Gur, D., Leshem, B., Pierantoni M., Farstey, V., Oron, D., Weiner, S., Addadi, L., (2015). Structural Basis for the Brilliant Colors of the Sapphirinid Copepods. Journal of the American Chamical Society. 137: 8408-8411. DOI: 10.1021/jacs.5b05289
Sapphirnid copepods or ‘sea sapphires’ as R.R. Helm from Deep Sea News has dubbed them, are marine crustaceans about half the size of a grain of rice and can be found worldwide within the first 300m of the ocean. Only males produce these intense iridescent colours (different species produce different colours) which they achieve through a unique spiral swimming behaviour. Trends show that species that flash warmer colours such as yellow, orange and red are typically found in shallow waters, whereas species that flash cooler colours such as blue and green are found deeper in the ocean—although colour can also vary among individuals of the same species. It is known that the bold, iridescent colours reflected from these male sea sapphires (as well as fish scales, silver spiders and chameleons) are caused by alternating microscopic layers of two transparent materials. In sea sapphires, these materials are thin plates of highly reflective guanine crystal and cytoplasm within the skin cells covering the organism’s backside (see high school refresher box if you’re fuzzy on the terms). However, what makes sea sapphires unique is the shape of their crystal plates: perfect hexagons in a tightly-packed, orderly arrangement (See Figure 1). Scientists previously postulated that crystal thickness was responsible for observed reflectance; however when studies failed to find correlation between crystal thickness and colour reflectance, it was clear there was a piece of the puzzle missing. Enter Gur et al., 2015 to crack the code!
Two species of male sea sapphires were examined: Sapphirina metallina—equipped with a 10-14 layer crystal & cytoplasm sandwich within their cells and Copila mirabilis—equipped with a 5-8 layer crystal & cytoplasm sandwich. First, the sea sapphires were photographed, and their reflectance was measured using a custom-built microscope. The same sea sapphires were then high-pressure frozen (a technique used to ensure optimal preservation of nanostructure of material), freeze-fractured (quickly frozen and cracked to achieve a clean break along the weakest portions of the specimen), then examined using a cryogenic scanning electron microscope or cryo-SEM (a microscope specifically designed to handle very cold objects—think below -150°C or -238°F). Using the images from the cryo-SEM, the thickness of the crystal and cytoplasm layers were measured.
Results & Significance
Crystal plate thickness was consistently ~70 nanometers (nm) among different coloured sea sapphires; however, the thickness of the cytoplasm layers ranged between 50-200 nm. This finding shows that it is thickness of the cytoplasm, and not the crystal thickness themselves that is responsible for the colours reflected! And finally, measured crystal reflectance matched simulated reflectance (Figure 2).
To take advantage of their deliberately spaced reflector crystals, male sea sapphires undergo a spiral swimming behaviour which increases in speed and rotation when exposed to more light. By doing so, they are changing the angle at which the light reflects off its reflective crystal plates. Interestingly, as the angle of light moves away from 90°, (think flashlight beam shining straight into a mirror, then rotating the beam away from center), the wavelengths reflected become shorter until in some cases, it is such that the reflectance shifts out of the visible light spectrum, into the UV (Figure 3). In otherwords, it becomes invisible.
Evolutionarily, this is a double-edged strategy, as it not only acts like a lighthouse one moment—beaconing to nearby lady sea sapphires that they are present, but they also seem to disappear at the next moment which helps with predator evasion. Why should humans care? Well there are many potential applications in optic technology, but most intriguingly, perhaps these findings can shift James Bond’s invisible car or Harry Potter’s invisibility cloak from fiction to reality.
What applications of invisibility are you most excited about? Let us know in the comments below!