Giant squids, giant eyes, but rather small brain lobes

NEW YORK – Giant squids, which roam the deep sea and grow bodies large enough to spoon a school bus, are the stuff of legend. Their rare visits to the ocean’s surface may have inspired the kraken of Norse mythology and the beasts in Jules Verne’s 20,000 Leagues Under the Sea.

Among the creature’s biggest claims to fame are its eyes – along with another species of squid, giant squids have the largest orbs in the animal kingdom. But the sophistication of the giant squid’s visual brain may not be commensurate with its basketball-sized peepers, according to a paper published this Wednesday in the journal Royal Society Open Science.

A recent, lucky opportunity to study part of a giant squid brain up close in Taiwan suggests that, compared with cephalopods that live in shallow waters, giant squids have a small optic lobe relative to their eye size.

Furthermore, the region in their optic lobes that integrates visual information with motor tasks is reduced, implying that giant squids do not rely on visually guided behavior like camouflage and body patterning to communicate with one another, as other cephalopods do.

“This is the first time we get to have any information about what the world might look like to a giant squid, and that’s just super cool,” said Dr Robyn Crook, an assistant professor of biology at San Francisco State University who was not involved in the study.

Usually when a giant squid is spotted, it has died and its decomposing carcass has floated to the surface. But when Taiwanese fishermen accidentally caught a giant squid while trawling for mullet in 2016, scientists for the first time were able to examine a giant squid optic lobe that was fresh off the boat and relatively intact, said Dr Chuan-Chin Chiao, a professor of neuroscience at National Tsing Hua University.

Dr Chiao, who helped lead the study, got a call shortly after the catch. His students rushed to the site, and the scientists were able to take a high-resolution MRI scan of the optic lobe before it was preserved at Taiwan’s National Museum of Natural Science with the rest of the squid.

They were interested in two structures that make up the optic lobe – the cortex, which mostly processes visual information, and the medulla, which helps guide camouflage and dynamic body patterning.

Compared with two more commonly encountered shallow-water species – pharaoh cuttlefish and oval squids – giant squids have rather small optic lobes, given how huge their eyes are. In particular, though their cortex is densely packed with neurons, their medulla is disproportionately small

This makes sense, said Dr Roger Hanlon, a senior scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts, because shallow-water cephalopods live in much more visually complex environments, where camouflage and body patterning are more useful for things like avoiding predation and communicating with members of the same species.

The giant squid’s “eyes are really adapting to something quite different”, namely, darkness, said Dr Hanlon, who was not involved in the study.

As for why giant squids even need such big eyes, previous research has suggested that their eyesight is uniquely adapted to spotting faint clouds of bioluminescence that indicate a sperm whale – their main known predator – is approaching from a distance.

This new study supports that conclusion, Dr Chiao said, by showing that the part of the giant squid’s optic lobe that processes visual information is indeed rich with neurons. It also shows that giant squids probably do not use that information to perform the complex and dramatic appearance changes other cephalopods are famous for.

After all, when you live in near-total darkness, what you are wearing likely does not matter, Dr Chiao said. THE NEW YORK TIMES