Structural Origins and Functions of Colour Diversity in Sea Slugs: Novel Photonic Architectures, Biological Roles, and Biomimetic Applications

Abstract

Sea slugs exhibit some of the most brilliantly diverse colours and patterns of all animals on Earth and are often described as the marine world’s answer to butterflies. Whilst the extraordinary variety of their colour palette is well documented, its physical origin has been largely overlooked. In this thesis, I demonstrate that the origin of this colour diversity stems from a wide range of photonic architectures and materials, including organic and inorganic nanostructures. Specifically, I investigate the origins of colour in nine species of sea slugs from across the phylogenetic tree, using a combination of optical and electron microscopy techniques, finding that all of them have a structural origin. Six of the nine photonic architectures observed have never been reported in marine organisms before, and four of those are entirely novel for biological systems. These highly varied photonic structures bestow the animals with splendid appearances beyond the scope of solely pigmentary colour. I speculate that these structural colours might have unique biological functions, from photoprotection in Sacoglossa to Batesian mimicry in Cephalaspidea and aposematism in Nudibranchia. In the case of Elysia viridis , I extended the study of the inorganic nanostructures producing the photonic colouration by examining its biogenesis. Additionally, as a separate project inspired by the ability of this species to photosynthesise, I designed a novel ‘kleptoplastic’ material. I have shown that by mimicking photoprotection mechanisms utilised by sacoglossan sea slugs, it is possible to extend the lifetime of isolated plant chloroplasts whilst simultaneously generating record photocurrent values for a biophotovoltaic system.