Cockeye squids face a conundrum. Living hundreds of meters underwater, they float between two worlds. Above them is the surface ocean, where a faint blue fraction of sunlight filters through. Below them is the deep sea, sunless and black. Which way to look?
Their mismatched eyes fix the problem, allowing them to look into both worlds at the same time.
Squids are born with two identical eyes, but the left eyeball grows much faster, eventually forming a long tube, often with a bright yellow lens. This huge, highly sensitive eye looks up and searches for the dark silhouettes of food or enemies passing overhead. The yellow color filters background light and breaks the camouflage of bright animals trying to blend in with the blue around them.
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The right eyeball, meanwhile, ends up being less than half the diameter of its partner. The squid points it down, looking for flashes of bright animals against the dark waters below.
“It’s a nice example of how they exploit and inhabit these two very different environments,” says Jon Ablett, senior curator of shellfish at the Natural History Museum in London.
Although the cockeye squid (Histioteuthis heteropsis) have been known to science for decades, scientists recently captured one on an expedition around the remote Atlantic islands of Ascension and St. Helena that brilliantly lived up to another name for the creature: the jewel squid. With an ultraviolet flashlight shining on it, the squid’s finger-long body glittered all over in ruby-red blotches.
“When you get within about 15cm suddenly everything glows red, and the closer you get, the stronger the color,” says James Maclaine, senior curator of fish at the Natural History Museum, who carried a UV flashlight on the expedition to see which animals perform this trick. “It’s really dramatic,” he says.
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As far as he knows, no other scientist has tried shining UV light on deep-sea animals. Maclaine found that some have bright red spots, including jewel squid and viperfish, and others, like lanternfish, don’t.
The red dots are light-emitting organs, known as photophores, that actually glow blue when squid swim hundreds of feet underwater. There is no UV light in the deep sea, so photophores do not glow red in their natural environment.
Like the animals they’re trying to spot with their large yellow eye, cockeye squid can use their bright photophores to disguise their silhouette. Flashing bioluminescent lights could also be a form of squid communication and a way to attract mates. Or they might attract prey that they mistake for something to eat. “It’s probably all of these reasons,” says Ablett, “a little bit of camouflage, a little bit of sexual selection, a little bit of prey attraction.”