After a 2-month break while I focused on relaxing, refreshing, and working on my forthcoming book "The Dynamics of Disaster," it took a Science NOW article to lure me back to posting here (i.e., spend some time trying to understand that article, which is really what the blog exercise is all about!).
Apparently biologists can measure the strength of a muscle, and have concluded that clams to not have enough muscles to plow more than 1-2 centimeters into the ocean floor. Winter and colleagues took an empty clam shell, filled it with epoxy, and tried to drive it into an exposed seashore, concluding that a clam could burrow no deeper than 2 cm.
Why is it then, that clam-collecting, is a rather challenging exercise--the clams shouldn't be able to get away from the collectors. In fact, they can dig themselves into the sand at a rate of roughly a centimeter per second or more. How do they do this? Amos Winter et al. have proposed a localized fluidization mechanism (Journal of Experimental Biology, 215, 2072, 2012). The authors say that the clam technique could be a model for engineering self-burying machines.
The physics of the clam defense is illustrated in the frame attached. First, the clam extends a foot, and then it lifts is shell up. The clam, then contracts the shell itself, sending blood into the foot to inflate it and provide an anchor. Then, with the foot acting as an anchor, the clam drags the shell down, a process repeated over and over about every second. Winter discovered the key to the clams success: when it contracts the shell, it relieves pressure on the soil and this pressure change results in water flowing toward the body of the clam. This "liquifies" the sand in the immediate vicinity of the clam. This liquefaction, in turn, reduces the drag on the clam, allowing it to pull itself down. The liquefaction process is local and ephemeral as the water then migrates back into the sand and the clam has to repeat the maneuver, over and over.
Clam digging isn't a recreational option in the midwest where I live, but this makes me really eager to try my hand next time I'm at the sea shore!
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