Instead, differences in feeding performance may be explained by other factors such as jaw musculature, tooth patterns, or perhaps feeding behavior. Some rays spend a lot of time handling prey. “They bite down and spit the prey back out and bite back down. We think they’re making localized fractures that eventually connect and then fail the whole skeleton,” Kolmann says. He has seen Rhinoptera rays spending more than 60 minutes handling single oysters. “They must be so conspicuous in the wild when they’re eating these things. They’re making a whole bunch of noise and they’re spitting sand and shells and stuff out of their spiracles.” Somehow, spending this time and energy while managing to avoid predators still pays off for cownose rays. In addition to discovering the different approaches of these freshwater and marine lineages to eating troublesome prey—linking ecology to evolution—Kolmann and his collaborators are advancing techniques that are useful for other researchers. Artificial shells cracked at the same place as the real ones when smashed between the aluminum jaws, although they took more force to break. The technique of 3D printing and using these artificial structures as a proxy for real ones is relatively new and may be useful for researchers studying failure mechanics based on shape alone. This is just another unusual tool Kolmann has employed in his research, making him as resourceful as his impressive rays. Kolmann presented this research at the 2016 annual meeting of the Society for Integrative and Comparative Biology in Portland, Oregon. Matt specifically thanks the following funding agencies for their contributions: Human Frontiers in Science Program (to M. Dean & J. Weaver) National Engineering and Research Council (to K. Welch & N. Lovejoy) National Science Foundation (to A. Summers) Rufford Foundation (to M. Kolmann) American Elasmobranch Society (to M. Kolmann) Ontario Trillium Scholarship (to M. Kolmann)
the Society for
Integrative &