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," Snke 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.
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.