The closest that most of us ever get to a squid is a pile of tasty calamari rings, but when Ian Bartol from the Old Dominion University, USA, looks at a squid he sees a remarkably agile swimmer. The nimble creatures propel themselves by rippling their fins and squirting jets of water from their mantles. However, very little was known about the fluid dynamics associated with these versatile swimmers' movements, and even less was known about how cephalopods' interactions with water change as they grow and develop. Curious to find out more about squid swimming techniques, Bartol and his student William Stewart, teamed up with Paul Krueger and Joseph Thompson to focus on how the animals jet around ().
Bartol decided to visualise the propulsive jets produced by squid using a technique called digital particle image velocimetry (DPIV), where swirling jets in the water are visualized by thin planes of laser light reflected off microscopic beads floating in the water. Bartol and Stewart trained the squid ranging in dorsal mantle length from 3.3 to 9.1 cm to swim against currents ranging from 2 to 22 cm s–1 while filming their propulsive jets and analysed the fluid flows.
After months of painstaking analysis, Bartol and Stewart realised that the animals produce two distinct types of jet: a short efficient jet and a long powerful jet. The team also noticed that the squid often used their fins more frequently when generating the short-pulsed jet, possibly to augment the weaker jet's thrust.
Having identified these two different propulsive jets, the team realised that the squid relied more heavily on the long powerful propulsive jet than the short efficient jet, as the longer jet provided the most thrust. However,the smaller squid (with dorsal mantle lengths less than 5 cm) used relatively more short jets, even though they are less powerful. The team suspect that the squid reduce their use of short efficient jets as their mantles grow larger and the muscle's mechanical properties change.
However, when the team compared the two jetting styles with the animals'speeds, they were surprised to find that instead of switching gear from the low power jet to the high power jet as they speeded up. The squid seemed equally comfortable using both jet styles at all speeds, possibly resorting to using their fins to increase thrust at times when the low powered jet produced insufficient thrust.
`When the flexibility of jet dynamics is coupled with highly versatile fins, which are capable of producing multiple hydrodynamic modes as well, it is clear that squid have a locomotive repertoire that is far more complex than originally thought,' says Bartol, and he is keen to find out whether the squids' jet modes correlate with their fin motions as they jet around.
