ADVERTISEMENT
  About the SA Blog Network













Observations

Observations


Opinion, arguments & analyses from the editors of Scientific American
Observations HomeAboutContact

Giant Eyes Help Colossal Squid Spot Glowing Whales

The views expressed are those of the author and are not necessarily those of Scientific American.


Email   PrintPrint



giant squid eye

Giant squid in ice; courtesy of Fir0002/Wikimedia Commons

Giant and colossal squid can grow to be some 12 meters long. But that alone doesn’t explain why they have the biggest eyeballs on the planet. At 280 millimeters in diameter, colossal squid eyes are much bigger than those of the swordfish, which at 90 millimeters, measure in as the next biggest peepers.

“It doesn’t make sense a giant squid and swordfish are similar in size but the squid’s eyes are proportionally much larger, three times the diameter and 27 times the volume,” Sönke Johnsen, a biologists at Duke University, said in a prepared statement. Why would these cephalopods evolve soccer-ball-size eyes?

The better to see you with, of course. Well, not you, exactly—unless you happen to be a hungry sperm whale. Scientists have found that having these extreme eyeballs likely allows these squid to spot whales when they’re still far enough away to escape the huge predators. The findings were described online March 15 in Current Biology.

Bigger eyes might seem an obvious solution for acquiring better vision. “For seeing in dim light, a large eye is better than a small eye, simply because it picks up more light,” co-author Dan-Eric Nilsson of Lund University said in a prepared statement.

But the low-light, low-contrast world of the pelagic oceans, where these squids and whales live and die, is much murkier than our airy environment here on land. “We have found that for animals living in water, it does not pay to make eyes much bigger than an orange,” Nilsson said. “For animals that live in the sea or in lakes, the optical properties of water will severely restrict how far away things can be seen.” And after a certain size—about 90 millimeters—bigger eyes don’t confer enough of a visual advantage to make up for their size, weight and energy demands. So given the diminishing returns, why such an extreme evolutionary development?

This is why the curious case of these jumbo eyes had scientists scratching their heads. The fact that these squids’ eyes are so much bigger than those of other animals suggest that they “use their eyes for a purpose not shared by other animals,” Nilsson, Johnsen and their colleagues wrote in the new study.

giant squid and whale

Giant, big-eyed squid battles a whale; courtesy of mercuriain/flickr

At that monster size, it turns out, the eyes don’t become uniformly better at seeing all objects—just those that are also jumbo, according to mathematical modeling described in the study. (In short: to see a subtle change in contrast at a long range demands both a big pupil and a big target—”to generate statistically detectable differences between object and background,” the researchers wrote.)

So, far from being extraneous, these extra-large eyes are nature’s answer for spotting something even bigger. And, as the authors noted in their paper, another big object “has a large chance of being important, either as a threat or as a potential for food or sex.” In other words, for these super-sized squid, size does matter.

All of this happens at depths where there is no sunlight at all; instead, the only light comes from the existence of glowing organisms. “Light is produced by small gelatinous animals when they are disturbed by the whale moving through water,” Nilsson explained. “Bioluminescence can reveal submarines at night, and diving sperm whales will become visible for the same reason.”

With adequate bioluminescence, these giant and colossal squid (Architeuthis and Mesonychoteuthis, respectively) can probably spot a sperm whale (Physeter macrocephalus) from 120 meters away—at a dark depth of 500 meters or more—and perhaps use its high-powered jet to squirt out of harm’s way. The spotting distance suggests that the squid can be on the lookout for moving predators over about seven million cubic meters. According to the new visual equations, this scenario would make an eye that is 90 to 300 millimeters optimal. Smaller than that, and they would miss the light show (and become dinner), and larger than that would again start the trend of diminishing returns and wouldn’t be worth the energy to develop—or drag around.

Whales, of course, don’t need big eyes to spot the squid. They have something that works even better to see in the dark: sonar.

Read more about cephalopods on my blog Octopus Chronicles.

Katherine Harmon Courage About the Author: Katherine Harmon Courage is a freelance writer and contributing editor for Scientific American. Her book Octopus! The Most Mysterious Creature In the Sea is out now from Penguin/Current. Follow on Twitter @KHCourage.

The views expressed are those of the author and are not necessarily those of Scientific American.





Rights & Permissions

Comments 4 Comments

Add Comment
  1. 1. jtdwyer 10:03 pm 03/15/2012

    The article states:
    “”We have found that for animals living in water, it does not pay to make eyes much bigger than an orange,” Nilsson said. “For animals that live in the sea or in lakes, the optical properties of water will severely restrict how far away things can be seen.” And after a certain size—about 90 millimeters—bigger eyes don’t confer enough of a visual advantage to make up for their size, weight and energy demands.”

    Visual acumen at distance is not the only reason for having large eyes, as improved vision should also enable squid to better locate nearby bioluminescence associated with prey animals.

    At any rate, the benefit derived from improved vision is not mathematically cost justified by a return on investment analysis represented by the specific “mathematical modeling described in the study.” Any significant survival benefit can lead to the adoption of new characteristics in successfully reproducing animals. In the case for improved prey location, increased nutrition might have more directly offset any increased energy demands.

    From the large number of squid consumed by certain types of whales, the larger eyes do not allow squid to successfully avoid their own predation.

    Link to this
  2. 2. scientific earthling 6:19 pm 03/16/2012

    Why don’t you mention that the squid is also one of the few animals who has its retinal photo-sensory cells pointing in the direction of incoming light, rather than like us who have our cones & rods pointing backwards and the little light that gets to them had to make it through a grid of nerves and blood vessels on the surface of the retina, then 2 further layers of nerve cells before being reflected of the back of the eyeball into the photo-sensory cones and rods.

    Link to this
  3. 3. Snowballsolarsystem 3:39 pm 03/17/2012

    The ability of cephalopods to see by the light of disturbed bioluminescence may stem from their origin in Oort-cloud compound-comet oceans of the Ordovician period, rather than from their transplanted home on earth.

    The Appalachian Basin Platform may be one such compound comet core from the inner Oort cloud that formed around the resonances of a companion star (Nemesis) orbiting our sun. Oort cloud comets may still be forming around the resonances of Nemesis today (such as the hypothesized Younger Dryas comet impact 12,900 B.P.) in the same way that asteroid-belt planetesimals formed around the resonances of Jupiter 4.5 Ga.

    http://hillscloud.wordpress.com

    Link to this
  4. 4. SquareState 7:48 pm 03/22/2012

    Goodbye, and thanks for all the fish.

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American Holiday Sale

Black Friday/Cyber Monday Blow-Out Sale

Enter code:
HOLIDAY 2014
at checkout

Get 20% off now! >

X

Email this Article

X