The Evolution of Colour Pattern Complexity in Nudibranch Molluscs

One of the most striking phenomena of animal signalling is called aposematism. It describes the use of highly conspicuous colours and colour patterns for the purpose of signalling unprofitability to potential predators. The design of aposematic signals evolves to increase the detectability of an unprofitable species, enhance predator avoidance learning and to decrease recognition error by predators. Complex colour patterns may be more difficult to recognise, whereas simple, very conspicuous colour patterns are expected to work more effectively as aposematic signals. However,there is little experimental studies to support this hypothesis, probably due to the difficulty of analysing complex pattern geometry. In this study, I used nudibranch molluscs to examine the hypothesis that increased colour pattern simplicity correlates with increased levels of secondary defence. With over 3000 species worldwide, nudibranchs are an extremely diverse group of animals. They are known for their diverse phenotypic variation in body colouration, patterns and chemical defences and are an intriguing and understudied model organism to examine the evolution of animal colour patterns. The nudibranch colour patterns were analysed using underwater photographs of the animals in situ. The obtained images were then quantitatively analysed using colour adjacency analysis, an analytical framework recently developed by Professor John Endler (Deakin University, Australia) in 2012. The resulting data was then compared to toxicity and unpalatability data of the analysed species in order to investigate the correlation of colour pattern complexity and chemical defence using phylogenetic regression analysis. I found a general trend which seems to suggest that a correlation between colour pattern complexity and toxicity could indeed exist. However, due to a limited sample size in toxicity data, the results were not significant. Ancestral state reconstruction indicates that aposematism in nudibranchs has evolved from moderately defended common ancestors with an intermediate level of colour pattern complexity. This study provides important insights into the evolution of aposematic signals and how animal colour pattern geometry can be studied in a quantitative way using a novel analytical framework.

Promotor(s) & Supervisor: Karen Cheney

ISCED Categories

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0511 - Biology", "0521 - Ecology