Deep-sea squid points a big, bulging eye up and a tiny eye down

A sight to behold

Kate Thomas

By Sam Wong

Here’s looking at you, squid. Cock-eyed squid have one huge, bulging eye and another normal-sized eye, but the reason has remained a mystery.

Now we have an answer. Kate Thomas of Duke University in North Carolina studied 161 videos of the creatures collected over 26 years by remotely operated submarines in Monterey Bay, California.

The findings provide the first behavioural evidence that the two eyes are adapted to look in different directions. The large one points upwards to spot prey silhouetted against the sky. The smaller one points downwards to spot bioluminescent organisms against the darkness below.

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The squid, from the histioteuthid family, live at depths of 200 to 1000 metres, where little light penetrates. The videos show that the squid normally swims with its tail end pointing upwards, but tilted so the large eye is consistently oriented towards the sky.

Bioluminescent camouflage

Based on measurements of the eyes and the light levels they would be exposed to, Thomas and her colleagues calculated that having a big upward-pointing eye greatly improves visual perception, while a downward-pointing eye would gain little from being large. “That gives you the context for how this trait might have evolved,” says Thomas.

Some of the squid’s prey, such as lanternfish and shrimp, have luminescent undersides so they are camouflaged against the sunlight when seen from below. Yellow pigmentation in the lens of the squid’s large eye may help it distinguish between sunlight and bioluminescence.

Double vision

Sonke Johnsen

Squid in this family are the only bilaterally symmetrical animals with two very different eyes, says Thomas, although other creatures have adapted their eyes in similar ways to exploit the unusual light sources of the deep sea. For example, there are fish with large upward-facing eyes as well as small, accessory eyes on the sides of their head to see bioluminescence.

“You do see eye adaptations, you just don’t see quite this strange dimorphism that you see in these squids,” says Thomas.

Journal reference: Philosophical Transactions of the Royal Society B, DOI: 10.1098/rstb.2016.0069

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