Small animals have a difficult life in the ocean, buffeted on all sides by waves and turbulence and pursued by predators much larger than themselves. It might seem that such environmental forces would completely overwhelm any locomotory efforts these tiny beasties could make.
Recent mathematical modeling work in the Physics of Fluids by Saverio E. Spagnolie and Michael J. Shelley indicates that small animals may not be quite so helpless – provided they can change their shape. In a simulation of a jellyfish-like creature, the researchers found that appropriately timed shape changes allowed the `animal' to harness some fluid energy from its environment and to propel itself up or down in the water column, or to keep itself from sinking.
The mathematicians imagined a creature with a symmetrical body suspended in a flow that oscillates up and down, similar to the flow underneath a wave. They allowed the creature to change its shape to become more or less flattened– ranging from a sphere to a pancake. By solving the equations of fluid motion around the animal, they were able to see what might happen when it synchronized its shape changes at various times in the oscillating flow.
The results showed a behavior the researchers called a `fluid ratchet'. If the beastie flattened into a pancake while the fluid moved up, it could ride that wave upwards. Then, if it squished into a ball as the fluid moved down, it could avoid being pushed back down too far. Surprisingly enough, after several pancake–ball cycles, the animal's wake came to resemble that of a jellyfish, even though the simulated creature wasn't able to jet fluid out the way a jellyfish can.
Further work showed that the ratchet works better for denser animals. The body's density helps to smooth out the ratchet, resulting in higher average speeds and lower fluctuations; but there is a caveat. Higher density is a problem if the creature is trying to use the ratchet to avoid sinking. Initially, the body's density helps to smooth the velocity, but after a while the gravitational force overcomes the ratchet and the animal sinks.
In the course of their simulations, Spagnolie and Shelley found another curious effect. Shape changes can also cause a velocity burst, which could be useful for escaping predators. If the creature is already moving, squishing the pancake into the sphere and on to something torpedo shaped reduces the drag force substantially, and results in a burst of speed – up to about 50% faster than the initial speed.
Although they show a number of interesting effects, the simulations do not address one key question: stability. The beastie was confined so that it could only move up and down. The researchers acknowledge that they do not know whether the fluid ratchet would be possible for a real animal that can move in any direction. Would the vortex shedding cause the animal to tumble, rather than moving steadily up or down? How would turbulence affect the movement? These are questions for future studies, but it might be that small, squishy animals can navigate their environments better than we once thought.