Swimming is hard work: ask anyone donning a bathing cap and goggles during morning laps in their local pool. Yet, life underwater is a reality for millions of organisms on our blue planet. To overcome the physical challenges imposed by life in the depths, sea creatures have evolved a variety of body shapes and swimming styles to help them get around efficiently. But not all solutions to the problem of swimming optimally are equal, and some strategies are favoured over others. So what's the secret to an effortless swim? Researchers at Northwestern University in Illinois, USA, say that for some animals, it's all about undulation.
Rahul Bale and his colleagues wanted to understand how the laws of physics shape swimming in aquatic animals. They suspected that similar hydrodynamic challenges faced by all swimming creatures have led to convergence towards a few optimal strategies for enhanced speed and efficiency. However, instead of tackling the techniques of all swimmers, the team decided to test their idea on a smaller group of 22 animals that use pairs of elongated fins for propulsion – the so-called median-paired fin swimmers, which include cuttlefish and rays. Rather than beating a tail back and forth, these creatures move by rippling their fins length-wise across their bodies. These ripples, or undulations, can be described by two measures: the wavelength (distance between two peaks) and the amplitude (height of each peak). The team analysed video footage of median-paired fin swimmers such as flatworms and knifefish, and quantified the ratio of the wavelength to the mean amplitude of the fin undulation, which is known as the specific wavelength. They then used computer simulations and a large robotic fin to analyse the forces generated by a range of specific wavelengths to see whether nature has, indeed, come up with the best solution to the problem of underwater locomotion.
So what do flatworms, cuttlefish, rays and knifefish all have in common? Despite not looking alike and being only distantly related to one another, these median-paired fin swimmers generally swim with a specific wavelength of 20, which Bale and his team call the optimal specific wavelength. Amazingly, the team found that undulations at the optimal specific wavelength were ideal for maximizing force and speed during their simulations and in their robot fin. This consistency in the optimal specific wavelength across such a diverse group of animals suggests that necessity rather than chance led to convergent evolution towards a mechanically superior way of moving through water. The researchers estimate that there are over 1000 different aquatic creatures that probably swim with the optimal specific wavelength.
Of course, median-paired fin swimming isn't the only way to move underwater: lots of animals swim with their body and/or tails. In fact, some of the fastest underwater athletes like tuna and dolphins use this more common swimming style, which, on the whole, is more effective than rippling fins. Just how median-paired fin swimming has evolved multiple times from tail-fin swimming ancestors is still unclear. In any case, the optimal specific wavelength may not help you trim seconds off of your lap times, but it's definitely made life in the depths easier for some aquatic animals!