The Strange Logic of the Strawberry Squid’s Lopsided Eyes

Our oceans are an endless cornucopia of weirdness. Today, we’d like to introduce you to a cockeyed deep-sea cephalopod called the strawberry squid. Researchers writing in the journal Philosophical Transactions of the Royal Society B say each of the squid’s unusual eyes serves a separate purpose in the black depths of the sea.

Histioteuthis heteropsis (literally “different eyes”) is pink, studded with bioluminescent spots, and quite content to cruise through the “twilight zone” some 650–3300 feet beneath the surface of the ocean. It has one enormous yellow eye and one normal blue eye—as normal as squid eyes get, anyway.

Biologist Kate Thomas of Duke University, lead author on the paper, says she was impressed by the strawberry squid’s strangeness. “You can’t look at one and not wonder what’s going on with them,” she says in the video above.

So she decided to find out. She pulled up 30 years' worth of undersea video recorded by remotely operated vehicles at the Monterey Bay Aquarium Research Institute and scrolled through, watching the squids’ behavior and looking for clues. She analyzed 152 different H. heteropsis sightings and another nine sightings of its cousin, Stigmatoteuthis dofleini.

The onscreen sightings revealed yet another strawberry squid eccentricity: They like to float along upside down, with their heads pointed toward the ocean floor. But this posture, while whimsical, appears to be practical. The head-down orientation aims the squid’s large eye toward the surface—which is where any light might originate. The little blue eye, better suited for spotting bioluminescence, can keep looking downward into the darkness.

Senior author Sönke Johnsen, Thomas's advisor, said the squid's blue eye could only have one job. “There is no way it is able to pick out shapes against the ambient light," he said in a statement. "And once it is looking for bioluminescence, it doesn’t really need to be particularly big, so it can actually shrivel up a little bit over generations. But the eye looking up actually does benefit from getting a bit bigger.”

It’s an ingenious solution to a low-light situation. “Eyes are really expensive to make and maintain,” Thomas added. “You want eyes just big enough to do what you need to do, but you don’t want to have any bigger eyes because then you are just wasting resources.”