Years ago, during a field trip to the Scottish island of Cumbrae, I came face to face with a stout bobtail squid. As I watched, rapt, the tiny mollusc blushed from ghostly pale to deep red and back again, like a magic performance. But this was no illusion.
Squid are part of a whole spectrum of species that are able to change colour – an ability that comes with several speed settings. At its more relaxed end there is a handful of birds and mammals – including the Arctic fox, willow ptarmigan and snowshoe hare – that undergo a seasonal whitening triggered by waning day length. The transformation occurs as pigment disappears from fur and feathers. In mammal fur, this makes space for more air, which provides the added bonus of extra insulation as temperatures plummet.
Other environmental factors, such as ultraviolet, diet and surroundings, can also trigger gradual colour changes, involving alterations to the type and concentration of pigments within skin, exoskeleton, feathers or fur. A diet-driven transition is performed by several species of crab spiders, which ambush flower-visiting insects. The arachnids take about a week to morph from white to yellow, hiding in plain sight against their preferred backdrop of golden blooms.
For crustaceans, a slow colour change allows them to adapt to alterations in their environment. Chameleon prawns, for instance, transition between green and red, tracking the seasonal appearance of seaweed in their rocky shore habitat; shore crabs take on a more uniform colour as they mature, allowing them to blend in as they migrate onto the seabed of the subtidal zone.
Coloration in animals is achieved in several ways. In birds and mammals, skin cells known as melanocytes contain packages of melanin pigments, which produce blacks, browns, yellows and reds that can be combined in varying patterns. Birds can also gather pigments such as carotenoids through their diet to produce yellows and oranges, alongside the vivid greens, blues and violets created by the structure of their feathers.
Awash with colour
The colour palette is expanded in fish, reptiles, amphibians, crustaceans and cephalopods (octopus, cuttlefish and squid), which are equipped with colour-producing cells known as chromatophores. These cells either contain pigment or produce iridescence, and there are several types, including melanophores (black, brown, red), xanthophores (yellow), erythrophores (orange and red), iridophores (iridescent colours) and leucophores (iridescent white).
Of these, the best understood are the melanophores. Pigment can be moved within the branch-shaped structure of the cell. When dark pigments are dispersed throughout the melanophore, they obscure the neighbouring chromatophores and make the animal appear darker; when concentrated into the centre, the surrounding colour becomes more visible.
Several species, including Caribbean reef squid and mourning cuttlefish, have mastered the art of multi-tasking colour change. They present different messages on each side of their body, simultaneously attempting to impress a female on one side while sparring with a male on the other.
The oval iridophores contain thin layers of crystal platelets that are mostly used to produce a range of shimmering blues, greens and silvers, according to how they are spaced and orientated. Iridescence is directional: a creature can appear utterly striking from one angle yet drab from another, allowing for targeted visuals. Hummingbirds, for example, use frenetic displays to appeal for a mate, and will position themselves at just the right angle to the sun in order to dazzle with their finery.
While birds rely on light to manipulate their iridescence, certain fish, including the paradise whiptail, can actually alter their appearance by varying the space between the layers of platelets in their iridophores. In doing so, they change the wavelength of light reflected by the cells and are thus able to rapidly switch their reflective head stripes from blue to red. Likewise, the diminutive blue-ringed octopus tweaks its iridophores to make a statement, throwing out a dazzling warning display when it starts to feel threatened.
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