Squid are remarkably versatile swimmers. While most fish find it difficult to reverse, squid are equally happy going backward and forward, swimming tail first at high speeds but arms first when hovering and swimming slowly to survey the area. Although squid generate most of their thrust at high speeds by squeezing jets of water out of the mantel, they are equipped with a versatile pair of mantle fins that ripple and flap at lower speeds. But it wasn't clear exactly how the fins contributed to the animal's agility. Were the fins simply acting as stabilisers, or could they generate thrust to propel the animals? Having previously investigated the way that squid jet around at higher speeds, William Stewart and Ian Bartol from Old Dominion University, USA, and Paul Krueger from Southern Methodist University, USA, decided to find out more about the way the squid use their fins while swimming (p. 2009).
After successfully catching the elusive animals, Stewart and Bartol rushed back to the lab ready to put them through their paces in a flume. Adding microscopic beads to the water and shining a plane of laser light on the tip of one of the squid's fins to reveal the water's motion the duo filmed the squid as they swam arms first (fins at the back) and tail first (fins at the front) at speeds ranging from 2 cm s−1 up to 23 cm s−1. Having collected the data, the trio spent another 6 months analysing it to find out how the squid use their fins and were pleased to see the tell-tale vortices that they had hoped to see spinning off the fins in four distinct modes when the squid swam tail first. However, the animals only used two of the four modes while swimming arms first.
When swimming tail first in the first mode, the squid flapped their fins up and down, but only exerted force on the water during the down-stroke. ‘We did not measure any detectible vorticity associated with the up-stroke,’ says Stewart and adds that instead of generating thrust the fins mostly produced lift to hold the squid's vertical position in the water column. When looking at the second mode, the team saw that instead of flapping the fins, the squid sent an S-shaped ripple along the fin edge, resulting in a chain of linked vorticity producing weak upward jets and stronger downward jets of water giving rise to a net lift force on the animal's body.
Analysing the third and fourth modes, the team realised that instead of generating lift alone, they both produced thrust to propel the squid forward. In the third mode the squid returned to flapping their fins, but the fin beat was relatively leisurely, producing independent vortices at the end of each up- and down-stroke, while in the fourth mode, the up-stroke followed rapidly after the down-stroke so that the shed vortices became linked in pairs.
Turning their attention to the squid's ‘arms first’ swimming style, the team found that the animal's fins also produced the second and third vorticity patterns, but this time neither pattern generated significant thrust – they both produced lift and often drag.
Stewart admits that he was surprised that the squid can produce two of the same vorticity patterns regardless of whether they swim arms or tail first. He explains that instead of being stiffened by rays the fins are muscular hydrostatic systems, and the squid's ability to produce the same vorticity patterns when swimming in opposite directions reflects the fin's versatility.
So squid can use their fins to generate lift and provide stability, but they can also use them to generate thrust to supplement their mantel jets, thanks to their versatile muscular structure.