For centuries, the enigmatic giant squid has been at the center of horror lore and sailor fright. With a body the length of a school bus and basketball-sized eyeballs, the cephalopod’s massive tentacles can grab prey from up to 9 meters (30 feet) away. Despite its enormous proportions, Architeuthis dux is rarely seen by humans and has never been kept alive in captivity, begging the question: how did this creature get so big?

Now, researchers from the University of Copenhagen and Woods Hole Marine Biological Lab (MBL) are on the hunt for answers – and they're starting with publishing the world’s first whole-genome sequencing of the giant squid’s DNA.

"We have so much to learn about the living world. Much of the world is covered in ocean, and we know relatively little about the deep sea, where these animals live," study author Caroline Albertin of MBL told IFLScience. "Their environment is so different from ours, we can learn about how they solved the challenges of living in that environment." 

Despite being found in deep ocean waters around the world, giant squid’s elusiveness makes them difficult to study. To this day, their biology, life history, and reproductive cycles largely remain a mystery. In order to begin piecing together the cephalopod puzzle, researchers drew comparisons to the human genome, as well as four other cephalopod species whose genomes have been sequenced. Cephalopods are thought to have evolved independently from vertebrates, Albertin said. Comparing their genomes can provide evidence for this. 

"As giant squid are hard to study, we hope to compare their genomes to other cephalopods, like the octopus, to better understand these little-studied animals," Albertin explained. "This is one of the best-assembled cephalopod genomes, so it will be very helpful in understanding both the giant squid, but also their cephalopod cousins." 

In terms of genes, the giant squid looks a lot like other animals. They have an estimated 2.7 billion DNA base pairs, making their genome 90 percent the size of the human one. Of particular interest were Hox and Wnt, two developmental genes that are found in almost all animals. However, only single copies were observed in the giant squid genome, meaning the giant squid did not get so large through “whole-genome duplication,” a strategy that has helped many vertebrates evolve to be larger.

"Vertebrates, like humans, mice, and fish, have a number of features that are similar to what we see in cephalopods – some famous examples are camera eyes, a closed circulatory system, and big brains," said Albertin. "Because it’s thought that these features are the result of independent evolutionary histories (convergent evolution), we can ask if cephalopods and vertebrates are making these features the same way or in different ways."

Researchers also found reflectins, a gene family unique to cephalopods that is responsible for encoding a protein involved in making iridescence. Additionally, the team noted more than 100 genes in the protocadherin family, which typically are not found in abundance in invertebrates. Mammals generally have between 50 and 60 while there are more than 160 in the octopus genome and an expansion in the giant squid too.  

"Protocadherins are thought to be important in wiring up a complicated brain correctly," said Albertin. "They were thought to be a vertebrate innovation, so we were really surprised when we found more than 100 of them in the octopus genome (in 2015). That seemed like a smoking gun to how you make a complicated brain. And we have found a similar expansion of protocadherins in the giant squid, as well."

This information is a “critical resource” to understanding the unique traits of this species, including its gigantism and key adaptations to deep-sea environments. Yet other mysteries still remain. How did A. dux evolve one of the largest brains among invertebrates? Where did its sophisticated behaviors and agility come from? How is it capable of instantly camouflaging its gigantic body? Those are all questions that Albertin hopes to answer through further analysis of this “bizarre” animal’s DNA.