New 3D Maps Reveal the Hidden Anatomy of Octopus Arms

Compared with an octopus’s arms, human bony limbs have roughly the flexibility of aged tree branches. It follows that the anatomy inside a cephalopod’s sucker-covered appendages should be every bit as distinctive as the creatures themselves.

Until recently, charting the network of nerves and muscles in octopus arms has been hampered by the scale and intricacy of the work. Research was largely limited to gathering two-dimensional sections and inferring how they fitted together.

Two new studies from the laboratory of evolutionary biologist Robyn Crook at San Francisco State University have now exposed the tissues of what some describe as the nearest thing to an alien on Earth in unprecedented detail.

"Having [these two papers] converging at the same time means the amount we can learn from any single experiment is just astronomically higher," says Crook.

"I would say these papers are really facilitating discovery in new ways."

Mapping the nerves and muscles of octopus arms

Watching an octopus hunt for prey can resemble ink moving with intention. With no bones, its muscles can bend, rotate, lengthen and reach, combining power and precision in a way that is almost unrivalled across the animal kingdom.

Earlier work has established a general picture of how oblique and longitudinal muscles interact. It has also shown that hundreds of millions of neurons form clusters known as ganglions, giving each arm a degree of independent control: much like soldiers in a highly disciplined unit, devoted to the wider purpose while still able to solve problems individually.

Yet, just as the human brain comprises varied neuron classes directed by a broad range of neurotransmitters, octopus arm nervous systems are expected to possess neurochemical organisation that enables them to move, feel and think with considerable autonomy.

Crook and her colleagues carried out two separate studies to rebuild the layout and identify the classifications of nerves extending through the arms of Bock’s pygmy octopus (Octopus bocki) specimens.

Two three-dimensional views of Bock’s pygmy octopus

In one experiment, led by neuroscientist Gabrielle C. Winters-Bostwick, the team used a DNA-based method to label and identify different nerve-cell types. High-resolution images, taken from arm tip to base with a newly acquired state-of-the-art microscope, mapped the three-dimensional distribution of every nerve-cell class and revealed differences in their populations across the limb.

"This allows us to start hypothesizing and posing new questions thinking about how the cells communicate with one another," says Winters-Bostwick.

"It's basically building our arsenal and our toolkit to better understand the behavior and physiologies of octopuses."

A second study, led by biologist Diana Neacsu, used electron microscopy to reconstruct the structure of neurons, muscles and skin, showing how these different tissues are connected and related.

This alternative 3D map uncovered unexpected features in the animal’s cortex, including oblique links between intramuscular nerve cords, repeating structures containing nerve ganglions and blood vessels that matched the positions of suckers, and an unusual pattern of scarce, exceptionally large nerve cells within the cell layers.

An atlas of octopus anatomy is only the starting point for understanding how a mollusc that followed such a different evolutionary route can behave in ways that feel so familiar.

"Why do you have an animal with this much complexity that doesn't seem to follow the same rules as our other example – humans – of a very complex nervous system?" says Crook.

"There's a lot of hypotheses. It might be functional. There might be something fundamentally different in the tasks octopus arms have to do. But it could also be an evolutionary accident."

The research was published in Current Biology here and here.