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Colour Change and Camouflage in Rockpool Fish

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Colour Change and Camouflage in Rockpool Fish Volume 1 of 1 Submitted by Samuel Smithers to the University of Exeter as a thesis for the degree of Masters by Research in Biosciences – C in September 2015 This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signature: ………………………………………………………….. Photo credit: Alice Lown S m i t h e r s | 2 Abstract Camouflage is one of the most widespread anti-predator strategies in nature. Many animals use a combination of both morphological and behavioural means. Camouflage can be particularly challenging in heterogeneous environments, and as such some animals have evolved under selection to change colour to enable them to camouflage on a range of different background types. One such species is the rock goby (Gobius paganellus), a common rockpool fish capable of rapidly (within one minute) changing its colour and luminance (perceived lightness) when placed on different backgrounds. The rock goby provides a good model for studying rapid colour change in fish inhabiting habitats such as rocky shores that tend to be highly heterogeneous, and where fish may be exposed to both terrestrial and marine predators depending on tidal level. I used digital image analysis and a model of predator vision to quantify changes in colour, luminance, pattern, and camouflage. In chapter 2 I investigate the ability of rock gobies to match the colour of sand and algae covered rock, and test whether a fish’s previous background affects their ability to match a new one. I also tested their ability to match a range of different background brightness. Finally, I ask whether rock gobies exhibit behavioural background matching in addition to adaptive colour change. In chapter 3 I ask whether rock gobies change their body pattern in response to their visual background, and then whether the spatial frequency of more natural backgrounds influences pattern change. I found that the gobies rapidly changed colour and luminance in response to the different backgrounds and an individual’s previous background had no effect on its ability to change colour and camouflage on a new background. The level of camouflage did however differ between backgrounds whereby some colours and brightness appeared easier to match than others. Rock gobies also showed a behavioural preference for darker backgrounds over lighter ones. Moreover, gobies are capable of rapidly changing their body pattern in response to their background, with high spatial frequency substrates such as sand inducing the greatest change in pattern. This thesis shows that small rockpool fish use a combination of rapid colour and pattern change, and behaviour background choice, to camouflage themselves against their background. S m i t h e r s | 3 Acknowledgements Doing research and writing a thesis with ADHD is challenging. It is like having a Ferrari engine for a brain with the brakes of a bicycle. This year had been one of the steepest learning curves of my academic life so far. I am very grateful to my supervisor Dr Martin Stevens for his guidance, imparted knowledge, and enthusiasm throughout my masters. I also extend my sincere thanks to my second supervisor Dr Alastair Wilson for his insightful comments and invaluable knowledge of statistics. It has been a pleasure and a privilege to be part of the Sensory Ecology and Evolution research group and I am thankful to everyone in the group, past and present, for their incredible kindness and for their willingness to help from the very start. I am particularly grateful to Dr Jolyon Troscianko for putting up with my endless list of questions, and for his help and guidance with the image analysis and vision modelling. Special thanks also go to Lina María Arenas, Jenny Easley, Emmanuelle Briolat, and Sara Mynott, as well as other members of the Sensory Ecology group for their help, advice, and friendship throughout my masters. I also thank Steve White and Owen Greenwood for advice and discussion about statistics, and Rebecca Rooney for her invaluable help in conducting the background choice experiments in chapter 2. Finally, I would not have been able to do this if it were not for the encouragement and loving support of my parents. Your struggles, worrying, and unwavering belief are paying off. Thank you so much. S m i t h e r s | 4 Table of Contents Abstract ..................................................................................................................................... 2 Acknowledgements .................................................................................................................. 3 Table of Contents ..................................................................................................................... 4 List of Tables and Figures ....................................................................................................... 6 Tables ..................................................................................................................................... 6 Figures .................................................................................................................................... 6 Chapter 1: Overall introduction ........................................................................................... 13 Camouflage and colour change in animals .......................................................................... 14 The functions of colour change in animals ....................................................................... 14 Camouflage in colour changing animals .......................................................................... 16 Crypsis through colour change ..................................................................................... 18 Behaviourally-mediated crypsis .................................................................................... 23 Purpose of this thesis ........................................................................................................ 24 Chapter 2: Colour change and behavioural background matching in a rockpool fish ... 26 Abstract ................................................................................................................................ 27 Introduction .......................................................................................................................... 28 Methods ................................................................................................................................ 31 Colour and luminance change experiments ...................................................................... 31 Generating the experimental backgrounds.................................................................... 31 Experimental set up....................................................................................................... 33 Experimental procedure ................................................................................................ 37 Background choice experiments ....................................................................................... 38 Experiments 3 and 4: experimental set up and procedure ............................................ 38 Image and video processing ............................................................................................. 42 Statistical analysis ......................................................................................................... 45 Results .................................................................................................................................. 47 Colour and luminance change experiments ...................................................................... 47 Experiment 1 ................................................................................................................. 47 Experiment 2 ................................................................................................................. 52 Background choice experiments ....................................................................................... 55 S m i t h e r s | 5 Experiment 3 ................................................................................................................. 55 Experiment 4 ................................................................................................................. 57 Discussion ............................................................................................................................ 59 Chapter 3: Rockpool gobies change the expression of their body pattern in response to changes in background markings ......................................................................................... 67 Abstract ................................................................................................................................ 68 Introduction .......................................................................................................................... 69 Methods ................................................................................................................................ 72 Preliminary experiment ...................................................................................................
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