Cockeyed squid have evolved with a bizarre form of sight

13 Feb 2017714 Views

The cockeyed squid has evolved with a mismatched set of eyes. Image: Kate Thomas

A bizarre-looking sea creature called the strawberry squid is cockeyed so it can track its food, with one big eye looking up and a smaller one looking down.

After trawling through 150 videos of the cockeyed squid swimming up to 1km below the ocean’s surface, a cockeyed mystery has been solved.

With an official name of Histioteuthis heteropsis, the strawberry – or cockeyed – squid is a bright pink creature with small photophores dotted across its body.

Their most prominent features, though, are their eyes; one big and yellow, the other small and black. Now we know why.

Cockeyed squid

The large eye is specifically adapted for looking up, searching for shadows against the faint sunlight, while the small eye is designed to look down in search of bioluminescence.

“You can’t look at one and not wonder what’s going on with them,” said Duke University biologist Kate Thomas, who investigated dozens of videos of cockeyed squid collated by the Monterey Bay Aquarium Research Institute (MBARI).

Comparing them to a similar squid called Stigmatoteuthis dofleini, she led a paper on the subject, finding that the squid prefer to drift through the sea in relatively constant pose, with tails up and heads down, therefore positioning the eyes in a constant vantage position.

“The deep sea is an amazing natural laboratory for eye design, because the kinds of eyes you need to see bioluminescence are different from the kinds of eyes you need to see the basic ambient light,” said Sönke Johnsen, who worked on the study. “In the case of the Histioteuthis, this cockeyed squid, they chose one eye for each.

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“The eye looking down really only can look for bioluminescence,” Johnsen said.

“There is no way it is able to pick out shapes against the ambient light. 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.”

The cockeyed squid has one normal eye and one giant bulging eye. Research shows these lopsided eyes evolved to spot two different sources of light available in the ocean’s ‘twilight zone’ – between 200m-1km beneath the surface. Image: Sönke Johnsen

The cockeyed squid has one normal eye and one giant bulging eye. Research shows that these lopsided eyes evolved to spot two different sources of light available in the ocean’s ‘twilight zone’ – between 200m and 1km beneath the surface. Image: Sönke Johnsen

MBARI being called upon for a study of such an odd creature should hardly be a surprise. Late last year, the group recorded the first ever footage of the elusive ghost shark.

Ghost sharks are pretty unusual, both in appearance and make-up. Their bodies are hardened by plates and cartilage rather than bone, with their scientific name taken from Greek mythology.

Like the chimaera from these myths, which had two heads – a goat’s and a lion’s – and a serpent’s tail, chimaeras are pretty weird looking. There are 38 species known throughout the world, with this particular oddity first discovered in 2002, through remains.

In August, a googly-eyed squid discovery was made by a team of researchers from the Ocean Exploration Trust aboard the Nautilus floating laboratory.

Called the stubby squid (Rossia pacifica), cartoon-like characteristics are present in its huge eyes, which one of the marine biologists in the team’s video described as looking “so fake”.

Gordon Hunt is senior communications and content executive at NDRC. He previously worked as a journalist with Silicon Republic.