Becoming invisible usually requires magic.

For some thumb-sized squid, though, all it takes is a little genetic tweaking.

Once these squid are genetically altered, "they're really hard to spot," even for their caretakers, says Joshua Rosenthal, a senior scientist at the Marine Biological Laboratory in Woods Hole, Mass.

"We know we put it in this aquarium, but they might look for a half hour before they can actually see it," Rosenthal says. "They're that transparent."

The see-through squid are offering scientists a new way to study the biology of a creature that is intact and moving freely.

"It changes the way you interpret what's going on in this animal," says Caroline Albertin, a fellow at the lab. "You can look through and see their three hearts beating, you can see their brain."

The transparent squid is a genetically altered version of the hummingbird bobtail squid, a species usually found in the tropical waters from Indonesia to China and Japan. It's typically smaller than a thumb and shaped like a dumpling. And like other cephalopods, it has a relatively large and sophisticated brain.

The see-through version is made possible by a gene editing technology called CRISPR, which became popular nearly a decade ago.

Albertin and Rosenthal thought they might be able to use CRISPR to create a special squid for research. They focused on the hummingbird bobtail squid because it is small, a prodigious breeder, and thrives in lab aquariums, including one at the lab in Woods Hole.

"You can see him right there in the bottom," Rosenthal says, "just kind of sitting there, hunkered down in the sand."

The squid is one that has not been genetically altered. So it is camouflaged to blend in with the sand. That's possible because of organs in its skin called chromatophores. They contain pigment that can be manipulated to change the squid's appearance.

Albertin and Rosenthal wanted to use CRISPR to create a bobtail squid without any pigment, an albino. And they knew that in other squid, pigment depends on the presence of a gene called TDO.

"So we tried to knock out TDO," Albertin says, "and nothing happened."

It turned out that bobtail squid have a second gene that also affects pigment.

"When we targeted that gene, lo and behold we were able to get albinos," Albertin says.

Because even unaltered squid have clear blood, thin skin, and no bones, the albinos are all but transparent unless light hits them at just the right angle.

The team described their success in July in the journal Current Biology.

Lots of labs would like to use the see-through squid. So in the lab at Woods Hole, a team of technicians is putting in long hours to create more of them.

Albertin lets me look over the shoulder of a technician who's looking through a microscope at a squid embryo smaller than a BB pellet.

She's using a pair of forceps to gently remove the "jelly layers" that surround the egg sac. Later, she'll use a quartz needle to inject the embryo with genetic material that will delete the pigment genes and create a transparent squid.

Early on, Albertin and Rosenthal realized these animals would be of interest to brain scientists. So they contacted Ivan Soltesz at Stanford and Cristopher Niell at the University of Oregon.

"We said, 'Hey, you guys, we have this incredible animal, want to look at its brain," Rosenthal says. "They jumped on it."

Soltesz and Niell inserted a fluorescent dye into an area of the brain that processes visual information. The dye glows when it's near brain cells that are active.

Then the scientists projected images onto a screen in front of the squid. And the brain areas involved in vision began to glow, something that would have been impossible to see in a squid with pigment.

"The evidence that they were able to get from this made all of us kind of jump through our skins," Albertin says. "It was really exciting."

Because it suggests that her see-through squid will help scientists understand not only cephalopods, but all living creatures.

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