So what about rays that feed on mollusks?
Someone recently asked me how I might go about studying the jaws of freshwater rays which consume hard prey - well my beginning interests were in these sorts of things - (turns out, I really like stingray jaws). I would be particularly fascinated to find that freshwater mollusk-eating rays are anatomically similar to mollusk-eating marine rays I've studied previously in my career.
One of my first interests was how rays use a cartilaginous skeleton, a skeleton presumably more ductile than their prey, to crush animals like mollusks. I was particularly taken by the diversity of jaw shapes that exist in myliobatid (eagle, cownose, and bat) rays - which are all durophagous - meaning that these rays have diets entirely composed of prey like snails, bivalves, and even big decapod crabs.
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So what does jaw shape mean for the ability to crush these robust kinds of shell-clad critters? One of my latest papers tried to find out.
Despite considerable variation in ray jaw shape, no particular jaw shape was found to perform better than any other at crushing hard prey. There are many types of hammers and they all perform equally well. This is unusual because we expect to see performance differences when there are large differences in shape. Consider for comparison the beaks of Galapagos finches - different shapes are better for dealing with different types of seeds.
But how did what prey were made of change the findings - now this is where things got interesting... are 'hard' prey really that different?
Turns out, shells ARE very different. Some shells are made of layered, or lamellar calcium carbonate mixed with protein. This is sometimes combined with nacre, or mother-of-pearl, which is a beautiful and incredibly strong substance. This sort of layered composite lends some shells a kind of rebar + concrete structural strength - the combination of layers with different properties allows some shells to be flexible AND strong.
Other shells like oysters lay down 'messy' unorganized calcium crystallites - but these shells can be produced very quickly. To account for some of this variation we crushed multiple kinds of shells, including 'faux' shells... why? Because nature is messy. So we measured the force it takes to crush 3D-printed model shells (in addition to live mollusks), in order to account for “noise” from natural variation. We used 3D printed shells that were identical in shape and structure to live shells. These faux shells broke in the same manner as the live shells. This suggests that shape was not very important in predicting when any shell would be destroyed… but what the shells are made of… well that was pretty important!
But how 'hard' (with how much force) can these rays bite anyway?
My previous research has found that these rays can be pretty bite-y. A cownose ray just 1 meter (3 feet) wide can generate more than 110 pounds of force (< 500 Newtons). Large eagle rays have been said to generate over 1000 pounds of force. These sorts of incredible forces may be enough to explain why all the different jaw shapes were equally well-suited to crushing a diverse array of prey - they over-perform consistently and perhaps jaw shape is more a reflection of resisting and protecting the jaws during biting.
Ultimately this phenomenon - where many different anatomies or morphologies - biological structures - behave in similar ways - suggests a greater theme in evolutionary thinking. One of the reasons why biodiversity can be so astounding is that not just any one shape is 'the best' at anything. Nature isn't selecting for the 'best' structures, behaviors, and animals - it's selecting for whatever is good enough to get the job done. An optimizing function that generates and preserves biological diversity. So I'm particularly taken by when unrelated animals start to look alike to perform similar tasks - because this shows a particularly strong relationship between form and function.
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