Octopuses can form an impressively tight grip—even on a rough surface. And recent detailed microscopic imaging of their suckers revealed an intricate landscape of fine grooves that make these improbable holds possible.
But how do these animals manage to hold their grip—for hours at a time—without getting tuckered out?
A new study, published earlier this month in the Beilstein Journal of Nanotechnology, finds tiny hairs, lining the top interior of the sucker, which might just help enhance the octopus's hold.
Octopuses are not always on the prowl for food or a mate. And in fact, they spend much of their time hanging out in the safety of a den, where, rather than tread water, they can suction themselves to a wall, ceiling or floor to stay put. But if you or I were dependent on, say, our fingers to hang onto a rocky wall, we probably wouldn't last too long.
Scientists recently found that the insides of octopus suckers are not a smooth dome, but rather, are crowned by a "protuberance" which juts out, creating a small air (or, rather, water) pocket around it when the sucker is pressed down and activated. This uses the cohesive forces of water to help minimize the energy needed to keep a sucker suctioned—at pressure differences up to 0.269 megapascals (standard air pressure is 0.101 megapascals).
But the new images from scanning electron microscopes found an additional octopus secret: hairs. Tiny hairs. And lots of them.
The researchers imaged the surface of this protuberance in common octopuses (Octopus vulgaris)—both males and females—caught by local fishermen off the coast of Livorno, Italy. The protuberance, they found, to be "completely covered with a dense network of brush-like hairs," they wrote in their paper. These hairs grew to approximately 50 micrometers long and two micrometers wide. And these main stalks then branched out "into very small filaments" that were closer to five micrometers long and 0.3 micrometers wide. That is to say, much thinner than a human hair.
Scans of the rest of the interior of the suckers showed them to be entirely hairless.
These micro-hairs may help the octopus in keeping an effortless grip on any surface. The hairs "might work in addition to the cohesive forces of water, assisting in keeping the original orifice closed for extended periods of time and significantly increasing the resistance," the researchers noted. A blend of hairs, water and mucus (all of which the octopus seems to have) seems to boost viscosity where the top of the sucker meets a surface.
A mollusk relative, the abalone, as well as clingfish, also uses microscopic hairs to improve its suction, the researchers noted. And all three of these underwater animals lack the strategy of clingy land animals, such as the gecko, which have been found to have different micro projections—known as setal structures—on their feet. This suggests that "biological structures operating underwater cannot exploit filament-like structures to generate van der Waals forces" (which relies on the creation of electrodynamic pull between molecules), the scientists wrote.
Lab tests have shown that creating fiberous surfaces improves attachment underwater by 20 times—and 25 percent more force is needed to pull them off—over flat surfaces. Considering the substantial interest of engineers in this biological system," the authors wrote, "our findings may provide an interesting idea for improving the adhesion capability of artificial devices."
So, the robot octopus uprising might prove to be a rather hairy affair.
Read more about the weird biology of octopus suckers in Octopus! The Most Mysterious Creature In the Sea.
Illustration courtesy of Ivan Phillipsen