The common octopus
is amazing. It can swim fast, or creep slowly across the ocean floor.
It can spread out to a large size or pack itself into the tiniest of spaces. It can change its coloring to mimic its surroundings (and one of its relatives can even mimic other creatures). The octopus can regrow any of its eight arms if one gets severed. And it’s clever, figuring out how to open jars, even when trapped inside. But the common octopus is also a freak: The animal is a cannibal and will sometimes even eat its own severed arm.
But
for all that we know about the octopus, there’s been one big question
left unanswered: Why don’t octopuses’ suckers stick to themselves? Now
Nir Nesher of the Hebrew University of Jerusalem in Israel and
colleagues think they’ve figured it out. The animals must have a
chemical in their skin that inhibits attachment, they report May 15 in Current Biology.
The
researchers began with severed octopus arms, which continue to act like
they’re alive for an hour or so after detachment. These arms would only
grab other amputated arms when those arms had been skinned. That led
the scientists to think that there was something in the skin that
signaled “octopus” to the suckers. They then coated Petri dishes in skin
extract. The severed arms grabbed the Petri dishes much more loosely
when covered in the extract than when covered with an extract of fish
skin or a control substance.
Nesher and colleagues also videotaped
live octopuses presented with severed arms to see what would happen. In
some instances, the octopuses would treat the arm like a typical piece
of prey, grabbing it and manipulating it with its arms, them bringing it
to its mouth. “But in others, octopuses showed a behavior
uncharacteristic of feeding,” the researchers note. “When presented with
amputated arms, the octopus would repeatedly rub its arms over the
amputated arm, touching it, but not attaching to it or grabbing it.”
Other times, the octopus would only grab the arm at the site of
amputation, where there was no skin.
The octopuses appear to know
when an amputated arm belongs to itself: They attached themselves to the
severed arms of other octopuses about 95 percent of the time but only
in 40 percent of trials that involved their own arms. And about
three-quarters of the animals treated other individuals’ arms as food
but only a quarter did so with their own. (But still — ew.)
When
an arm was skinned, though, it was always treated as a typical piece of
food, regardless of it was its own arm or one from another octopus, the
researchers write.
The
results suggest that there is something in the skin that tells the
octopus that the object they are touching is an octopus. And while
severed arms always avoided attaching to octopus skin, live octopuses
didn’t, which indicates they probably have the ability to override that
initial signal, the researchers write.
The signaling system may
help the animals with motion control. It’s already known that they
simplify motor control of their many arms by limiting the degrees of freedom for them to just three and through the use of “predefined motor programs”
embedded in the neuromusculature of the arms themselves. Having an easy
way for the suckers to be able to identify “octopus” and “self” may be
another method for simplifying the complications that come with
eight-limbed life.
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