Investigating Defensive Colouration in Nudibranch Molluscs using a Novel Analytical Framework for the Study of Animal Colour Patterns​​​​​​​

Abstract

Animal colour patterns are complex and highly diverse traits that are used for intra- and inter-specific communication, thermoregulation and predator avoidance, including aposematism, camouflage and mimicry. To understand the evolution and design of such colouration, it is essential to quantify animal colour patterns and how they appear against their natural background. Visual signals comprise multiple elements, such as luminance, chromaticity and pattern geometry, and the integrated perception of these elements against the background determine how an animal appears to predators, prey and conspecifics. However, despite considerable attention in the literature, the ability of visual ecologists to achieve a comprehensive analysis of complex visual signals has been limited, mainly due to quantitative and qualitative limitations of data acquisition. Importantly, existing pattern analyses failed to integrate the perception of colour-, luminance- and pattern geometry contrast of complex animal colour patterns against their natural background.In this thesis, I have overcome many limitations of colour pattern analyses by combining calibrated digital photography, visual modelling and comprehensive colour pattern analysis. I first developed a new colour pattern analysis framework (QCPA) (Chapter 2) in a collaborative effort. This resulted in user friendly and open source software running on two separate software platforms (Matlab & ImageJ), accompanied by a website hosting manuals, user guides, worked examples, tutorials and videos, and a user forum.I used this new methodology to investigate the evolution and design of colour patterns in nudibranch molluscs (Chapter 3). I investigated morphological differences in colour patterns between dorid nudibranchs exclusively active during the night and those active during daytime at dive sites in rocky shore sites in Nelson Bay, NSW, Australia. This chapter investigated a key assumption underlying the use of nudibranch molluscs for the study of defensive colouration, namely that selection pressure by visual predators is the primary cause for colour pattern evolution in nudibranchs. To do this, I obtained a calibrated image database of 23 species of dorid nudibranchs against their natural backgrounds. I found distinct differences in colour pattern morphology between species active at either time of the day, supporting the hypothesis that colour pattern morphology is indeed correlated with daytime activity.To test assumptions on parameter and model choices using QCPA and deepen our understanding of animal vision, I conducted behavioural experiments using Picasso triggerfish (Rhinecanthus aculeatus) to determine thresholds of luminance contrast detection and discrimination of reef fish (Chapter 4). This study found profound context dependant differences in luminance discrimination thresholds, questioning the current use of visual models to describe luminance contrast perception in animals.The work in this thesis greatly contributes to the ability of researchers to investigate visual perception in non-humans at a quantitative scale, while contributing to our understanding of how animal might perceive spatiochromatic information. My thesis greatly reduces current boundaries to visual modelling and colour pattern analysis by creating a comprehensive and user friendly open-source software and online user platform. My work innovates and expands upon currently used tools for the study of animal vision, while investigating the use of the receptor noise limited model. It also provides specific solutions while outlining potential for future research and development. I also provide a specific example of how to calibrate parameter choice for the QCPA. Finally, the thesis provides a first application of the QCPA framework while highlighting the great suitability of nudibranchs as a model organism of growing importance for the study of defensive animal colouration.