Thursday, August 20, 2015

A World From Dust (Plus): This is Water, Part 2: Water Swims Around Worms

Ever looked in the wrong place for something? It happens to scientists all the time.

It happened in a previous post, where it was noted that very different animals swim with the same motion through water, because they have evolved to converge on an identical, efficient movement through the fluid. The eye is drawn to the animal, but the common denominator and the explanation is found in the water surrounding the animal. The properties of the water shape the movement of the animal, and evolution is just the method of search for the movement of greatest efficiency.

This flips what you expect on its head. If you want to understand how an animal swims through water, you look at the animal, not the water, right? Wrong. It's more accurate to say the water is swimming around the animal, as it guides the animal and shapes its movements.

This is shown even more clearly in another paper, a recent PNAS study titled "Propensity of undulatory swimmers, such as worms, to go against the flow." Worms swimming near a surface turn against the flow, and the question is why and how they do this.

The eye is drawn to the worm. Maybe it has a sensor, "sees" the wall, and turns, right? Wrong. The flow field in the water turns the worm. In the sense, the worm is as passive as a leaf flowing in the river -- an undulating leaf, perhaps, but the water is the active agent and the cause here. Because the water causes the turning, this turning happens in widely different organisms. As the study puts it, there is no involvement of the worm's nervous system, and the turning of the worm "results from purely mechanical interactions."

As Adrian Bejan puts it when describing the similarities among flying animals, where the fluid that shapes the movements is the air, but the idea is the same:

"It is the inanimate fluid in the wake of the leading body that organizes itself. It does so using no brain power whatsoever, so that it may travel and spread itself the fastest through the stationary fluid." p. 240, Shape and Structure

This is air, and this is water. Each flows where it wills. Even when you don't see it, it shapes complex behavior into an efficient and predictable biology.

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