The Cephalopod Nervous System: Convergent evolution...or something way cooler than we vertebrates got?!

by Heather Price

Octopuses have long been renowned as the planet’s most intelligent animals lacking a backbone. Their capacity for learning and problem-solving, (and penchant for escaping from tanks and consuming their neighboring aquarium mates), combined with their uniquely flexible, color-changing, eight-appendaged body, make these cephalopods some of the most mysterious and fascinating creatures in the sea.  

The octopus belongs to a group of molluscs called Cephalopods (meaning “head-foot”), which also includes squid and cuttlefish.  There are currently 289 recorded species of octopus, ranging in size from 2 cm (our very own California Lilliput Octopus) to the Giant Pacific Octopus which can reach a diameter of 9 meters!  In nature, the octopus is the master of inventive disguises, both physical and behavioral, that allow it to surprise its prey.  In the lab, octopuses have shown their ability to associate tactile and visual cues with rewards, to navigate mazes, and to teach each other more rapidly than a human researcher can teach an octopus. This high level of intelligence has often caused the octopus brain to be compared to that of vertebrates.

In addition to their intelligence, octopuses also exhibit strikingly rigid, jointed-appendage-like movements, despite the practically unlimited range of flexibility of their arms.  This has also led many people to draw comparisons with the vertebrate nervous system, and to declare that the similarities between the two are an exemplary model of convergent evolution.

While octopuses seem to share more in common with vertebrates than their more closely related invertebrate cousins, the central nervous system of our tentacled friends is actually arranged quite differently than those of backbone-endowed organisms.

The Cephalopod nervous system is divided into three main parts:

1. a central brain surrounded by a capsule made of cartilage

2. two large optic lobes connected to camera-like eyes

3. a peripheral nervous system connected to the arms

The octopus brain is comprised of about 40 lobes, each with its cell body on the outside, and its dendrites arranged into a central mass.  Unlike in most vertebrate brains, stimulation of a particular neuron in the octopus brain does not activate movement of one particular part of the octopus body, but instead triggers an entire series of movements comprising a complex behavior. Researchers have found that octopus arms actually cannot be stimulated individually, and that at least two arms move at a time in response to any one neuronal trigger.

Octopuses has over 500 million neurons, (which is similar to the number our canine companions possess), but unlike Fido, the octopus has about two-thirds of its nerve cells contained within its peripheral nervous system.  Concentrations of ganglia at the base of each arm control most of neuronal processing involved in movement.  The central brain will send an initial message to the peripheral ganglia, which will then coordinate a suite of complex movements. So unlike their human counterparts, male octopuses can legitimately claim that their hectocotylus (a modified arm used for inseminating females) has a mind of its own!

In addition to having arms with eight minds of their own, the octopus nervous system is unique in that it controls color-changing cells in their skin, known as chromatophores.  These chromatophores allow octopuses to rapidly change their appearance in order to camouflage with their environment, or communicate with one another.  Chromatophores consist of a central pigment-containing molecule surrounded by radial muscles that are controlled by the brain. Recent studies have shown that it is actually the rate of expansion and contraction of these radial muscles that determines the pattern on the octopuses’ skin, and not the intensity or type of pigment. While there are several species of vertebrates that exhibit color-changing abilities, none compares in rapidity or complexity to that of Cephalopod chromatophores.

Our tendency as Homo sapiens is to compare anything we regard as highly intelligent to ourselves.  Thus, the frequent comparisons between the octopus nervous system and our own.  However, when one looks more closely at the actual structure and function of the octopus brain and body, it becomes clear the convergence between these clever creatures and ourselves isn’t all that close.  In fact, the octopus may have some adaptations that are much cooler and more complex than we vertebrates could dream of evolving!

Also, in case you’ve been kept up at night wondering whether to say “octopuses” or “octopi” when telling your nerdy science friends about your recent aquarium visit...a Webster’s editor clears it up here nicely:

References:

Gray, E.G., and J.Z. Young. 1964. Electron microscopy of synaptic structure of octopus brain. The Journal of Cell Biology. 1:87-103.

Hochner, Binyamin. 2012. An embodied view of octopus neurobiology (minireview). Current Biology. 22:887-892.
Suzuki, M., T. Kimura, H. Ogawa, K. Hotta, and K. Oka. 2011. Chromatophore activity during natural pattern expression by the squid Sepioteuthis leesoniana: contributions of miniature oscillation. PLoS One. 6(4): e18244.