Cephalopods have the most complicated central nervous system of all invertebrates. In combination with their complex visual systems and tactile limbs, this makes squid and octopuses some of the most “voracious” predators under the sea.
"The modern cephalopods, a group including octopus, cuttlefish, and squid, have famously complex brains, approaching that of a dog and surpassing mice and rats, at least in neuronal number,” Dr Wen-Sung Chung in a statement.
"For example, some cephalopods have more than 500 million neurons, compared to 200 million for a rat and 20,000 for a normal mollusk.”
Now researchers at the University of Queensland's Queensland Brain Institute have created MRI-based mapping of the bigfin reef squid (Sepioteuthis lessoniana), a squid endemic to the Indo-Pacific region with a lifespan between four and six years. More than 280 known connections were found and an additional 145 undescribed pathways, among them were neural circuits specifically dedicated to camouflage and visual communication, which gives the animal a unique ability to evade predators, hunt, and communicate using “dynamic color changes.”
"This the first time modern technology has been used to explore the brain of this amazing animal, and we proposed 145 new connections and pathways, more than 60 percent of which are linked to the vision and motor systems," said Chung, who writes in iScience that the findings provide new insights that may reveal functional morphology and interactions between brain regions.
Reef squid are famous for their “simple eyes,” which are used for their “vision-dominated lifestyle” and are notably similar to the structure of eyes seen in vertebrates. In addition to their eyes, reef squid also have a similar organization, functionality, and development of their brains as to vertebrates. The findings add to a growing body of evidence that finds parallels between vertebrate and invertebrate physiological similarities, supporting the theory of convergent evolution – the theory that organisms independently evolved similar traits. Earlier this year, researchers sequenced the entire genome of the giant squid and found that cephalopods and vertebrates exhibit similar features, further providing evidence of convergent evolution.
"The similarity with the better-studied vertebrate nervous system allows us to make new predictions about the cephalopod nervous system at the behavioral level," said Chung. "For example, this study proposes several new networks of neurons in charge of visually-guided behaviors such as locomotion and countershading camouflage – when squid display different colors on the top and bottom of their bodies to blend into the background whether they are being viewed from above or below."
Limitations with MRI-based visualizations exist, and it is important to note that the findings do not necessarily show what connections are responsible for certain tasks. Furthermore, the researchers note that their mapping is unable to reveal synaptic connections and label neural tracts. Even so, the findings demonstrate just how complicated cephalopods are.
For starters, squid have the ability to camouflage themselves even though they are colorblind – their unique cloaking ability has even inspired technological breakthroughs. Octopuses, a cousin of the reef squid, have long been touted for their extreme cleverness and intellect. They have been known to problem solve and even react in much the same way to MDMA as humans.
The researchers note that understanding how cephalopod species have evolved different subdivisions of the brain may someday inform our understanding of their diverse behavior.