Highly defended nudibranchs do not ‘escape’ to visually diverse background habitats

The ‘escape and radiate’ hypothesis predicts that once species have evolved aposematism, defended species can utilise more visually diverse background habitats as they ‘escape’ the need to be well camouflaged. This enables species to explore new ecological niches, resulting in increased diversification rates. To test the ‘escape’ component of this hypothesis, we examined whether the background habitats of 13 nudibranch mollusc species were more variable for highly defended species (in terms of unpalatability), compared to undefended (palatable) species. We obtained 157 colour pattern statistics from the Quantitative Colour Pattern Analysis (QCPA) framework to analyse backgrounds viewed through the eyes of a potential predator (triggerfish, Rhinecanthus aculeatus). This was done at viewing distances simulating an escalating predation sequence. Contrary to the hypothesis, we found no significant positive correlations between the diversity of visual backgrounds and the strength of chemical defences, but instead found numerous significant negative correlations. Highly defended species were viewed against backgrounds up to 56.9% less variable than non-defended species. Furthermore, the strongest correlations were found at viewing distances coinciding with likely prey detection and identification. Our results provide empirical evidence against background generalisation in aposematic nudibranch species. Instead, we suggest that less variable visual backgrounds may strengthen aposematic signalling.