Walking along Little Sugar Creek one day, something flashed and caught my eye in the stream. It was the shiny underbelly of a fish that was probably 8 or 10 inches long. The weird thing about this fish was that it was moving completely sideways rather than headfirst. As I got closer to the fish and could see through the murky (it wasn’t so clear that day) water, I could see the problem. There was a good-sized snapping turtle carrying that fish. I watched it for awhile until it climbed under some rocks under the bridge at East Boulevard.
One of the things that struck me about this turtle was that none of its movements were very fast. Even underwater, it slowly pushed itself along the rocks, pausing from time to time. A swimming fish is definitely faster moving than anything I saw that turtle do. I know that snapping turtles are sit-and-wait predators. Rather than tracking and chasing prey, they hide and grab it at an opportune moment. For a turtle, there are still two issues with this kind of hunting: 1. The prey grab has got to be faster than the reaction time of the fast moving fish and 2. The problem of water. Water is, unfortunately (depending on your perspective) viscous. When you push yourself forward in the water, you push the water in front of you away, which can make it hard to grab things (try pinching a fly out of your drink!). A snapping turtle that pushes its head and mouth forward to grab a fish would also push the water and so, push the fish out of its way. So how does the turtle manage this?
Well, now we get to the fun bits about mouth parts. In order for a turtle to get prey into its mouth (and avoid warning prey about an impending attack), it needs to reduce the movement of water forward. The only good way that a turtle can do this task is to pull water into its mouth. In a fish (with gills), water can be brought into the mouth and expelled through the gills, creating a great degree of suction. A turtle can’t do this kind of displacement, though. What a turtle can do, however, is to drop its tongue in its mouth and expand its throat. With this movement, a turtle can ram its head forward (at 150 cm/s) without pushing its food out of the way. Cool stuff. Slow body, but very fast (and flexible) head. Snap!
Lauder, G.V. and T. Prendergast. 1992. Kinematics of aquatic prey capture in the snapping turtle Chelydra serpentina. Journal of Experimental Biology 164: 55-78.