UC Santa Barbara

Although pigments contribute much of the brilliant purple and orange coloration of the aeolid nudibranch, Flabellina iodinea, the brightness of both colors was found to be enhanced by unusually sparkling iridocytes that exhibit rapid temporal variation in the brightness of their reflectivity. Electron micrographic examination revealed these epidermal cells to contain numerous multilayer stacks of crystals, both within vesicles and apparently free in the cells. High-resolution light microscopy showed that these structures tumble freely to produce the observed sparkling reflectivity. Most abundant near the epithelial basal lamina, the perceived color of these sparking iridocytes appears dependent on the size and thickness of the crystal platelets that comprise the stacks, with those that exhibit silver reflectance in the orange cerata being larger and thicker than those that reflect blue in the epithelium of the purple body. Thin layer chromatography and UV spectrometry show that the crystals isolated from all epithelial regions are identical in composition, with guanine as the major component and its derivative, hypoxanthine, a minor component. Electron diffraction of the crystals isolated from the orange and purple tissue exhibit nearly identical lattice parameters of a = 6.57 ± 0.05 Å, b = 13.65 ± 0.07 Å and c = 18.76 ± 0.06 Å - unit cell dimensions that closely match those measured for the non-tumbling guanine crystals widely distributed in other biophotonic systems ranging from marine invertebrates to terrestrial vertebrates. The epidermal iridocytes of F. iodinea are thus capable of manipulating crystal growth to produce multilayer stacks of purine crystals with optical properties specific to different locations in the body. This specificity parallels and apparently augments the animal’s striking pattern of pigmentation.