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Neurobiology

No nerves lost: Octopuses can regenerate their nervous system.

Reference: Imperadore, Pamela, et al. “From injury to full repair: nerve regeneration and functional recovery in the common octopus, Octopus vulgaris.” Journal of Experimental Biology 222.19 (2019): jeb209965.

The nervous system of an octopus is nothing short of amazing. Each of the octopus’ eight tentacles is controlled by a cluster of nerve cells, called ganglia, and the ganglia are in turn controlled by the centralized brain. As it turns out, the octopus can also regenerate parts of its nervous system after an injury.

Cutting this nerve turns the skin on its side of the mantle white. Image source: Imperadore et al., 2017 (1).

A team of scientists, headed by Graziano Florito of Stazione Zoologica Anton Dohrn in Naples, Italy, investigated whether the common octopus, or Octopus vulgaris, can regenerate its pallial nerves. These nerves run from the centralized brain in the head to ganglia in the octopus mantle on both sides of the body. The scientists found that if an octopus pallial nerve was cut, it repaired itself within five months.

The two pallial nerves have two very important functions for the octopus. First, they regulate breathing by controlling a respiratory muscle in the mantle. Second, they help change the color of the mantle, which is how octopuses communicate with each other. To live a good life, octopuses need to both breathe and talk to others, so the regenerative potential of the pallial nerve was an exciting finding for the scientists – and good news for Octopus vulgaris.

Octopuses on their last nerve

To study regeneration of the pallial nerves, the researchers caught fourteen wild octopuses in the Bay of Naples. When the research team anesthetized the animals and cut one of their two pallial nerves with fine scissors, leaving the other one intact, they saw that the respiratory muscle on the corresponding side of the mantle stopped contracting. They also saw that on that side, the color of the mantle skin went from brown to white.

To see if the octopuses were doing well after the surgery, the researchers tempted them with a tasty snack. After the animals came out of anesthesia, the researchers put live crabs in their tanks. The octopuses promptly attacked the crabs, showing their normal predatory behavior and not showing any distress about the surgery.

Over the next several weeks, recovery of the pallial nerve could be observed with the naked eye, as its two behavioral functions slowly returned to the octopuses. First, the white skin of the mantle developed some brown spots that formed color patterns when the animals were at rest. After a month and a half, the mantle skin returned to normal, forming color patterns when the octopuses were hunting prey or moving around the tank.

Breathing returned as well. Four weeks after the pallial nerve was lesioned, the mantle opening resumed its normal expanding and contracting, which showed that the paralyzed respiratory muscle started working again. For the first time, regeneration of the octopus pallial nerve was confirmed by the recovery of the normal behavior that depends on this nerve.

Making repairs 

The return of breathing and skin patterning suggested that the transected pallial nerve had managed the repair itself, but the only way to make sure of this discovery was to see directly what the nerve looked like in the octopus tissue. The scientists found that the two stumps of the lesioned nerve, one on the mantle side and one on the head side, had reconnected by 45 days after the injury. The fibers of the nerve had crossed the site of the lesion and formed a network with the local nerve cells in the mantle.

Octopuses communicate with each other by forming color patterns on their skin. This ability is lost when the pallial nerve is cut, but returns as the nerve stumps reconnect. Image source: Taken from Pixabay

To test if those regenerated fibers were indeed functional nerve fibers, the researchers exposed the regenerated nerve and placed an electrode on each side of the lesion. The electrode on the head side stimulated the nerve and the electrode on the mantle side recorded the activity of the regenerated nerve fibers. If electrically stimulated, the pallial nerve itself produced electric current and carried it to the other side of the lesion – confirming for the first time that the reconnected nerve stumps form a perfectly functional nerve.

Regeneration on the tree of life

It has been known for a long time that octopuses are master regenerators – the first study describing regeneration of the pallial nerve was published in 1932 (2). Besides their nervous system, octopuses can also regenerate their tentacles (3) and parts of the eye (4).

This exceptional regenerative ability is not unique to octopuses – other members of Cephalopod species, like cuttlefish, can also regenerate arms (1). But even among vertebrates, some animals can rival their reputation of incredible regenerators. 

Many fish and amphibian species rely on regeneration to recover from injuries to their organs and body parts. Zebrafish can replace missing heart tissue, and the Mexican axolotl salamander can regenerate an entire limb (5, 6). 

By contrast, mammals are notoriously bad at regeneration, land and ocean species alike. Mice and young human children can sometimes regenerate severed fingertips (6), and the regenerative abilities of ocean mammals, like whales and dolphins, are still very poorly understood, possibly because it is difficult to work with these animals in a biology research lab.

While octopuses do not reign as supreme regenerators in the animal kingdom, they certainly have an upper tentacle here when compared to humans.

 

Additional references: 

  1. Imperadore, P., Shah, S. B., Makarenkova, H. P., & Fiorito, G. (2017). Nerve degeneration and regeneration in the cephalopod mollusc Octopus vulgaris: the case of the pallial nerve. Scientific reports, 7, 46564
  2. Sereni, E., & Young, J. Z. (1932). Nervous degeneration and regeneration in cephalopods. Pubbl. Staz. Zool. Napoli, 12, 173-208.
  3. Lange, M. M. (1920). On the regeneration and finer structure of the arms of the cephalopods. Journal of Experimental Zoology, 31(1), 1-57.
  4. Dingerkus, G., & Santoro, E. D. (1981). Cornea regeneration in the Pacific giant octopus, Octopus dofleini, and the common octopus, O. vulgaris. Experientia, 37(4), 368-369.
  5. Jopling, C., Sleep, E., Raya, M., Martí, M., Raya, A., & Belmonte, J. C. I. (2010). Zebrafish heart regeneration occurs by cardiomyocyte dedifferentiation and proliferation. Nature, 464(7288), 606.
  6. Payzin-Dogru, D., & Whited, J. L. (2018). An integrative framework for salamander and mouse limb regeneration. International Journal of Developmental Biology, 62(6-7-8), 393-402.

I am a PhD candidate at Northeastern University in Boston. I study regeneration of the nervous system in water salamanders called axolotls. In my free time, I like to read science fiction, bake, go on walks around Boston, and dig up cool science articles.

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