Iridescence

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Iridescence is a widespread phenomenon in the natural world. It is common in invertebrates including in the eyes, wings and other parts of insects such as flies, wasps, bees, beetles, butterflies and bugs; the setae and other parts of crustaceans; in squid, octopus and cuttlefish; and in arachnids, especially the jumping spiders. Iridescence is also common among vertebrates such as on the scales of fishes, the scales of some reptiles and on the plumage of birds. It is present in some minerals, on the inside of sea shells, on soap bubbles, oil slicks and the surfaces of CDs and DVDs. Recently it has been shown to occur in flowers where it is visible to pollinating insects, but invisible to humans because the iridescence is masked by pigments. It also occurs in many myxomycetes.
Iridescence is not caused by a pigment but by the physical interaction between light and the nanometer-scale structure of a surface. These structural colours can be produced by either interference, diffraction or scattering, depending on the characteristics of the surface. (In many cases in the natural world, colour is produced by both structural characteristics and pigments.)
Put simply, the interference of many light waves reflected from different layers within an object creates iridescence. The light may be reflected from the front and back surfaces of thin layers, from layers of particles or air pockets, and/or from the faults and boundaries in crystalline materials. Light waves interfere positively with each other and produce a light wave with double the intensity of colour. Diffraction occurs when light waves bend around the corners rather than continue straight ahead from a surface made up of parallel grooves or depressions (e.g. CDs and DVDs). Because some wavelengths are diffracted more than others, light is separated into the spectral colours.

Functions of iridescence

Iridescence has evolved many times in numerous different organisms so it probably has a range of different functions, some of which are yet to be scientifically tested. They include: visual communication, camouflage, indications of age or environmental stress, assistance in flocking or schooling, predator avoidance or deterrence, warning, thermoregulation, friction reduction, water repellence, strength and vision enhancement – among others. Here are some examples:

Age and condition

Iridescent surface structures such as feather barbules may be more vulnerable to wear and tear, thus indicating the age and environmental stresses experienced by an individual.

Iridescent surface structures such as butterfly wing scales and feather barbules are likely to be more vulnerable to wear and tear than pigmented areas and thus could indicate the age and environmental stresses experienced by an individual.
Colour-producing nanostructures might change in response to an animal’s physiological state. For instance, in Black-winged damsel flies the fatter males are iridescent blue and the thinner males are green. The fatter males achieve their blue colour through compression of the chitin and melanin layers that produce iridescence.

Thermoregularion

The reed frog (Hyperolius viridiflavus) inhabits the seasonally hot dry African savannas where it aestivates unhidden on plants. (Aestivation is dormancy or sluggishness that occurs in some animals when conditions are hot and dry. It is analogous to hibernation and usually lasts an entire season.) Increases in temperature during the dry season induces changes in its colour from white, through a copper-like iridescence to green iridescence. This results in higher overall reflectance and is thought to help the frogs regulate their temperature.

Camouflage and warning

Blue-ringed Octopus

The well documented colour-changing abilities of cephalopods (cuttlefish, squid and octopus) are used for camouflage and advertising. For example, the flashing blue rings of the blue-ringed octopus often observed by CNFN members during field trips to Penguin shelf are a warning to steer clear of one of the most venomous animals in the world.
When at rest, the pale yellow-brown of the blue-ringed octopus blends with its surroundings. But when warning the unwary, an octopus can flash 50-60 iridescent blue rings in a third of a second.
Cephalopods change their appearance by using chromatophores, i.e. small sacs of pigments that they compress or stretch using their muscles, or by using iridophores which are light reflectors composed of stacked thin plates that produce iridescent colours. In most cephalopods, the iridophores are switched on and off by chemical signalling of neurotransmitters. Experiments showed that, despite the blue-ringed octopus’s ability to make its rings visible or invisible, they had no chemical ‘off’ switch.
The researchers found that the iridophores are tucked into modified skin folds so the iridescence is not visible when the octopus is relaxed. When threatened, the octopus relaxes one set of muscles and contracts another set to move the skin folds and expose the iridescence, a mechanism of producing iridescent signals that has never been seen before. The blue-ringed octopus is the first reported cephalopod to employ this mechanism, but it may occur in other cephalopods.
The iridescent colours of many insects may warn predators that they contain unpalatable or toxic substances. Known as aposematic colours, iridescence may be aposematic in butterflies, some beetles, and frogs. Aposematic colours are often mimicked by non-toxic species (Batesian mimicry).
The bluestriped fangblenny fish (Plagiotremus rhinorhynchos) uses iridescence in aggressive mimicry. They mimic juvenile cleaner fish but instead of cleaning their target, they attack them by removing scales and other skin tissue.

