23. Oct, 2018

Cuttlefish' brain controls colour change

The  cuttlefish  (Sepia) belongs to the family of Sepiida,  that in turn are part of the Cephalopoda which also include squids octopuses and nautiluses. Cuttlefish (see also my earlier Blog) form an enormous variety of species of which some, like Sepia officinalis and Sepia elegans, have -sadly enough- gained great popularity as appetizers and tasty entries on dinner tables. Sepia has a  cuttlebone used for buoyancy control. They feed themselves by extending two hidden feeding tentacles, which snag prey and pull it back to its strong beak. When Sepia mates the male grabs the female by the face and inserts a specialized tentacle into an opening near the females’ mouth and then inserts sperm sacks. The male then guards the female until the eggs are laid a few hours later. 

Giant Cuttlefish  (picture taken by Dave Abbott)

 The reputation of Sepia as ‘ chameleon of the sea’  is based on their unique capacity to rapidly change color by controlling pigment cells in their skin. Their skin functions like an extended brain, a bit like the eight arms of the Octopus, with two-thirds of its nerve cells located in the nerve cords of its arms (see my earlier Blog on Octopus). But in Sepia, the brain is reflected in its pigment carrying cells in the skin. These chromatophores produce colors varying between orange, red, yellow, brown or black. They are like color pixels on a computer screen,  but instead of a computer program their skin cells are controlled by motor neurons in the brain, that in turn activate muscles around the chromatophores. The muscles constantly expand and contract in a complex choreography in direct response to the activity of motor neurons, which is displayed in rapid changes in the color of the chromatophores. 

The color changes reflect the inner state of the cuttlefish. They may mirror certain  ‘emotions’  like fear (for predators),  sexual attraction (spotting a potential partner),  anger (spotting a rival) or when using camouflage by mimicking the color seagrass of coral. The mating ritual of the Australian giant cuttlefish is particularly impressive with respect to color changes.

Groups of cuttlefish use complex codes, changing rapidly in often synchronous patterns of pigment changes reflecting processes of social communication, of which the meaning still remains a mystery for biological  science.

Sam Reiter and his team from The Max-Planck Institute for Brain research, recently published an article in the journal Nature, describing in more detail the properties of cells in the cuttlefish skin, and neurons extending from the brain that control color changes. The team monitored changes in the state of the color-changing cells in living cuttlefish with high-resolution cameras, which allowed them to track the activity of tens of thousands of neurons simultaneously, for the first time. They also used electrical stimulation of muscles of the mantle of a small number of cuttlefish to produce artificial color changes and track motor units (nerve cells that make a muscle contract).  Analysis of the pattern dynamics and rules that govern the development of skin pattern, showed that new pixels of the chromatophores (that changed recently)  were mostly yellow and more mature pixels black.