Ever wondered why squids have an array of colours and patterns? Probably not, but here’s why.
Firstly, we need to learn the words chromatophores and iridophores as they will be used frequently. Chromatophores are cells present in the skin of squids. Inside them, they contain a pigment which comes more visible as small radial muscles pull the sac open. As they do so, this causes the pigment to expand. Iridophores are very thin cells that are stacked up. They reflect light back at varying wavelengths (almost all wavelengths on the visible light spectrum) and different polarities. Interestingly, the colour that is reflected is dependent on the angle you are observing the squid at. So perhaps from a bird’s eye view you might see a squid that appears blue, but from a frontal view it may look red. The combination of iridophore reflection and the correct patterning of chromatophores is what allows camouflage to occur, this is a useful adaptation for survival.
Chromatophores can contain brown, red or yellow pigments. Whilst iridophore cells can be red, orange, yellow, green or blue. Chromatophores are innervated by the brain directly. When the muscles contract, the chromatophores can overlap underlying iridophore cells. Using a spectrometer, you can acquire highly focused reflectance measurements of the cells and the quantity and quality of light when the two interact. Thus, it is the expansion and retraction of chromatophores which enables the production of an array of body patterns like bands, stripes and spots.
Experimental procedure is used to demonstrate the amount of light transmitted by different groups of cells. Iridophore have a significantly higher reflection peak than chromatophores when compared on a graph. But where the two overlaps, there is a compromise, thus the peak is not as big as iridophores only, but not as low as chromatophores only. You can compare results produced to the visible light spectrum, there should be a match between the colour produced at a certain wavelength and the colour shown on the spectrum at the same wavelength. For example, an iridophore may appear red at 650nm in wavelength. If you look on the light spectrum, wavelengths between 650-700nm are red.
Iridophores act as multilayer reflectors. Under different physiological conditions, the wavelength can be changed. Transmission of light can also be switched on and off through physiological control.
So, what is the point of all of this? Well, colour change can be a useful survival mechanism. The squid can change colour to match its environment. If an animal is in a brown, gloomy environment, and that animal is bright fluorescent colours, its predators will spot it from a mile away! Not good! Thus, the organism is adaptable. In bright daylight, with lots of greenery, it makes sense for the organism to be shades of blue and green.
Acetylcholine is a neurotransmitter (a chemical) realised by nerve cells or neurons. It functions in causing muscle contraction. Therefore, acetylcholine presence is a physiological control mechanism for colour. If there is a lot of acetylcholine, a lot of chromatophores pigments will expand, overlapping with more iridophores. This means more colour, more patterns!