School of BioSciences - Theses
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ItemLight manipulation by Christmas beetles: quantification, mechanisms and ecological relevance( 2022)The brilliant and colourful appearances of Christmas beetles (Scarabeidae - Rutelinae) are famous for decorating eucalyptus trees during the Australian summer. Millions of years of evolution perfected the design of their hardened fore wings (elytra) to manipulate light and create striking optical effects. However, little is known about the exact underlying nanostructures or the ecological variables driving their evolution. Despite the recent increase in biophotonics studies, this remains the case for many organisms with striking colourful appearances. By studying light manipulation in Christmas beetles, I aim to develop methods and concepts generalisable to other organisms. Christmas beetles produce a wide variety of optical effects including saturated colours, black, and pearlescent white. Some species look mirror-like and metallic gold or brass. Moreover, these colours change with the viewing or illumination angle. Christmas beetles can also interact with long wavelengths outside of the human-visible spectrum, in the near-infrared (NIR), and reflect circularly polarised light, which makes them appear as different colours through the right and left lenses of 3D cinema glasses. To better understand this diversity, I proposed a method for its quantification, studied some of its underlying mechanisms and tested if it can be explained by ecological differences. To characterise the beetles’ optical effects, I proposed a generalization of existing spectroscopy methods and parameters to describe reflection profiles across the visible and NIR spectrum, regardless of their underlying mechanism. Ultimately these methods break down complex optical effects into simpler traits and can facilitate comparison between studies. The proposed terminology attempts to conceptually unify disparate fields (biology and optics) and allows a clear distinction with terms used to describe colour perception. To study the optical mechanisms in Christmas beetles, I analysed the architecture of their elytra. I discovered that unlike scarab beetles studied to date, three different Christmas beetle species use multicomponent photonic systems, with an upper layer acting as a green pigment-based filter and an underlaying broadband reflector, that particularly enhances NIR reflectance. For beetles with spiral nanostructures, I found a trade-off between polarisation and NIR reflectance. This diversity of photonic structures shows that beetles are a promising model for understanding complex optical properties of natural materials. To investigate ecological correlates of the optical effects in Christmas beetles, I used a biophysical essay and a phylogenetic comparative analysis. High reflectance reduced heating of the beetle elytra under controlled conditions, but I did not find any evidence that highly reflective species occur in hotter environments. Christmas beetles do not follow a simple eco-geographical pattern, possibly because their optical effects respond to species-specific combinations of environmental challenges. I demonstrated that diversity in optical effects and photonic mechanisms is more than meets the eye, even for a small group such as Christmas beetles. My thesis highlights the value of a cross-disciplinary approach where optical methods can spark stimulating biological questions and the comparative study of phylogenetically related species can inform species selection for photonics studies. The resulting conversation between the two disciplines accelerates progress in the search for the biological function of striking optical effects.
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ItemThe evolution of salmon lice (Lepeophtheirus salmonis) in response to parasite management strategies in salmon aquaculture( 2022)Parasites, pathogens and other pests have a major impact on global agriculture. Pest populations are able to adapt to novel selection pressures on farms, making long-term pest management a challenge. This is exemplified by the repeated evolution of pesticide resistance over the last century. Resistance can be slowed or avoided by applying evolutionary principles to pest management. This first requires understanding the evolutionary dynamics of the pest under contemporary farming conditions. Such knowledge is especially needed in rapidly emerging and changing farm systems, such as aquaculture. Salmon farming – among the biggest aquaculture industries – is significantly affected by parasitic sea lice (especially the salmon louse Lepeophtheirus salmonis). Efforts to control this parasite have been undermined by salmon lice evolving resistance to most chemical pesticides. An abundance of non-chemical technologies have been introduced as alternatives, but there has been little research into whether they may also drive louse evolution. This thesis explores the potential for L. salmonis to adapt to the pressures faced in salmon aquaculture today; in particular, possible resistance to non-chemical management strategies. Chapter 1 outlines the need for evolutionary perspectives in pest management, and provides an overview of the thesis. In Chapter 2, I review the literature and conclude that salmon lice could, theoretically, adapt to non-chemical methods. This chapter explores how non-chemical resistance might arise, how it might have ripple effects on wild salmon, and how it could be prevented using evolutionarily-enlightened pest management. Chapters 3 and 4 focus on one non-chemical strategy: using physical barriers to prevent lice from entering salmon cages. In Chapter 3 I describe an empirical study that identified significant inter-family variation in louse swimming behaviour, with some families preferring deeper water than others. Chapter 4 addresses whether preventative cage barriers could impose selection on this behaviour, by inadvertently allowing deeper-swimming lice to pass underneath barriers to infest salmon. This was tested using a particle-tracking model, which predicted strong directional selection on swimming behaviour. Taken together, Chapters 3 and 4 offer the first evidence that lice could adapt to physical barriers. Chapter 5 describes a metapopulation model designed to predict evolutionary dynamics of lice across the larger, interconnected farm network. This model will be a versatile tool for future research exploring how various management regimes could limit the spread of resistance through the population. Finally, Chapter 6 ties the threads of this thesis together into a research framework that can be applied to the wider aquaculture sector to assist with managing resistance. This thesis combines theoretical, empirical and modelling techniques to predict the trajectory of L. salmonis evolution in response to different pest management strategies. Key areas for future study are highlighted – more research is crucial if the salmon industry wishes to find ways to stymie louse resistance. More broadly, this thesis illustrates that pests can adapt to management strategies – especially non-chemical methods – in surprising ways. By looking at the world from the parasite’s point of view, we can discover new techniques for safeguarding farms against undesirable evolutionary processes.