![]() Under a higher magnification, it is observed that a single iridophore contains mainly two stacks of the thin light-reflecting platelets (RPs). The lateral stripe of the neon tetra consists of many iridophores, which are arranged like pavement tiles. Schematic of the ( a) neon tetra and ( b) iridophore (reproduced from with permission). Consequently, the colour variation is thought to originate from the varying thickness of the cytoplasm part. Thus, there is now little doubt that the stack produces structural colours through multilayer optical interference phenomena. In fact, Lythgoe & Shand experimentally confirmed the correspondence between the spacing of the platelets and the wavelength of the reflected light. Since the platelets are observed to be periodically stacked, it is indicated that multilayer interference phenomena are the origin of the structural colour a multilayer system can selectively reflect light whose wavelength satisfies the interference condition. As illustrated in figure 1, it is known that one iridophore contains mainly two, occasionally one or three, stacks of thin platelets of guanine crystals. The microstructures inside the iridophores forming the lateral stripe have been already revealed by electron microscopy. It can even assume a yellow colour when the fish is excited or under stress. The lateral stripe of the neon tetra looks brilliant blue in the day time, while the colour changes to deep violet at night. However, the exact mechanism responsible for the change in structural colours of fish has not been fully understood, even for one of the most striking examples, the neon tetra ( Paracheirodon innesi). In fact, an attempt has recently been made to synthesize materials that have submicron periodic structures with the capability of mechanical transformation. Physiological colour change is possible because colour-producing microstructures are inside living cells, called iridophores, which are motile.įrom the view point of biomimetics, it seems quite interesting to learn how these fish realize the colour tunability, since such a property is expected in various types of tunable optical applications like lasers, band-pass filters and laser-reflecting mirrors. Such colour changes have been reported for the Atlantic killifish ( Fundulus heteroclitus, ), the cardinal tetra ( Paracheirodon axelrodi, ), the blue damselfish ( Chrysiptera cyanea, ), the common surgeonfish ( Paracanthurus hepatus, ), the paradise whiptail ( Pentapodus paradiseus, ) and so on. On the other hand, some fish have very different structural colours from those of butterfly wings and bird feathers: the colour can vary depending on the conditions of the surrounding environment. Thus, the brilliancy of these structural colours can last for a very long time unless the microstructures are destroyed. In these examples, the microstructures mainly consist of chemically stable materials such as dried cuticles in butterfly wings or keratin and melanin granules in bird feathers. Some of them possess very high reflectance with saturated colours that are produced by highly sculpted microstructures. The experimental results and detailed analysis are found to quantitatively verify the model.īutterfly wings and bird feathers are well-known examples of structural colour in nature. ![]() In particular, we have prepared a new optical system that can simultaneously measure both the spectrum and direction of the reflected light, which are expected to be closely related to each other in the Venetian blind model. In order to quantitatively evaluate the validity of this model, we have performed a detailed optical study of a single stack of platelets inside an iridophore. ![]() As a mechanism of the colour variability, the Venetian blind model has been proposed, in which the light-reflecting platelets are assumed to be tilted during colour change, resulting in a variation in the spacing between the platelets. It has been known that an iridophore of the neon tetra contains a few stacks of periodically arranged light-reflecting platelets, which can cause multilayer optical interference phenomena. This fact clearly indicates the variability of the colour-producing microstructures. The structural colour of the neon tetra is distinguishable from those of, e.g., butterfly wings and bird feathers, because it can change in response to the light intensity of the surrounding environment.
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