The blue-spotted stingray produces a unique blue colour
Blue-spotted stingrays are rather shy sea creatures, but are sought-after photo subjects by divers - thanks to their blue spots. For the first time, researchers have investigated how they produce this colour.
Blue is rarely found in nature and has to be elaborately produced by animals and plants, as it is not a pigment but a structural colour. The blue-spotted stingray (Taeniura lymma) is no exception. And as its eponymous spots always appear the same bright blue colour, even when viewed from different angles, researchers were particularly interested in the structure of the skin cells responsible. And indeed, the fish produces its characteristic features in a unique way, as reported by a team led by Mason Dean from the University of Hong Kong. It reminded the research group a little of the bubble tea pearls that are stored in the liquid.
Dean and co x-rayed the animals' skin using microcomputed tomography, scanning and transmission electron microscopes to decipher the skin's "delicate architecture", as they write. The rays have unique skin cells with a stable three-dimensional arrangement of nanoscale spheres, which in turn contain reflective nanocrystals. The sizes of the nanostructures and their spacing correspond to a multiple of the wavelength of blue light, which is why they specifically reflect blue radiation.
The arrangement of the spheres ensures that the blue always appears the same, regardless of the angle from which the skin is viewed. A thick, melanin-containing layer beneath these cells in turn absorbs all other colours, resulting in extremely light blue skin. Only both cell types together ensure the intensity of the colour. Although the blue spots appear very conspicuous to the human eye, they help the fish to camouflage itself. "In water, blue penetrates deeper than any other colour and helps the animals blend in with their surroundings," explains Dean in a statement. This is especially true when the animal is swimming or manoeuvring quickly with undulating fin strokes.
Next, the working group - which is also hoping for applications of the blue spotted ray principle in the colour industry - would like to investigate the blue shark. Initial analyses indicate that it relies on completely different mechanisms than the ray to produce its blue tones.
Spectrum of Science
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