The tiny nautilus may help illuminate the history of brain development

Even in a family tree of full of eccentrics, the tiny nautilus is still an oddball. 

Unlike most other soft-bodied cephalopods — cuttlefish, squid and octopus — the nautilus has no camouflage, no ink and relatively poor vision.

That's because the nautilus adapted differently to threats in its environment. It developed an external shell for protection and the ability to live deep in the ocean — up to 700 meters below the ocean surface, where few of its predators want to go.

At that depth, where there's very little light, the nautilus has to rely on smell to survive. 

“They use smell to get around," says Dr. Jennifer Basil, an evolutionary biologist and Associate Professor of Ecology at Brooklyn College. "They use it to find each other; they use it to find food and carcasses. So they have a greatly expanded olfactory lobe — and there's a lot of organization in that lobe … There are layers, like you see in an octopus brain, which is considered to be the most complex of all invertebrates.” 

The nautilus' brain is surprisingly complex: it can be trained, and it has good short- and long-term memory.  “The Nautilus has up to 40 dedicated, obvious lobes in its brain,” Dr. Basil explains. “Some of those are dedicated to learning and memory.” 

Basil believes that these lobes evolved to enable the nautilus to compete with the highly visual, fast-moving bony fish that were evolving around it. Based on their brain and their behavior, Basil thought, "These guys are probably really good at remembering smells.”

So she and a colleague, Dr. Robyn Crook, created a series of experiments to test the nautilus’s memory. They devised a conditioning experiment similar to the famous experiment on Pavlov’s dog.

“Often, in the deep sea, the wavelengths of light that make it that far [down] are blue, and there also blue bioluminescent bacteria that are often found on decaying food," Basil explain, "So we thought, ‘Okay, why don’t we see if they can associate a blue light with the smell of food?’”

They put the animals into a “little nautilus car seat,” flashed a light, and released a food odor a second later. When exposed to the odor, the nautilus’s tentacles would come out and try to sense where the food was. Then the researchers would give them the food. Now they needed to see if they would learn that the blue flash of light meant food was coming.

They tested the nautilus’s response to the blue light at intervals between 30 minutes and 24 hours later. The results were surprising. For the first hour, if they flashed the light, the nautilus’s tentacles would come out and look for the food reward. After that hour, they wouldn’t react at all.

“Finally, after about six hours or so, the animals started responding again, which is an expression of long-term memory," Basil says. "So they have short-term memory and long-term memory.”

They then tested to see if this demonstration of learning also applied to spatial awareness.

They trained the nautiluses to find a hole in a maze that led to deeper waters. Then they waited several weeks and put them into the maze again. “They swam straight for the hole,” Basil says.

Basil says these results rival similar tests with the octopus, impressive given that the species is considered a living fossil. “The nautilus line is hundreds of millions of years old," Basil says. "The entire lineage has come up with a unique solution for having a lot of pressure in their environment: a big brain and complex behavior. And it's an old brain, older than the vertebrate brain. That in itself, I think, is interesting and a bit humbling.”

See more on the unique world of cephalopods, including the fascinating and wrongly-named Vampire Squid, from Science Friday.