Freshwater rays: a study system for adaptive evolution?
I became interested in freshwater stingrays and how they feed through a couple of happenstance revelations. The first was learning about the family Potamotrygonidae. I was an MSc student at Florida State University studying fisheries-related feeding ecology of a very different group of rays – cownose rays. For a project in my macroevolution course, I decided on the biogeography of freshwater rays in South America – and I was hooked. Several of the references I kept coming back to were publications by my PhD advisor, Nathan Lovejoy. So after graduating the MSc and migrating up to Canada to study with Nate, I started reading anything I could get my hands on regarding freshwater rays… which wasn’t much.
Tangentially, I was reading about the evolution of Darwin’s finches in the Galapagos – birds which are descended from a single species, from which they have diversified into numerous species, all with variable beak sizes and shapes. These beaks help them eat a variety of seeds, some harder, some softer, etc. Almost as a side note, the paper mentioned that some of these birds had evolved or re-evolved to feed on various kinds of insects, but these weren’t as interesting to the authors as the seed-eating birds.
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So I looked those papers back up and sure enough, a couple of freshwater rays ate insects. Some even ate snails. Other species (Paratrygon) only eat fish. Did these rays adapt to novel kinds of food when they invaded freshwater? Could part of their evolutionary story be tied up with adapting to eat these sorts of prey?
I knew that the first thing I’d need to do was determine how aspects of the jaws and jaw muscles – their shape and size, primarily – related back to how they fed on certain prey. Were the jaw muscles larger and did the jaws have greater leverage in rays that ate snails? Some of my previous work on other sharks and rays could support this hypothesis. But what about the insect feeders? What did I think they would look and act like? I expected them to have cusped, or pointed teeth for one – and I expected that they’d be able to shear their upper and lower jaws against each other – after all, these behaviors are what insectivorous mammals use to process insects. These ideas are what drove me to ask how freshwater rays might eat tough (insects) and stiff (snails) prey – and how their anatomies might differ in accordance with their diet.
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