Anti-predator defence

Research has found that iridescence in insects can confuse potential predators because of the extremely rapidly changing colours that occur when the prey moves (e.g. flicks its wings or moves other body parts) or as the predator approaches. This makes pinpointing the exact location of prey difficult.

Iridescence in Myxomycetes

Lamproderma “obovoid”

Many species of myxomycetes (acellular slime moulds) have an iridescent membrane (a peridium) that encases the spore mass. The function of the iridescence has intrigued me since I started collecting slime moulds.
It is unlikely that the intense colours are for attracting invertebrates to assist in spore dispersal as most species occur in dark places where there is little chance of illumination. Furthermore, the spores of most species are wind dispersed and the invertebrates that are observed feeding on slime moulds are usually seen on the larger species such as dog’s vomit slime Fuligo septica that are not iridescent.
It could be that the complex nanostructure of the peridium provides a protective water-repellent layer in some species. The raison d’être of slime mould fruiting bodies is to produce and disperse spores. The spores themselves are hydrophobic, i.e. they resist wetting, so the peridium possibly provides added protection.
The structure of the peridium may also strengthen it, although in some species the peridium splits as soon as the fruiting bodies mature, so strength is unlikely to be an advantage. However, in other species a strong water-repellent peridium may help to resist fungal attack that is a common feature of many fruiting bodies. Those species with a fugacious non-iridescent peridium (i.e. one that rapidly falls away) such as Comatricha and Stemonitis species are particularly prone to fungal attack. Members of the order Trichiales have a peridium that displays minimal iridescence, and they too are vulnerable to being colonised, especially by the fungus Polycephalomyces tomentosis. Once infected by fungi, myxo spores are rendered nonviable.

The fungus Polycephalomyces tomentosis infects species such as this Trichia that show little or no iridescence on their peridium.

Seeing Blue

Iridescent signals enable animals to produce short wavelength colours ranging from blue to ultraviolet. (e.g. all the photos accompanying this article). Blue pigments are rare in animals and are only found in a few invertebrates and a couple of fish species. And yet, most animals can see blue wavelengths and many can detect ultraviolet wavelengths.
Some animals may use these short wavelength colours as a private communication channel if their primary predators lack ultraviolet vision.

Visual communication

Iridescent surfaces appear to change colour as the angle of view or illumination changes, making the colours highly directional. Animals can use the intense colours for species recognition, to communicate with mates, and to avoid potential predators.
Hundreds of species of iridescent hummingbirds such as the Sparkling Violetear (above) live in South America. Many have dark plumage containing melanin pigments which allows them to remain well camouflaged in the shaded rainforest where most of them dwell. To advertise their presence to potential mates-or to startle potential predators-they move their bodies to catch the light and display their highly contrasting, iridescent colours.
Mate choice
Some iridescent butterflies with intact Ultra Violet reflectance achieve greater mating success than males with experimentally reduced UV reflectance. In addition, the brightness and degree of iridescence of the eyespots on peacocks’ tails has been found to be an important cue for mate choice.

References:
Doucet, S.M. & Meadows, M.G. (2009) Iridescence: a functional perspective. Downloaded from: rsif.royalsocietypublishing.org/
Farrant, P. (1999) Colour in nature, a visual and scientific exploration. Blandford, London.
https://en.wikipedia.org/wiki/Integumentary_system (accessed February 2017)
https://i.ytimg.com/vi/SO7ZlMJv5ZM/maxresdefault.jpg (accessed March 2017)
http://jeb.biologists.org/content/215/21/3752 (accessed 17 April 2017)
http://www.iflscience.com/plants-and-animals/invisible-butterflies/ (accessed 20 April 2017)