Researchers have painstakingly mapped the octopus’ optic lobe cell by cell to understand how these animals see — and the data showcases some remarkable similarities, alongside important differences.

Octopi… really don’t resemble humans much. But the optic lobes in their brains resemble ours quite a bit, according to new research. Such findings showcase just how similar very different animals or bits of their bodies can become through the seemingly-coincidental process of convergent evolution.

To give some perspective on the similarities that the team describes, humans and octopuses come from lineages that diverged over 500 million years ago. That being said, our sight and that of octopuses evolved to solve the same issue in incredibly similar ways, despite our different environments, general body structure, and lifestyle.

Big brains in the big ocean

Soft-bodied cephalopods such as squids, octopuses, and cuttlefish, have the largest brains of any invertebrate. Around two-thirds of its volume is reserved for the processing of vision. This gives them excellent eyesight even in low-light conditions. Octopuses even use their skin to provide data for their optical lobes. This tissue contains the same pigmented cells as the retinas. They perceive the animal’s surroundings and help it better blend in with its active camouflage.

This paper is the first to map out the octopus visual system in detail. It involved an analysis of over 26,000 cells. Data for the study was collected during the dissection of two juvenile California two-spots octopuses (Octopus bimaculoides). Although the brains of these animals were healthy and fully-functioning, they also appear to still have been developing. Nearly one-third of the neurons in the octopuses’ visual lobes appeared immature, the team explains.

Still, the team identified four main populations of cells in these brains, each producing and releasing a chemical signal: these included dopamine, acetylcholine, glutamine, or a mix of dopamine and glutamine. These compounds also play a role in vertebrate brains. However, a smaller population of cells that formed clusters in the cephalopod brain also secreted unique chemicals.

For instance, a group of cells forming a ring around the optic lobe was found to produce octopamine, which is similar in structure to the hormone noradrenaline in our bodies. Exactly what function octopamine performs is still unknown. That being said, it has been noted to be active in the brains of fruit flies while they are flying, and in the bodies of other invertebrates it plays a key role in preparing their bodies and nervous systems for bouts of activity.

Beyond this, the team identified several genetic transcription factors and signaling compounds that have so far only been seen in octopuses. The team believes these elements have a role to play in the development of the cephalopod brain; further research will be needed before we can say for sure what function they perform, however.

“The atlas we present here provides a roadmap for such studies, and more generally provides a path forward towards cracking the functional, developmental, and evolutionary logic of the cephalopod visual system,” the authors write.

The octopus visual system is structured in layers, the team explains. While our own are also structured in layers, the octopus visual system employs a fundamentally-different architecture and diversity of various cell types. Although the neurons here are arrayed in different ways and use different neurotransmitters than our own, it’s likely that they are performing the same kinds of computations as neurons in the human visual system.

The paper “Cell types and molecular architecture of the Octopus bimaculoides visual system” has been published in the journal Current Biology.