More surprising things about the octopus, our favorite cephalopod

First, don’t call them “octopi.” That is incorrect. The correct plural is octopuses or, more infrequently, octopodes.

Second, an octopus’ eight appendages are called arms, not tentacles.

“Tentacles are a really special kind of thing, which is an interesting kind of neural innovation and functional architecture that are really useful and commonly spread across the cephalopods, but octopuses didn't get one of them,” explains Frank Grasso, an associate professor of psychology at Brooklyn College in New York who studies octopuses in their Biomimetic and Cognitive Robotics Lab.

An octopus’ arms are kind of amazing, Grasso says. “Each octopus arm has roughly 3,000 suckers apiece,” Grasso explains. “Each one of those has on the order of 10,000 sensory neurons. Their suckers can discriminate not only textures, but shapes. Their suckers are chemical sensors; they provide a sense of taste.”

There is a lot of “intelligence” in an octopus arm, Grasso says. In fact, the arms make up about three-fifths of an octopus brain — insofar as this rather unique organ can be defined as a brain, as we typically understand it.

“We talk about the central nervous system of these animals because there's a distinction between the peripheral nervous system and the central nervous system,” Grasso explains. “To call it the brain is technically incorrect, but in terms of the computational function of the nerve cords that run through the arms, they really are acting like eight brains that are wired together to the central brain, which is the one that we would consider the cerebral ganglia.”

The octopus’ oddly diffuse brain has a reflex to “attach first, ask questions later,” Grasso says, which makes sense in most situations an octopus will encounter. But an octopus arm does not reflexively attach to its own tissue. They must have a specialized set of receptors for self-recognition, which comes in handy because of the large risk of self-entanglement.

The octopus also shows a rather human-like intelligence: It can show a kind regard or a dislike for people it encounters. According to one account, an octopus took a particular distate for one volunteer researcher and would squirt her with water any chance it got. 

Scientists uncovered a few other surprises when they sequenced the genome of the octopus. The octopus genome is unusually large, says Carrie Albertin, a graduate student in biology at the University of Chicago.

“Octopus bimaculoides [has] about 2.7 billion base pairs,” Albertin says. “That’s about 90 percent of the size of the human genome, which is about 3.2 billion. This is many times bigger than the genomes of other animals.”

Researchers presumed the octopus genome became so large by making multiple copies of itself, like mammals and fish. “But when we were looking through the octopus genome, we didn't actually see that signal, and we were really surprised by that,” Albertin says.

Albertin says the researchers were also surprised by “how completely normal at the level of gene families the octopus genome looked. When we sequenced it, we went through and looked for different families of genes, and we found that they have the normal number and kinds of genes that are found in other animals, like snails and worms.”

But about 10 percent of the genes didn't have a match in any other animals, a finding Albertin calls “really exciting.” Also interesting to note: Octopuses and squids diverge genetically about 270 million years ago. "To put that in perspective, that's before the first dinosaur was walking here," Albertin says. "Therse are really old families."

Frank Grasso says this new set of studies is “opening the floodgates of a whole bunch of knowledge. For a very long time, people didn't understand where octopuses came from, evolutionarily, in terms of their genetic composition and so forth. So many fundamental questions about these really bizarre and unusual animals, in terms of their bodies and their brains, will begin to be put on a solid footing with this research.”

This article is based on an interview that aired on PRI’s Science Friday with Ira Flatow.