Parallels are particularly interesting because they speak of the seemingly serendipitous nature of convergent evolution.
Humans and octopuses diverged from a common ancestor 500 million years ago, yet the ways in which our visual systems have evolved to solve the same problems are strange. Despite our different shapes, lifestyles, and habitats, vertebrates and octopuses independently developed a pupil and lens that directs light into the retina, for example.
Soft cephalopods–squids, octopuses, and cuttlefish–have the largest brains of any invertebrates, with two-thirds of their central processing tissues reserved for sight only.
As you would expect from all that room, these ocean creatures have a really good view, even in the dark. An octopus’ skin contains the same pigment proteins as its eyes, allowing the octopus’ dermis to “see” the details of its surroundings and camouflage accordingly.
Current research by researchers at the University of Oregon is the first to comprehensively map an octopus’ visual system. It required analysis of more than 26,000 cells, collected during the autopsy of two events california two locations (octopus bimaculoides) octopuses.
Although the brains of these octopuses were fully functional, they seemed to be developing. In fact, about a third of the neurons distributed throughout the optic lobes looked as if they were still developing.
When the researchers sequenced cephalopod cells, they found four main groups, each of which released a different chemical signal — some that released dopamine, some that released acetylcholine, some that released glutamine, and some that released a signal with both dopamine and glutamine.
These neurotransmitters are also seen in vertebrate brains, like ours, but there were many smaller groups of neurons in the cephalopod brain that express unique chemicals.
A ring of cells around the optic lobe, for example, has been found to produce octopamine, a neurotransmitter closely related to a hormone in our bodies called noradrenaline.
What exactly octopamine does in octopuses is a mystery that requires more research to solve. However, it is known to be active in the brains of fruit flies when they fly, and is important in many other invertebrates for functions related to preparing their bodies and nervous systems to function.
A new octopus brain map could aid these future efforts. Researchers have identified several gene transcription factors and signaling molecules unique to octopuses, which may help shape neural development in some way.
Further studies could omit or mask these factors to see their possible role in the cephalopod brain.
“The atlas we present here provides a roadmap for such studies, and generally provides a path forward toward breaking the functional, developmental, and evolutionary logic of the cephalopod visual system,” the authors wrote.
Similar to vertebrates, the octopus’ visual system is made up of layers, but not in the same way as our visual system. The diversity of cell types and the way they are organized in the brain of cephalopods is fundamentally different.
“On the obvious level, neurons don’t connect to each other – they use different neurotransmitters,” explains biologist Chris Neal of the University of Oregon.
“But maybe they do the same kinds of calculations, just in a different way.”
One of the biggest questions is how the cephalopod visual system develops. Octopuses spend years growing huge brains, but how does information from their retina help direct that growth?
In vertebrates, the photoreceptors in the retina do not connect directly to the brain. Instead, they deliver messages to other neurons. But in cephalopods, the photoreceptors connect to the visual lobes of the brain.
Future work is needed to explore how these direct messages affect the development of immature neurons, and how many immature neurons are finally incorporated into a mature visual system.
Neil and his colleagues are now continuing their work to map the remaining third of the octopus’ brain.
The study was published in current biology.