For sea slugs, bright colours may warn off predators—but only in the daytime

Research Highlight: van den Berg, C. P., Hassler, G., Blomberg, S., Wilson, N., Suchard, M., & Cheney, K. (2025). Diel activity correlates with colour pattern morphology of heterobranch sea slugs. Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.70036. The ecological drivers of colour evolution in marine systems provide an important comparison to studies in terrestrial species that traditionally receive more research attention. However, many studies have historically been limited by lack of (1) standardised colour pattern data across well-resolved phylogenies and (2) methodological approaches that objectively assess colouration as a highly multidimensional trait. In a recent study, van den Berg et al. (2025) combined a novel phylogenetic tree with Quantitative Colour Pattern Analysis (QCPA) to test hypotheses about ecological correlates of colouration in highly charismatic Australian sea slugs. The authors found that diurnal activity was tightly correlated with bold patterning and high contrast of colour, luminance and overall pattern elements, and that a reliable set of 55 colour traits could accurately predict diel activity in nearly 90% of species. These results provide an exciting case study for future analyses of multidimensional colour patterns and advance our understanding of how ecological interactions can shape colour phenotypes that span highly variable signalling environments, especially in marine organisms so profoundly understudied relative to terrestrial taxa.