Abstract
The marine world is incredibly rich in brilliant and intense colours. Photonic structures are found in many different species and provide extremely complex optical responses that cannot be achieved solely by pigments. In this study we examine the cuticular structure of the red alga Chondrus crispus (Irish Moss) using anatomical and optical approaches. We experimentally measure the optical response of the multilayer structure in the cuticle. Using finite-difference time-domain modelling, we demonstrate conclusively for the first time that the dimensions and organisation of lamellae are responsible for the blue structural colouration on the surface of the fronds. Comparison of material along the apical-basal axis of the frond demonstrates that structural colour is confined to the tips of the thalli and show definitively that a lack of structural colour elsewhere corresponds with a reduction in the number of lamellae and the regularity of their ordering. Moreover, by studying the optical response for different hydration conditions, we demonstrate that the cuticular structure is highly porous and that the presence of water plays a critical role in its ability to act as a structural light reflector.
Highlights
The marine world is incredibly rich in brilliant and intense colours
Structural colour is widespread in nature[2,3,4,5,6,7,8,9,10], and has been observed in a range of organisms, where it is employed for functions including visual communication, mate attraction and camouflage[11,12,13]
Photonic structures responsible for structural colouration have been well documented in underwater organisms[14,15,16,17,18], land animals, including insects and birds[3,19,20,21,22,23] and recently in plants[24,25], within leaves[26,27,28], flowers[29,30,31] and fruits[32,33]
Summary
The marine world is incredibly rich in brilliant and intense colours. Photonic structures are found in many different species and provide extremely complex optical responses that cannot be achieved solely by pigments. The most common mechanism for structural colour production in nature is multilayer interference Light propagating in such structures is reflected at interfaces between adjacent layers that differ in refractive index and can constructively interfere[1]. Studies focusing on Chondrus crispus found that structural colour only occurred in the gametophyte stage of the lifecycle (Fig. 1a), and was never observed in tetrasporophytes. Gerwick and Lang[41] were able to remove structural colouration by mechanically scraping off the cuticle layers, suggesting that the laminated cuticle could be involved in the production of colour Taken together, these studies have identified structural features capable of producing colour, but lack both clear and quantitative elucidation of the mechanistic basis and an understanding of the biological purpose(s) of such colour
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