We've known for centuries that octopuses get around one of two ways: one, by crawling over surfaces with their arms, or, two, swimming with the help of their siphon's jet.
But a new study (pdf) shows us that their movement is not quite so simple—and is far more fascinating.
A team of researchers has been keenly studying the way these weird, eight-armed animals swim in hopes of better understand their unusual nervous system—as well as in an effort to help us build a lifelike robotic octopus.
This group, led by Binyamin Hochner, a neuroscientist at Hebrew University in Israel, used four high-definition cameras to film swimming octopuses from different angles. They tested six adult common octopuses (Octopus vulgaris) in large tanks and found that there were in fact three very distinct modes of swimming.
"This is the first time that the swimming patterns of Octopus vulgaris have been recorded systematically in the laboratory," the researchers noted in their poster, presented at the Society for Neuroscience meeting earlier this month in California and last year at the Meeting of Neural Control of Movement Society in Italy.
They found that sometimes the octopuses swam headfirst, waving their mantle and arms behind them; other times the animals jetted in the more familiar squid-projectile shape with arms trailing behind them; and occasionally they seemed to swim mantle first, but using a strange waving of their arms—rather than a strong siphon jet.
This third swimming style is particularly intriguing. In it, the octopuses create thrust by closing outward extended opposing arms, the researchers proposed. And an extra push seems to come from the arm tips, which stay curled up until the final portion of the stroke, during which they whip straight into line with the rest of the arm.
In this stroke, the octopus's web also seems to play an important role. This fleshy, extendable tissue that connects the upper portions of the octopus's arms, contracts during the stroke, adding additional thrust.
The researchers now are trying to figure out how the octopus, with its somewhat decentralized nervous system, might be coordinating this strange movement.
This swimming pattern also adds an exciting wrinkle in the challenge of building a fully lifelike octopus robot. In Italy a couple years ago I visited a lab where they were attempting to recreate the octopus's jetting abilities. For this, they had cast a silicone replication of an octopus's mantle in hopes of better emulating the thrust created by the siphon in a simple jet-propelled swim.
But to coordinate so many infinitely flexible arms to keep the octopus robot moving forward (or backward, as it were), we will need a bit more time studying these strange cephalopods.
In the meantime, these new multi-dimensional recordings "may assist in the design and control of robotic arm prototypes," the researchers noted.
Read more about octopuses and all of their oddities in Octopus! The Most Mysterious Creature In the Sea.
Illustration courtesy of Ivan Phillipsen