How Did The Giant Squid Get So Big?

The giant squid is an enigmatic deep-sea dwelling species that more than lives up to its name. However, despite being one of the largest living cephalopods – the group that includes squid, octopus, cuttlefish, and nautilus, the giant squid remains elusive, and so little is known about their biology.

Now, researchers from the University of Copenhagen, Denmark, have revealed the complete genome sequence of this legendary being, Architeuthis dux. But, what does this mean for us?

Well, having the genes assembled means we will have a better snapshot on the anatomy and evolution of this mysterious giant, like – how they got so big, and perhaps crack open several unanswered evolutionary questions.

Led by Rute da Fonseca of the University of Copenhagen, the team discovered that the giant squid has big genome consisting an estimated 2.7 billion DNA base pairs, which is 90 percent the size of the human genome.

Caroline Albertin of the Marine Biological Laboratory (MBL), Woods Hole, who was involved in the research, is a connoisseur in cephalopod genome sequencing. In 2015, she and her team sequenced the first genome of a cephalopod. She explains that the genes of the giant squid bear a striking resemblance with other animals, and studying these truly bizarre creatures will help us learn a lot about ourselves.

For the study, the team analyzed several ancient gene families in the giant squid and compared the genome to four other cephalopod species that have been sequenced with the human genome.

They found that Hox and Wnt – two of the most important genes which manage growth and development of almost all organisms – to be present in single copies only in the giant squid genome. This indicates that the ginormous mollusk did not get so big through whole-genome duplication, unlike the process that evolution strategized the vertebrates to adopt eons ago to achieve gigantism.

To give you an idea of how enormous the giant squid is, they can grow to be more than 50 feet in length and weigh nearly a ton.  Because of their deep-sea habitat, they are rarely sighted, and almost everything we know about them comes from the carcasses that washed up on the shore or were found by fishermen.

So, in part, we have an answer to how the giant squid got so big. However, to come up with concrete explanation for its enormity, we need further probation of its genome.

A genome holds answers to a great many questions about the biology of the species such as the giant squid. Answers to questions such as: How did they acquire the largest brain among the invertebrates? How did they develop their sophisticated behaviors and agility, and their mastery in the art of camouflage?

“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?” explains Albertin.

Albertin and her team were also able to single out more than 100 genes in the protocadherin family in the genome of the giant squid. These genes are rarely found in invertebrates.

“Protocadherins are thought to be important in wiring up a complicated brain correctly,” she says. “They were thought they were 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.”

Genetic assessment of reflectins – a gene family which has so far been considered unique to cephalopods – was also carried out. This family of gene encodes a protein responsible for making iridescence.

“Color is an important part of camouflage, so we are trying to understand what this gene family is doing and how it works,” Albertin says.

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

A full genome sequence of the elusive giant squid, Architeuthis dux, is available at the journal GigaScience.