A valiant effort to map the optic lobe of the octopus brain, cell by cell, revealed a visual system with remarkable similarities and differences to our own.
The parallels are particularly interesting because they testify to the seemingly serendipitous nature of convergent evolution.
Humans and octopuses diverged from a common ancestor 500 million years ago, and yet how our respective visual systems evolved to solve the same problems is strange. Despite our different body shapes, lifestyles and habitats, vertebrates and octopuses have independently developed a pupil and a lens that guides light onto a retina, for example.
Soft-bodied cephalopods – squid, octopus and cuttlefish – have the largest brains of any invertebrate, with two-thirds of their central processing tissue reserved solely for sight.
As you would expect throughout this room, these ocean creatures have very good vision, even in the dark. An octopus’ skin contains the same pigment proteins as its eyes, allowing its dermis to “see” details of its surroundings and camouflage itself accordingly.
The current research by researchers at the University of Oregon is the first to comprehensively map the octopus’ visual system. It required the analysis of more than 26,000 cells, collected during the dissection of two juveniles California colons (bimaculoid octopus) octopuses.
Although the brains of these young octopuses were fully functioning, they appeared to be growing. In fact, almost a third of the neurons distributed throughout the visual lobes appeared to be still developing.
When the researchers sequenced the cephalopod cells, they found four main populations, each releasing a different chemical signal – some releasing dopamine, some releasing acetylcholine, some releasing glutamine, and some signaling to both dopamine and glutamine.
These neurotransmitters are also seen in vertebrate brains, like ours, but there were several clusters of smaller neurons in cephalopod brains that expressed unique chemicals.
A ring of cells around the optic lobe, for example, has been shown to produce octopamine, a neurotransmitter closely related to a hormone in our body called norepinephrine.
What exactly octopamine does in octopuses is a mystery that will need more research to solve. However, it is known to be active in the brains of fruit flies when they are flying, and is important in many other invertebrates for functions related to preparing their bodies and nervous systems for action.
This new octopus brain map could help future efforts. The researchers identified several octopus-specific genetic transcription factors and signaling molecules that likely help shape neuronal development in one way or another.
Further studies could remove or stifle these factors to understand their possible role in the cephalopod brain.
“The atlas we present here provides a roadmap for such studies, and more generally a way forward to break the functional, developmental, and evolutionary logic of the cephalopod visual system,” the authors write.
Similar to vertebrates, the octopus’ visual system is layered, but not in the same way as ours. The diversity of cell types and the way they are organized in the cephalopod brain is fundamentally different.
“On the obvious level, neurons don’t map to each other — they use different neurotransmitters,” says biologist Cris Niell from the University of Oregon.
“But maybe they’re doing the same kinds of calculations, but in a different way.”
One of the biggest questions is how the visual system of cephalopods develops. Octopuses spend years developing massive brains, but how does information from the retina help direct that growth?
In vertebrates, photoreceptors in the retina do not connect directly to the brain. Instead, they transmit messages to other neurons. But in cephalopods, photoreceptors connect directly to the optic lobes of the brain.
Future work is needed to explore how these direct messages influence the development of immature neurons and how so many immature neurons ultimately integrate into a mature visual system.
Niell and his colleagues are now continuing their work to map the remaining third of the octopus’ brain.
The study was published in Current biology.
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