Giant nerves key to cuttlefish's incredible camouflage capability
The cuttlefish is often called the chameleon of the sea, but where the land-based version can only change its color, the sepia-squirting, tentacled one can change its skin texture as well as its tint in seconds. How it does this has been a mystery, but scientists from the Marine Biological Laboratory and the University of Cambridge have found that the cuttlefish controls its stealth ability using a giant neural circuit similar to the one that squids use to control their iridescence.
For a creature that dates back to the Cretaceous era, the cuttlefish has done pretty well for itself. For many people it's better known as a denizen of budgie cages and seafood entrées, but the cuttlefish is one of, if not the, most intelligent molluscs. It also has eyes that are in some ways better than a humans, and it has a unique inner shell to help it keep its shape.
But the cuttlefish's most dramatic talent is its ability to seemingly vanish from its enemies and prey by altering its color and skin texture to blend into the background – a remarkable feat for an animal that can't see color. Also used to communicate with other cuttlefish, how it does this is still largely unknown, but the international team funded by the US Air Force Office of Scientific Research and the UK Biotechnology and Biological Sciences Research Council has found a vital neurological clue.
The cuttlefish manages its disappearing act by means of tiny muscular organs called chromatophores that contain hundreds of thousands of pigment granules and a folding membrane to hide or reveal them. These, along with similar organs, provide the cuttlefish with an amazing degree of control not only over its color, but also its iridescence and even how it polarizes reflected light. Alongside these are the papillae that allow the cuttlefish to alter its skin texture in three dimensions.
According to the researchers, the cuttlefish manages to do this by sending instructions from its brain to its skin though a peripheral nerve center called the stellate ganglion. Specifically, it goes through the axon system. Axons are nerve fibers that act as the main transmission lines of the nervous system. They're usually about one micrometer wide and in some mammals they can get as large as 20 micrometers, but in the cuttlefish these giant axons are up to one millimeter in diameter – large enough to be seen with the naked eye.
Similar to the nervous system of the squid, the giant axon allowed the researchers to trace the path of the cuttlefish commands and better understand its camouflage abilities. In addition, it raises the question of whether these abilities found with other cephalopods is due to a common ancestor or if they developed independently.
Along with seeking to solve the conundrum of the chromatophores, the team also studied the papillae that let the cuttlefish change texture. Like any muscular activity, this minor shape shifting should require the cuttlefish to expend energy and continuous nerve impulses to maintain the desired texture, but the scientists found that the papillae have a catch-like mechanism similar to that found in bivalve molluscs like oysters and mussels that can close their shells and then keep them sealed without further effort.
"This research on neural control of flexible skin, combined with anatomical studies of the novel muscle groups that enable such shape-shifting skin, has applications for the development of new classes of soft materials that can be engineered for a wide array of uses in industry, society, and medicine," says Professor Roger Hanlon of the Marine Biological Laboratory.
But how do cuttlefish know what changes in color and texture to make in order to mimic their surroundings? At this stage, that question remains unanswered?
The research was published in iScience.
Source: University of Cambridge