In the book I draw a parallel between the octopus and our gut. Here are some of the highlights:
Number of neurons
Octopus: 500 million
Our gut: 500 million
Where are these neurons?
Octopus: 60-70% reside in the body and arms. Some of the decision making and coordination is performed directly in the limbs by neuron clusters (ganglia, at the base of the arms). The Octopus can taste with its arms
Our gut: the gut brain, also called the enteric nervous system (ENS), surrounds our digestive tract. (i) It can operate independently from our brain and spinal cord, but communicates with the brain via the vagus nerve. The ENS governs our gut's physical reflexes and chemistry (e.g., digestion movement and enzymes) and has an influence on many complex themes such as mood (ii). It communicates just like our brain via neurotransmitters (e.g., dopamine, serotonin), hormones and electric signals.
Does this mean that our gut is as smart as an octopus?
Difficult to say, because the definition of "smart" has to be seen in context of the function and the two have very different functions and goals. From a raw computing power, they are in the same ball park.
Octopus: My speculation is that the octopus is better at targeted problem-solving because it has to hunt its food or escape from predators, navigate in a three dimensional world, and has been shown to be an excellent "out-of-the-box" thinker (iii).
Our Gut: My speculation is that our gut is better at multitasking. Our gut-brain is highly interconnected with every body system (i.e., it is our primary hormonal system and a critical component of our immune system) and supports our overall metabolism. In a sense it has to solve many micro-problems in parallel.
(i) For a more detailed review see: John B. Furness (2007) Mark A. Fleming, Lubaina Ehsan, Sean R. Moore, Daniel E. Levin, (2020), "The Enteric Nervous System and Its Emerging Role as a Therapeutic Target",Gastroenterology Research and Practice,vol. , Article ID 8024171, 13pages.
(ii) more than 90% of the serotonin in our body is in our guts (serotonin is a neurotransmitter involved in mood, learning, memory, and physiological reactions such as vomiting and narrowing of blood vessels). Close to 50% of the dopamine in our body is found in the gut (similar to serotonin, dopamine is involved in learning, mood motivation, sleep, pain processing, but also in the function of blood vessels, vomiting, kidney and lactation). The ENS uses wide a range (30 and counting) of different neurotransmitters.
(iii) Here is a fun recent video summarizing the marvelous intelligence of octopuses