The Cuttlefish, Camouflage And Colourblindness
The Cuttlefish (Sepiida) is a mollusc of the Cephalopoda class which includes other similar species such as squid and octopuses. There are several species of cuttlefish such as the common cuttlefish, the hooded cuttlefish and largest species the giant cuttlefish. They can grow upto 50cm and weight as much as 11kg. They are commonly found in the tropical/temperate waters of Western Europe, The Mediterranean, East and South Asia along with Africa and Australia, however, they are completely absent from North and South America. Usually found at sublittoral depths between the edge of the continental shelf and the low tide up to 180m but have been known to reach depths of 600m.
Cuttlefish eat a wide variety of prey from crabs, fish, worms and even other smaller cuttlefish, they are also preyed upon by sharks, dolphins, seals and seabirds. The have a rather short life expectancy with an average of one to two years with females usually dying shortly after they lay their eggs, they are also thought to be one of the most intelligent invertebrates with the largest brain to body size ratio of any invertebrate. The cuttlefish achieves buoyancy through an internal system known as the cuttlebone. This “bone” is porous and made of aragonite, buoyancy is regulated by changing the gas to liquid ratio in the chambers of the cuttlebone via ventral siphuncle. The size, shape and pattern of the cuttlebone is distinct for each species and has been used by jewellers for moulding small objects.
Cuttlefish have the ability to quickly change their skin colour, shape and texture when needed, they are even able to change the polarisation of reflected light waves, this can be done for several different reasons such as communication and courting. Cuttlefish have been trained to change colour when presented with certain stimuli implying that this colour change ability isn’t completely innate. Their ability to change the polarisation of light waves can be used to signal other animals which can sense polarisation, seeing that cuttlefish lack colour vision this may provide an alternate mode of receiving information that is just a defined as colour. Cuttlefish have three categories of colour patterns, these are, uniform, mottle and disruptive.
The cuttlefish change its colour to camouflage itself through the use of chromatophores, these are a sac containing thousands of tiny pigment granules and a large retractable membrane. These are connected to hundreds of muscles which are under neural control, this means that the cuttlefish are able to contract and relax the muscles around individual chromatophores, causing different levels of pigment to be exposed. There are three types of chromatophores, yellow/orange, red and brown/black.
Video showing the Cuttlefishes active camouflage in its natural environment. Author panvorax. Taken from https://www.youtube.com/watch?v=7ujRgSRYE9A
As mentioned previously the cuttlefish is seemingly colourblind, this begs the question whether the cuttlefish is merely guessing the colour of the object its attempting to hide against or whether the camouflage behaviour is merely an automated response that the cuttlefish doesn’t actually think about or decide what colour to change to. Like other cephalopods the cuttlefish have fairly complex eye structures, but only possess one kind of colour sensitive protein meaning they can only see the world in black and white.
Studies into how the cuttlefish is able to camouflage so accurately without seemingly knowing what they’re camouflaging against actually looks like. Cephalopods have the ability to focus their eyes at different depths using a lensing property known as chromatic blur. How this works is that each colour has a different light wavelength, due to lenses bending some wavelengths more than others it means that one colour of light basing through the the lens will be in focus whilst others would still appear blurry. So with several lenses a quick sweep of an area would indicate what colour an object was based off when it blurs. This hypothesis was tested by the National Academy of Science that built a computer model of an octopus eye and used to determine the colours of certain objects by changing its focus, it was found that this was an effective method, this theory is currently still theoretical and would require testing on live cephalopods to determine whether they use this method to aid with camouflage.