An international team of scientists has sequenced the genome of the giant squid (Architeuthis dux), the species which has inspired generations to tell tales of the fabled Kraken. The Architeuthis dux genome allowed the team to investigate the unique traits of this species, including its gigantism and key adaptations to deep-sea environments.
Cephalopods, an intelligent group of ocean dwelling creatures, include the eight-armed octopuses, the ten-armed squids and cuttlefishes, and the shelled nautiluses.
These animals show a wide variety of lifestyles and behaviors, but with the exception of the nautiluses they are characterized by rapid growth and short lifespans.
They range in size from the tiny pygmy squids (average length 2 cm, or 0.8 inches) to animals that are nearly three orders of magnitude larger, such as the giant squid Architeuthis dux (average length 10-12 m, or 33-39 feet), to the colossal squid Mesonychoteuthis hamiltoni (a recorded weight of 500 kg makes it the largest known invertebrate).
Cephalopods can rapidly alter the texture, pattern, color, and brightness of their skin, and this both enables a complex communication system, as well as provides exceptional camouflage and mimicry. Together these allow cephalopods to both avoid predators, and hunt prey highly efficiently, making them some of the top predators in the ocean.
These remarkable adaptations of cephalopods also extend to their genome.
“In terms of their genes, we found the giant squid look a lot like other animals,” said Dr. Caroline Albertin, a researcher in the Eugene Bell Center for Regenerative Biology and Tissue Engineering at the Marine Biological Laboratory, Woods Hole.
“This means we can study these truly bizarre animals to learn more about ourselves.”
Dr. Albertin and colleagues found that the Architeuthis dux genome is 2.7 billion DNA base pairs long — about 90% the size of the human genome.
They analyzed several ancient, well-known gene families of the giant squid, drawing comparisons with the four other cephalopod species that have been sequenced and with the human genome.
They found that important developmental genes in almost all animals (Hox and Wnt) were present in single copies only in the Architeuthis dux genome.
That means this gigantic, invertebrate creature didn’t get so big through whole-genome duplication, a strategy that evolution took long ago to increase the size of vertebrates. So, knowing how this squid species got so giant awaits further probing of its genome.
The scientists also identified more than 100 genes in the protocadherin family — typically not found in abundance in invertebrates — in the Architeuthis dux genome.
“Protocadherins are thought to be important in wiring up a complicated brain correctly,” Dr. Albertin said.
“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. 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.”
The team also analyzed a gene family that is unique to cephalopods, called reflectins.
“Reflectins encode a protein that is involved in 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,” Dr. Albertin said.
“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.”
The team’s results were published in the January 2020 issue of the journal GigaScience.
Rute R. da Fonseca et al. 2020. A draft genome sequence of the elusive giant squid, Architeuthis dux. GigaScience 9 (1): giz152; doi: 10.1093/gigascience/giz152