New Facts About the Legendary Giant Squid

The researchers have uncovered hints about how the cephalopod’s brain became so huge, and how it accomplishes such impressive camouflage.

Up until 2006, most of our clues about the giant squid’s very existence came in the form of their washed-up, slimy corpses on beaches or through their beaks, which have been uncovered from the bellies of sperm whales. When the first-ever live footage of a giant squid was captured that year, just off of Japan’s Ogasawara Islands, we caught our first real glimpse of the magnificent cephalopod while still alive.

Fourteen years later, we’ve seen footage of the giant squid in its natural habitat, deep below the ocean’s waves, where the sun can’t reach—but the beasts are still fleeting and still difficult to study.

That’s why it’s such a huge deal that researchers have finally put together a draft genome sequence of the giant squid. It could help answer questions about how this mollusk grew to become over 40 feet long. The stuff of nightmares. Fodder for sailors’ tales of krakens.

“The elusiveness of the species makes it difficult to study,” the authors of the genome research, published in the scientific journal GigaScience, wrote in the abstract. “Thus, having a genome assembled for this deep-sea dwelling species will allow several pending evolutionary questions to be unlocked.”

The research team, led by Rute da Fonseca at the University of Copenhagen, found that just as the giant squid is absolutely humongous, so is its genome. With about 2.7 billion DNA base pairs, its genome is about 90 percent the size of the human genome. Still, that size in and of itself is not that telling. A rare Japanese flower called the Paris japonica, for instance, has 149 billion base pairs, which makes it about 50 times larger than the human genome.

The scientists extracted DNA for this analysis from a single giant squid, also known by the scientific name Architeuthis du. Because a giant squid has never been caught and kept alive before, the DNA sample came from a dead giant squid. While genome analysis points to some of the giant’s secrets—like how its brain grew to become the largest of the invertebrates and how it manages to camouflage its huge body so well—there’s still plenty that the researchers say can’t be told through genome sequencing just yet. For example, we still don’t know how the giant squid mates or what it eats.

Becoming Massive

Caroline Albertin, Hibbitt Fellow at Marine Biological Laboratory in Woods Hole, Massachusetts, is an expert on slippery creatures like the octopus and the squid. In 2015, she led the team that uncovered the very first cephalopod genome for the California two-spot octopus, Octopus bimaculoides.

Working on the giant squid genome, she noticed some similarities between it, the two-spot octopus, and the other three cephalopods that have had their genomes sequenced to date. In almost all animals, important developmental genes like Hox and Wnt are present.

In the giant squid, these were only present in single copies, which means the huge invertebrates didn’t become huge through whole-genome duplication, a biological strategy that allowed vertebrates to grow larger throughout the course of evolution. Indeed, it looks like invertebrates take a whole different approach to growth than vertebrates.

Whole-genome duplication is a process by which an organism creates additional copies of the entire genome of its species. Most living things that reproduce sexually, including humans, contain two copies of their full genome, inherited from each parent. This means humans are diploid, or contain two full sets of chromosomes. Whole-genome duplication increases the rate and efficiency by which organisms can acquire new biological traits.

So to fully understand why these cephalopods grow to over 40 feet in length, researchers will have to further probe the giant squid genome.

“A genome is a first step for answering a lot of questions about the biology of these very weird animals,” Albertin said in a press release. “While cephalopods have many complex and elaborate features, they are thought to have evolved independently of the vertebrates. By comparing their genomes we can ask, ‘Are cephalopods and vertebrates built the same way or are they built differently?'”

A Little Brain

One of the truly remarkable characteristics of the giant squid is its brain, which is rather complex and strangely shaped. Smithsonian describes its brain as “shaped like a donut.” And weirdly enough, its esophagus actually runs through that hole.

But here’s the catch: While the brain is colossally complicated, it’s actually pretty small as a share of the giant squid’s full body mass. While a giant squid can weigh up to one ton, based on the carcasses that have washed ashore and the limited video evidence we’ve seen of the them, their brain only weighs about 100 grams, which is just over one-fifth of a pound. Still, that’s the largest invertebrate brain on record.

Albertin and the rest of the research team working on the giant squid genome noticed the presence of more than 100 genes in the protocadherin family, which “are thought to be important in wiring up a complicated brain correctly,” she said. Usually, these genes aren’t found in invertebrates, so it’s hard to say what exactly this finding means. However, Albertin also identified protocadherins in the octopus genome she studied in 2015.

Camouflage for Days

Unique to cephalopods, reflectins are a gene family present in the giant squid’s genome that can help to camouflage them while stalking prey or trying to escape from one of their few natural predators, the sperm whale.

“Reflectins encode a protein that is involved in making iridescence,” Albertin said. “Color is an important part of camouflage, so we are trying to understand what this gene family is doing and how it works.”

Since only a few types of cephalopods have had their genomes sequenced—and because reflectins are so far only present in this class of animals—it’s difficult to say exactly how the proteins work. Finding them in the giant squid will help to propel research on them.

“Having this giant squid genome is an important node in helping us understand what makes a cephalopod a cephalopod,” Albertin said. “And it also can help us understand how new and novel genes arise in evolution and development.”

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