Highly defended nudibranchs ‘escape’ to visually distinct background habitats

Abstract

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.

Chemical defences indicate distinct colour patterns with reduced variability in aposematic nudibranchs

“…3), thus further supporting our assumptions of significant connectivity between sampling locations. Furthermore, it is reasonable to assume similar visual and cognitive abilities of the predator community across the studied area (Losey et al, 2003), as well as the overall similarity of the visual appearance of rocky reef habitats across the area (van den Berg, Endler, & Cheney, 2022a). Together, these factors indicate our study to quantify variability between species with low levels of phylogenetic confound, consisting of well-connected meta-populations in a comparably homogeneous landscape of selective pressures across the study area.…”

“…Two individuals of Chromodoris magnifica were provided by an aquarium supplier (Cairns Marine, Pty Ltd, Cairns, QLD). A sub-sample of these images were previously used to investigate background habitat variability of nudibranch molluscs (van den Berg, Endler, & Cheney, 2022a) and distance-dependent signalling in terms of colour pattern detectability and boldness (van den Berg, Endler, & Cheney, 2022b). These species were selected as they were relatively abundant at our study sites and cover a broad range of visual appearance and strength of chemical defences.…”