Scientists have figured out why the strawberry squid has such a lopsided face: in order to see different light sources deep below the ocean’s surface, the squid has evolved two very different-looking eyes.
With one small and sunken blue eye on the right side of its face and a huge yellowy-green eye bulging out of the left side, the Histioteuthis heteropsis looks every bit its nickname: the cockeyed squid. But as with all the rest of nature’s strange and wondrous creatures, there’s an evolutionary method to this apparent madness.
“You can’t look at one and not wonder what’s going on with them,” said Duke University biologist Kate Thomas, part of a team of scientists that closely observed the heteropsis in over 30 years worth of videos collected by the Monterey Bay Aquarium Research Institute (MBARI) in Monterey, California.
Bright pink in colour, the heteropsis cephalopod can be found in the ocean’s “twilight zone,” that area between 200 and 1000 metres below the surface, where light coming from above is scant and dim. It gets its “strawberry” tag not only due to its colour but from the fact that its body is covered in photophore spots, dotting its surface somewhat like the seeds of a strawberry. One would think that in the dark of the ocean depths, having light-producing spots all over oneself would be a bad idea, basically acting as a neon drive-thru sign for predators. But the heteropsis actually uses its photophores to hide itself from predators lurking further below. By altering its photophores so that they flash much like the shimmers of light coming down from the surface, the heteropsis can blend into the background – a defence mechanism known as counter-illumination.
Bioluminescence plays a huge role for creatures of the deep, both in searching for prey but also for reproduction (in the dark, you need to tell your own kind where to find you). Thus, the heteropsis’ one smaller eye is habitually pointing down, searching for flashes of bioluminescent light to help identify prey, say the new study’s researchers. At the same time, the one big bulging eye scans up above, using what faint light filters down from the surface to literally keep an eye out for both prey and predators.
The researchers surmise that the mismatched eyes are a perfect example of evolutionary efficiency. In reality, having two huge eyes would give the squid more sensing power than it actually needs and would use up valuable energy and bodily resources in the process. Thus, the one shrunken eye helps cut down on wasted energy while still doing its job. “Eyes are really expensive to make and maintain,” Thomas said. “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.”
The heteropsis is one of over a dozen squid in the Histioteuthis genus, all of which have similar dimorphism in the eyes. Heteropsis’ geographic range is wide, being found in waters all along the west coast of North and South America, from Canada down to Peru and Chile, but not in more tropical waters close to the equator.
The new study is published in the journal Philosophical Transactions of the Royal Society B.
Below: Wonky eyed squid, Histioteuthis